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2002
Sedimentology and stratigraphy of the Upper
Cretaceous-Paleocene El Molino Formation,
Eastern Cordillera and Altiplano, Central Andes,
Bolivia: implications for the tectonic development
of the Central Andes
Richard John Fink
Louisiana State University and Agricultural and Mechanical College
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Fink, Richard John, "Sedimentology and stratigraphy of the Upper Cretaceous-Paleocene El Molino Formation, Eastern Cordillera and
Altiplano, Central Andes, Bolivia: implications for the tectonic development of the Central Andes" (2002). LSU Master's Theses. 3925.
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SEDIMENTOLOGY AND STRATIGRAPHY OF THE UPPER CRETACEOUSPALEOCENE EL MOLINO FORMATION, EASTERN CORDILLERA AND
ALTIPLANO, CENTRAL ANDES, BOLIVIA: IMPLICATIONS FOR THE
TECTONIC DEVELOPMENT OF THE CENTRAL ANDES
A Thesis
Submitted to the Graduate Faculty of the
Louisiana State University and
Agricultural and Mechanical College
in partial fulfillment of the
requirements for the degree of
Master of Science
in
The Department of Geology and Geophysics
by
Richard John Fink
B.S., Montana State University, 1999
August 2002
ACKNOWLEDGEMENTS
I would like to thank Drs. Bouma, Ellwood and Byerly for allowing me to present
and defend my M.S. thesis in the absence of my original advisor. Drs. Brian Horton
(University of California, Los Angeles) and William Devlin (ExxonMobil Exploration)
were instrumental in my development as a scientist and critical thinker. I would like to
thank Brian Hampton for providing valuable advice and commentary at all stages of this
project. Brian Lareau supplied additional remarks during the project development and
analysis phases of this thesis. I am immensely thankful to Dr. Sohrab Tawackoli of the
Servicio de Geologia y Minas de Bolivia (La Paz) for logistical support and advice in
support of fieldwork. Pedro Chuvata was extremely helpful as a driver, liason and field
assistant. James Graber also provided valuable assistance with fieldwork. Conoco Inc.
provided petrographic laboratory support.
This research was supported by National Science Foundation funds awarded to
Dr. Brian Horton. Additional funding was provided by the Geological Society of
America, American Association of Petroleum Geologists and Sigma Xi national grantsin-aid programs. Lastly, I would like to thank my parents, Henry J. Fink Jr. and Ilse R.
Fink, and Bruce Miller for moral support and encouragement.
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS..............................................................................................ii
LIST OF TABLES............................................................................................................ v
LIST OF FIGURES ......................................................................................................... vi
ABSTRACT...................................................................................................................viii
1. INTRODUCTION ........................................................................................................ 1
1.1 Tectonic Overview................................................................................................ 2
1.2 Upper Triassic(?)-Paleogene Stratigraphic Overview .......................................... 5
1.2.1 Lower Succession Strata .............................................................................. 5
1.2.2 Middle Succession Strata............................................................................. 5
1.2.3 Upper Succession Strata ............................................................................ 10
2. CRITERIA FOR DISTINGUISHING TECTONIC SETTING ................................. 13
2.1 Rift Basin Systems.............................................................................................. 13
2.1.1 Rift Basin Evolution .................................................................................. 13
2.1.2 Syn-Rift Recognition Criteria........................................................................ 16
2.1.3 Post-Rift Basin Recognition Criteria ......................................................... 19
2.2 Retroarc Foreland Basin Systems ....................................................................... 19
2.2.1 Foreland Basin System Evolution.............................................................. 19
2.2.2 Foreland Basin Recognition Criteria ......................................................... 22
3. EL MOLINO FORMATION...................................................................................... 26
3.1 General Description ............................................................................................ 26
3.1.1 Regional Correlations ................................................................................ 29
3.1.2 Age Control................................................................................................ 30
3.1.3 Depositional Environment-Previous Work................................................ 30
3.2 Depositional Environment-This Work................................................................ 32
3.2.1 FA1 Open Water Facies............................................................................. 32
3.2.2 FA2: Nearshore Facies............................................................................... 41
3.2.3 FA3 Beach, Bar and Shoal Facies.............................................................. 49
3.2.4 FA4: Floodplain Facies.............................................................................. 55
3.2.5 FA5: Fluvial Facies.................................................................................... 62
3.3 Discussion ........................................................................................................... 68
4. PALEOCURRENT DIRECTIONS AND PROVENANCE....................................... 72
4.1 Methods and Background Data........................................................................... 72
4.2 Description.......................................................................................................... 76
4.3 Interpretation....................................................................................................... 81
4.4 Discussion ........................................................................................................... 82
5. CONCLUSIONS......................................................................................................... 85
iii
REFERENCES ............................................................................................................... 89
APPENDIX A. STRATIGRAPHIC SECTIONS ......................................................... 101
APPENDIX B. STRATIGRAPHIC SECTION COORDINATES AND NOTES....... 112
VITA ............................................................................................................................. 116
iv
LIST OF TABLES
2-1 Recognition criteria for syn-rift, post-rift and foreland basin settings .................. 14
3-1 Table showing lithofacies codes used in subsequent stratigraphic columns ......... 36
4-1 Table showing point count parameters.................................................................. 74
4-2
Table showing raw thin section point count data .................................................. 75
4-3 Table showing raw clast count data....................................................................... 77
v
LIST OF FIGURES
1-1 Digital elevation and schematic maps showing the field area................................. 3
1-2
Cross-sectional diagram of the Central Andes ........................................................ 4
1-3 Chronostratigraphic column showing Cretaceous Bolivia strata ............................ 6
1-4
Photo of La Puerta Formation ................................................................................. 7
1-5 Photo showing eolian cross-bedding within the La Puerta Formation.................... 8
1-6
Photo showing igneous intrusion within the La Puerta Formation ......................... 9
1-7 Photo of middle succession strata at the Miraflores Syncline ............................... 11
1-8 Photo of the Chaunaca and El Molino Formations................................................ 12
2-1
Illustration showing the evolution of active and passive rifts ............................... 15
2-2
Illustration showing the key components of a foreland basin system ................... 21
3-1 Photo showing silicified gastropods used for stratigraphic correlations ............... 27
3-2 Close-up photograph of silicified gastropods........................................................ 28
3-3 Base map showing the location of stratigraphic sections ...................................... 33
3-4 Graphic representation of FA1 .............................................................................. 34
3-5
Legend for stratigraphic columns .......................................................................... 35
3-6
Regional distribution of FA1................................................................................. 38
3-7 Graphic representation of FA2 .............................................................................. 42
3-8 Photo showing mud cracks in massive limestone beds ......................................... 43
3-9 Photo showing a theropod trackway in FA2 strata................................................ 44
3-10 Photo showing mammilated stromatolites in FA2 strata....................................... 45
3-11 Regional distribution of FA2................................................................................. 46
3-12 Graphic representation of FA3 strata..................................................................... 50
vi
3-13 Photo of horizontally-laminated and low angle trough cross beds in FA3 ........... 51
3-14 Photo of fossiliferous packstone in FA3................................................................ 52
3-15 Regional distribution of FA3................................................................................. 54
3-16 Graphic representation of FA4 .............................................................................. 56
3-17 Photo of FA4 paleosol ........................................................................................... 58
3-18 Photo of amalgamated carbonate nodules ............................................................. 59
3-19 Regional distribution of FA4................................................................................. 60
3-20 Graphic representation of FA5 .............................................................................. 63
3-21 Photo of FA5 lenticular sandstone bed.................................................................. 64
3-22 Photo of horizontally-laminated sandstone ........................................................... 65
3-23 Regional distribution of FA5................................................................................. 67
4-1 Map showing paleocurrent data and clast count summaries ................................. 78
4-2
Ternary diagrams showing sandstone composition............................................... 79
5-1
Summary diagram showing facies and distributions............................................. 86
vii
ABSTRACT
The Upper Cretaceous-Paleocene El Molino Formation of the Bolivian Central
Andes consists of mixed siliciclastic and carbonate strata alternately interpreted as synrift, post-rift thermal sag and foreland basin deposits. These deposits can be divided into
two lithostratigraphic sequences. The first sequence consists of a carbonate, carbonate
sand and mudrock lower member, a middle member consisting entirely of mudrock and
an upper member containing carbonates and mudrocks. The second lithostratigraphic
sequence contains a lower member composed of carbonate sands, carbonates and
mudrocks, and an upper member consisting of a coarsening upwards sequence of
sandstones and mudrocks.
Within these lithostratigraphic sequences, five facies associations can be
identified: 1) an open water facies; 2) a nearshore facies; 3) a beach, bar and shoal facies;
4) a floodplain facies; and 5) a fluvial facies. A regional study of El Molino Formation
stratigraphic stacking patterns and facies association geographic distributions suggests
that deposition occurred within a dominantly lacustrine basin. For most of El Molino
Formation deposition, the lacustrine system remained hydrologically-closed and
perennial, although evidence indicates that depositional systems experienced periodic
ephemeral lacustrine conditions as well as hydrologically open lacustrine and/or shallow
marine depositional environments. While lacustrine systems exist in syn-rift, post-rift
thermal sag and foreland basin systems, sedimentological and stratigraphic data, in
addition to an absence of key indicators of tectonic activity (e.g. faulting, growth strata)
limit the El Molino Formation tectonic setting to post-rift thermal sag and/or foreland
basin back-bulge settings. Paleocurrent and provenance data further support the
viii
interpretation for post-rift thermal sag and/or back-bulge basin deposition, indicating
flow of continental block provenance sediment into a central depositional basin while
clast count data show a simple unroofing sequence indicative of the tectonic quiescence
associated with post-rift thermal sag and foreland basin back-bulge tectonic settings.
ix
1. INTRODUCTION
Sedimentary basins form in response to subsidence of the earth’s crust (Allen and
Allen 1990). Different subsidence mechanisms vary with different tectonic settings,
producing different characteristic basin geometries. For example, normal fault-controlled
subsidence characteristic of rift settings produces relatively narrow, elongate, simple
fault-bound basins normal to the axis of maximum extension. Alternately, flexural
subsidence in compressional fold-thrust belt systems creates broad foreland basin systems
that thin away from the fold-thrust belt load.
Sedimentary basin geometry is a prominent factor governing distribution of
sedimentary facies, basin fill provenance, and paleoflow within the basin (Eisbacher et al.
1974; Cant and Stockmal 1989; Flemings and Jordan 1989; Owen and Crossley 1989;
Jordan and Flemings 1991; Large and Ingersoll 1997). When preserved, these basin fill
characteristics act as a proxy record of basin evolution, allowing the reconstruction of
past tectonic events (DeCelles 1986; Crossley et al. 1992; Horton and DeCelles 2001).
The tectonic events governing deposition of the Upper Cretaceous-Paleocene El
Molino Formation of the Central Andes (Bolivia) have been attributed to syn-rift
(Viramonte et al. 1999), post-rift (Welsink et al. 1995; Mertmann and Fiedler 1997), and
foreland basin settings (Sempere 1994, 1995; Sempere et al. 1997). This study tests these
three competing hypotheses by reconstructing the El Molino Formation depositional
system, basin geometry, and tectonic evolution from 16 regionally-distributed measured
stratigraphic sections totaling 4440 m, 19 paleocurrent measurement stations (n = 185),
10 conglomerate clast counts (n = 1107) and 23 sandstone thin-sections.
1
1.1 Tectonic Overview
The modern Central Andes (Figs. 1-1, 1-2) consists of five distinct physiographic
provinces: 1) the Western Cordillera, an active magmatic arc; 2) the Altiplano Plateau, a
crustal-scale piggyback basin; 3) the Eastern Cordillera, the deformed thrust belt core; 4)
the Subandean zone, the active fold-thrust belt; and 5) the Chaco and Beni Plains and
Pantanal Wetlands, the modern, undeformed foreland basin (Jordan et al. 1983; Isaacks
1988; Horton and DeCelles 1997; McQuarrie and DeCelles 2001).
The evolution of these physiographic provinces began with the onset of
subduction-related east-stepping magmatic arc development during late Paleozoic-Early
Mesozoic time (Forsythe 1982; Herve et al. 1987; Ramos 1988; Coney and Evenchick
1994; Viramonte et al. 1999). During the late Permian-Triassic periods, extensional rift
basins developed in western Chile and central Peru, continuing into Cretaceous time with
new rift basins forming in northern Argentina and Bolivia (Gallinski and Viramonte
1988; Cominguez and Ramos 1995; Sempere 1995;Welsink et al. 1995; Sempere et al.
2002). Paleozoic basement rocks and Mesozoic pre-rift, syn-rift and, post-rift strata were
subsequently uplifted during the late Cretaceous-Paleocene formation of an eastwardmigrating retroarc foreland basin system (Sempere 1994, 1995; Horton and DeCelles
1997; Sempere et al. 1997; Horton et al. 2001). During late Eocene to early Miocene
time, west vergent backthrusting at the western edge of the Eastern Cordillera created a
crustal-scale piggyback basin between the backthrust belt and the Western Cordilleran
volcanic arc (McQuarrie and DeCelles 2001). Meanwhile, the east vergent foreland basin
system migrated to its current position in the Subandean Zone, Chaco and Beni Plains
and Pantanal Wetlands.
2
3
PERU
70
Be
ni
Pl
Ea
a
ste Su
W
ba
es
r
n
t
SB Zone
Puna
60
ARGENTINA
PARAGUAY
BOLIVIA
Figure 1-1. Digital elevation and schematic map showing study area (boxed area) and the physiographic
provinces of the Bolivian Central Andes. Cool colors (blue and purple) represent low elevation topography
while hot colors (yellow and red) represent high elevation. The flat green topography adjacent to the South
American orocline represents the ~3.8 km-high Altiplano Plateau (DEM image produced by Cornell Andes
Project). Graphic map modified from Hampton (2002).
80
30
20
10
BRAZIL
Chaco Plain
Zone
an
in
e
d
llera
rdi lano
o
C Altip
n
illera
Cordcordillera
e
n
er Pr
5 km
0 km
-5 km
Chile
Trench
Western
Cordillera
Altiplano
Plateau
Eastern
Cordillera
Subandean
Zone
Vertical Exaggeration = 10x
MOHO
?
100 km
Naz
ca P
200 km
300 km
0 km
200 km
South American
? Lithosphere
1300°C
late
600 km
400 km
800 km
1000 km
Figure 1-2. Cross-sectional diagram showing the relative elevation of Central Andean
physiographic provinces (names on top) and the general tectonic scenario (Modified from
Hampton 2002).
4
1.2 Upper Triassic(?)-Paleogene Stratigraphic Overview
Upper Triassic(?)-Paleogene basin deposits of the Puca Group in the Eastern
Cordillera and Altiplano (Fig. 1-3) include: 1) a lower succession dominated by
regionally extensive, Upper Triassic(?) to Lower Cretaceous sandstone; 2) a middle
mudrock, carbonate, and evaporite succession deposited in restricted, localized basins;
and 3) a regionally extensive upper succession consisting of carbonate rocks, mudrock
and sandstone.
1.2.1 Lower Succession Strata
Lower succession rocks consist of the poorly dated Triassic-Lower Cretaceous La
Puerta Formation and equivalent units (Kosmina, Macha, Ravelo and Sucre Formations)
are present throughout the study area (Viramonte et al. 1999). These rocks
unconformably overlie Paleozoic basement rocks, consisting of up to 1350 m of
sandstone with rare >200 m thick basal conglomerate rocks. Basal conglomerate rocks
include basalt and Paleozoic basement rock clasts while sandstone beds exhibit
ubiquitous trough cross-stratification interpreted as braided fluvial deposits and ~10 m
thick cross-beds suggesting an eolian environment (Figs. 1-4, 1-5) (Cherroni 1977;
Sempere, 1994; Mertmann and Fiedler, 1997). Basalt flows (Fig. 1-6), and rare volcanic
dikes interbed with and crosscut lower succession rocks. In addition, Sempere (1994)
reports normal faulting of lower succession rocks north at Otavi.
1.2.2 Middle Succession Strata
Strata of the middle succession unconformably overlie lower succession rocks and
include the Lower to Upper Cretaceous Condo, Tarapaya, Miraflores, Aroifilla, Torotoro
5
I
I
I
I
P
V
V
V
V
N
6
Valanginian
Berriasan
Hauterivian
Barremian
Aptian
Albian
Cenomanian
Campanian
Santonian
Coniacian
Turonian
Maastrichtian
Thanetian
Selandian
Danian
Stage
Senonian
140
130
120
110
100
90
80
70
60
Volcanism Faulting Ma
Epoch Period
b
32n
33n
31n
30n
29n
28n
27n
Upper
V Volcanics
N Normal Faulting
72 Ma
70 Ma
68 Ma
66 Ma
64 Ma
62 Ma
60 Ma
Legend
M Magnetostratigraphy
I Isotopic Date
Mammal Fossils
Marine/Lacustrine Fossils
P Palynology
Middle
Lower
Cretaceous
Lower
Neocomian
Figure 1-3a. Chronostratigraphic column relating Cretaceous stratigraphy to age data and episodes of volcanism and
faulting.Grey shaded area highlights the El Molino Formation. Note that the formations listed consist of the prevalent
formations in the area of interest and does not contain the formation names of all stratigraphic equivalents in the Bolivian Central Andes. Figure 1-3b, Magnetostratigraphic column comparing magnetostratigraphic chrons with relative
stratigraphic level within the El Molino Formation. Note that the magnetostratigraphic column is hinged on two Ar-Ar
dates of 71.6 and 72.6 Ma below the 25m level of the El Molino Formation, and lacks correlative age control above
these two isotopic dates. Paleomagnetic, isotopic and palynological age data compiled from Viramonte et al. (1999)
and Sempere et al. (1997). Fossil information derived from Gayet et al. (1991, 1993, 2001) and Marshall et al. (1997).
Fault data taken from Sempere (1994). Figure 1-3b is modified from Sempere et al. (1997).
M
Age Data
Upper
La Puerta
Formation
Tarapaya
Miraflores
Aroifilla
Chaunaca
El Molino
Santa Lucia
Formation
Tertiary
Paleogene
a
Puca Group
Paleocene
El Molino Formation
Figure 1-4. Outcrop exposure of lower succession La Puerta Formation at the Miraflores Syncline northwest of Potosi. Note that the obvious cliff-former in the foreground, as well as the less exposed sandstone strata forming the ridgeline, both represent La Puerta Formation strata.
7
Figure 1-5. Eolian cross-bedding in La Puerta Formation sandstones at the town of Betanzos located approximately 35 km west of Potosi. Scale person is approximately 1.8 m tall.
8
Figure 1-6. Igneous intrusion within the La Puerta Formation at Incapampa.
9
and Chaunaca Formations with a total thickness >1800 m in some locations (Figs. 1-7,
1-8) (Sempere 1995; Viramonte et al. 1999). These rocks can be divided into four
lithofacies groups: 1) brown mudrock (Tarapaya Formation) with a rare basal
conglomerate (Condo Formation) interpreted as alluvial to lacustrine deposits (Sempere
1994); 2) carbonate beds with minor mudrock interbeds (Miraflores Formation)
representing a rapid shallow marine transgression (Sempere 1994); 3) red to green
mudrock intercalated with minor evaporite beds, sandstone, carbonate rocks,
conglomerate, volcanic flows and tuff beds (Aroifilla and Chaunaca Formations)
interpreted as distal alluvial to playa lake deposits (Mertmann and Fiedler 1997; Sempere
et al. 1997); and 4) red sandstone with volcanic flows (e.g., Maragua) and conglomerates
containing basalt clasts (Torotoro Formation) representing alluvial plain deposition
(Marshall et al. 1997; Sempere et al. 1997).
1.2.3 Upper Succession Strata
Upper succession rocks conformably overlie middle succession rocks or
unconformably onlap lower succession or Paleozoic basement rocks (Fig. 1-8). This
stratigraphic interval reaches thickness >1000 m and includes the Upper CretaceousPaleocene El Molino, Santa Lucia and Impora Formations (Sempere et al. 1997). Upper
succession rocks consist of mudrock, carbonate, sandstone, evaporite, and tuff beds, and
have been interpreted as lacustrine, shallow marine, fluvial and alluvial deposits
(Sempere et al. 1997; Gayet et al. 2001).
10
La Puerta Formation
Tarapaya Formation
Miraflores
Formation
Figure 1-7. Photograph of middle succession rocks including reddish-brown Tarapaya
Formation mudrock and whitish-yellow Miraflores Formation limestone at the Miraflores
syncline northwest of Potosi. The lower succession La Puerta Formation forms the upper
ridgeline and part of the dip-slope.
11
Basal El Molino
Formation
Chuanaca Formation
Basal "Green" Layer of the
Chaunaca Formation
Figure 1-8. Foreground rocks represent the basal "green" layer of the middle succession
Chaunaca Formation. Reddish-violent background rocks belong to the Chaunaca Formation while resistant yellow-beige rocks form the base of the upper succession El Molino
Formation. Scale person is ~1.8m.
12
2. CRITERIA FOR DISTINGUISHING TECTONIC SETTING
Distinguishing tectonic environments from the rock record can be difficult. The
contrasting tectonic interpretations for El Molino Formation deposition underscore this
statement. In order to discern between tectonic settings for the El Molino Formation, an
understanding of tectonic basin evolution and associated recognition criteria is required.
The following describes the tectonic evolution and recognition criteria (Table 2-1) for
syn-rift, post-rift and retroarc foreland basin deposits.
2.1 Rift Basin Systems
2.1.1 Rift Basin Evolution
Continental rifts represent the crustal manifestation of lithospheric extension.
Lithospheric extension results from active or passive driving forces within the
asthenosphere (McKenzie 1978; Sengor and Burke 1978; Ruppel 1995). Active rifting
(Fig. 2-1) occurs in response to the “active” impingement of asthenospheric material
upon the base of the lithosphere (Sengor and Burke 1978; Ruppel 1995). Buoyant
magmatic plumes rise through denser asthenospheric material, exerting a normal force
upon the base of the lithosphere (Fig. 2-1a) (Ruppel 1995). The lithosphere responds to
this normal force by upwarping and thinning, forming a characteristic crustal dome (Fig.
2-1b) (Neugebauer 1978; Sengor and Burke 1978; Ruppel 1995). As the magmatic plume
continues to impinge upon the lithosphere, large volumes of melt are produced and
subsequently extruded (Fig. 2-1b) (Ruppel 1995). Crustal extension follows magmatism,
forming rapidly subsiding normal fault bound syn-rift basins (Fig. 2-1c) (Ruppel 1995;
Crossley and Cripps 1999). Examples of active rifts include the West Antarctic, East
African, Ethiopian and Rio
13
14
Tholeiitic and alkaline volcanism.
Basins typically consist of a
series of narrow (20-50 km),
long (50-100 km) basins comnected by transform zones. Segments reach depths of 6-10 km.
Rapid subsidence.
Continental block provenance
characterized by high quartz
content.
Volcanism
Basin Geometry
Subsidence Curve
Provenance
Foredeep depozones tend to
form deep, elongate basins.
Forebulge depozones form topographic highs. Back-bulge
basins are commonly broad and
shallow.
Possible volcanism but not
characteristic of environment.
Thrust faulting common in
wedge-top depozone. Localized
normal faulting reported for
forebulge depozones.
Foreland Basin
Continental block provenance
characterized by high quartz
content.
Recycled orogen provenance
characterized by quartz and
plagioclase feldspar content.
Subsidence curve shows inverse Subsidennce curve appears
logarithmic geometry.
approximately sigmoidal in
geometry.
Basins show lens-like geometry
in cross-section normal to and
centered over syn-rift depositional centers.
Possible volcanism but not
characteristic of environment
Normal faulting common within Devoid of structures.
half- and full-graben basins.
Post-Rift Thermal Sag Basin
Structural Style
Syn-Rift Basin
Table 2-1. Table showing recognition criteria for discriminating between syn-rift, post-rift thermal sag and foreland basin systems.
15
Lithosphere
Crust
Asthenosphere
Zone of
Preexisting
Weakness
Incipient Rift Zone
Rising Magmatic
Plume
Secondary
Convection??
Early Stage
Magmatism
e
Lithospheric and
Sublithospheric
Magmatism
Incipient Normal
Faulting
Upwelling
Lithosphere
Crust
Onset of Crustal Rifting
Passive Rifting
Asthenosphere
b
Asthenosphere
Lithosphere
Magmatism
Lithospheric Thinning
Crust
f
Asthenosphere
Lithosphere
c
Normal Fault-Bounnd
Crustal Rifts
Upwelling
Potential Crustal
Underplating
Mature Rift
Asthenosphere
Lithosphere
Crust
Late-Stage
Magmatism
Normal Fault-Bound Syn-Rift Formation
Figure 2-1. Figure showing the differences between active and passive rifting processes. Active rifting processes
begin with a phase of mantle impingement on the base of the crust, magmatism, uplift and subsequent fault-related
extension (steps a through c). Passive rifting processes start with the lithospheric extension, then fault-related extension followed by mantle upwelling and associated uplift and magmatism (steps d through f). Figure is modified
from Ruppel (1995).
d
a
Asthenosphere
Lithosphere
Long-wave Thermal Doming
Active Rifting
Grande rifts (Sengor and Burke 1978; Fairhead 1986; Reading 1986; Leeder 1995;
Ruppel 1995).
Passive rifting (Fig. 2-1) initiates in response to shear stress caused by plate
boundary interactions or sublithospheric convective drag (Ruppel 1995). Crustal normal
faulting follows lithospheric extension, generally forming along preexisting zones of
weakness (Fig. 2-1d). As the lithosphere thins in response to extension, asthenospheric
material upwells beneath the thinned lithosphere (Fig. 2-1e). Magmatism commonly
follows asthenospheric upwelling, underplating the crust or emplacing magma at or near
the surface (Fig. 2-1f). Lastly, late-stage epeirogenic doming may occur (e.g., Oslo Rift)
although doming is not a defining characteristic of passive rifting (McKenzie 1978;
Rohrman et al. 1994; Ruppel 1995). The Baikal Rift, Oslo Rift, West and Central African
Rift System, Gediz Graben, Basin and Range Provinces, and Rhine Graben are all
examples of passive rifting (McKenzie 1978; Sengor and Burke 1978; Reading 1986;
Rohrman et al. 1994).
Post-rift basins result from the thermal relaxation of the lithosphere after syn-rift
thinning and heating. During syn-rift crustal thinning, lithospheric temperatures are
elevated, causing lithospheric expansion. Once the syn-rift heat source abates, the
lithosphere cools and contracts, creating broad, post-rift basins centered over the syn-rift
grabens (Sclater and Christie 1980).
2.1.2 Syn-Rift Recognition Criteria
Several recognition criteria allow for the identification of syn-rift tectonic
settings: 1) a pre- or post-rift unconformity; 2) tholeiitic to alkaline volcanism; and 3)
normal faulting. The pre-rift unconformity forms during the initial doming phase of
16
active rift systems while rare post-rift unconformities occur during late stage passive
rifting. Although tholeiitic to alkaline volcanism commonly occurs in many rift
sequences, volcanism alone is not a reliable indicator of rift development (Sengor and
Burke 1978; Reading 1986; Friedmann and Burbank 1995; Crossley and Cripps 1999).
Many rift systems lack significant volcanic rocks, and where present, volcanism is
concentrated along the rift axis (Crossley and Cripps 1999; Le Turdu et al. 1999).
Normal faulting occurs within both active and passive rift systems, however fault
timing varies between systems. Active rift systems form normal faults towards the end of
the rift cycle, subsequent to the onset of crustal doming and magmatism. Normal faulting
in passive systems represents an early stage of rift development, preceding magmatism
and possible epeirogenic uplift. In either case, normal faults constitute a primary
characteristic of rift basins, defining basin geometry, acting as the major subsidence
mechanism and exerting considerable control over the location of sedimentary facies
(Gupta et al. 1998).
Intracontinental rift basins typically consist of a series of long (50-100 km),
narrow (20-50 km) fault-bound asymmetric half-graben segments linked by strike-slip
fault zones (Reading 1986; McHargue et al. 1992; Friedmann and Burbank 1995). Halfgraben rift basin extension usually persists for >10 Ma (30-40Ma), potentially evolving
into full horst-graben systems (Wilson 1989; Friedmann and Burbank 1995; Scholz et al.
1998; Le Turdu et al. 1999). However, total rift basin extension rarely exceeds 20% or a
stretching factor (β-factor) of 1.2. Subsequent basin subsidence initiates with a very brief,
early stage of slow subsidence followed by a lengthier phase of rapid, fault-induced
17
subsidence generating basins with up to 6-10 km of accommodation space (Friedmann
and Burbank 1995; Gupta et al. 1998).
Sediment accumulation within subsiding syn-rift basins varies with distance from
the basin-bounding normal fault. The hanging wall immediately adjacent to the footwall
marks the axis of maximum syn-rift subsidence (Cohen 1990). Due to rapid subsidence,
this zone consists of short, steep, footwall-derived drainage systems, commonly
producing thick, small area, rarely coalescing fan delta and alluvial fan deposits
(Ingersoll 1988; Cohen 1990; Mack and Seager 1990). Footwall-derived fan deltas and
alluvial fan deposits commonly contain poorly sorted, monomictic pre-rift sediment
(Cohen 1990; Crossley and Cripps 1999). In deepwater systems, footwall-derived,
dominantly coarse-grained turbidity flows are also common (Cohen 1990; Lambiase
1990; Scholz 1990; Leeder 1995).
Footwall-derived fan deltas, turbidity flows and alluvial fans interfinger with
hanging wall-derived fine-grained deepwater, playa lake or fluvial deposits (Owen and
Crossley 1989; Mack and Seagar 1990; Johnson and Ng’ang’a 1990; Friedmann and
Burbank 1995). Hanging wall-derived deposits vary considerably, including lacustrine
and marine siliciclastic and carbonate sediments in addition to fluvio-deltaic, fluvial and
alluvial fan deposits (Ingersoll 1988; Cohen 1990; Friedmann and Burbank 1995).
Hanging wall alluvial fans tend to be broader in dimension and finer-grained than
footwall-derived alluvial fan strata, receiving sediment from large, low relief drainage
systems (Ingersoll 1988; Cohen 1990; Mack and Seagar 1990). Because of low hanging
wall relief, changes in climate or subsidence rates trigger rapid changes in basin facies
distribution (Cohen 1990).
18
2.1.3 Post-Rift Basin Recognition Criteria
Post-rift basins consist of broad, relatively shallow saucer-shaped basins centered
over syn-rift deposits (Sclater and Christie 1980; McHargue et al. 1992; Henry et al.
1996; van Wees et al. 1996). For example, North Sea post-rift basins contain 1100-1400
m of strata at basin center, thinning to 600-700 m towards the flanks and cover an area
200-300 km wide by 600 km long (Sclater and Christie 1980). Normal faults associated
with syn-rift tectonism are absent (Sclater and Christie 1980; Henry et al. 1996). Post-rift
basin strata include shallow marine, lacustrine or fluvial facies deposits (Sclater and
Christie 1980; McHargue et al. 1992; Crossley and Cripps 1999). Other examples of
post-rift basins include the West African Kwanza, and East Brasil Rift basins (Sclater and
Christie 1980; Henry et al. 1996).
2.2 Retroarc Foreland Basin Systems
2.2.1 Foreland Basin System Evolution
Retroarc foreland basin systems represent regions of potential sediment
accommodation that develop on continental crust between subduction-related
compressional fold-thrust belts and cratons (Dickinson 1974; Beaumont 1981; Jordan
1981; DeCelles and Giles 1996). Sediment accommodation space in retroarc foreland
basin systems results from flexure of the continental lithosphere by orogenic and
sediment loading (Jordan 1981; Beaumont 1981) in addition to dynamic subsidence
(Mitrovica et al. 1989; Gurnis 1992). Orogenic and sediment loading generates a flexural
wave normal to the fold-thrust front, generally migrating cratonward with the advance of
the orogenic belt over 10’s of millions of years (Flemings and Jordan 1990; DeCelles and
Currie 1996). In addition to wave effects resulting from orogenic and sediment loading,
19
the rate of fold-thrust belt advancement also effects foreland basin wavelength (Flemings
and Jordan 1989). Rapid thrusting produces short wavelength, high amplitude foreland
basins while slow thrust rates generate long wavelength, low amplitude foreland basins
(Flemings and Jordan 1989).
The basin geometry produced by the flexural wave can be divided into wedge-top,
foredeep, forebulge and back-bulge depozones (Fig. 2-2)(DeCelles and Giles 1996). The
wedge-top depozone consists of depositional basins within the orogenic belt and extends
cratonward to the forward-most extent of frontal thrust deformation (DeCelles and Giles
1996). Foredeep deposits accumulate within the high amplitude, flexurally down-warped
basin adjacent between the orogenic load and the positive amplitude forebulge (DeCelles
and Giles 1996). The positive amplitude forebulge is a theoretical model-driven
construct. Load-driven flexural models using an elastic lithosphere indicate the existence
of a forebulge at a distance of πα (for an infinite plate) or 3πα/4 (for a broken plate)
normal to the axis of loading, where α is defined as the flexural parameter that describes
the relationship between flexural rigidity and the density contrast between mantle and
basin fill (DeCelles and Giles 1996). Evidence for forebulge existence consists of
Bouguer gravity highs, stratigraphic onlap patterns and normal faulting present at or
about the predicted axis of the forebulge (Coudert et al. 1995; Horton and DeCelles
1997; Currie 1998; Hudson 2000). The forebulge depozone consists of strata deposited
upon or immediately adjacent to this theoretical positive relief feature (DeCelles and
Giles 1996).
Cratonward of the forebulge depozone lies the broad wavelength, shallow,
negative amplitude back-bulge depozone (DeCelles and Giles 1996). While foredeep
20
21
FRONTAL TRIANGLE ZONE
FOREDEEP
FOREBULGE
CRATON
BACK-BULGE
Figure 2-2. Illustration showing the key components and theoretical depozones within a foreland basin
system. Note the overall wedge-shaped basin fill geometry (dotted area). Modified from DeCelles and
Giles (1996).
HINTERLAND
DUPLEX
FOLD-THRUST BELT
OROGENIC WEDGE
WEDGE-TOP
TF
FORELAND BASIN SYSTEM
depozone accommodation space primarily results from orogenic and sediment loading of
continental lithosphere, some deposits attributed to back-bulge deposition show (e.g.,
Morrison Formation in Utah) thicker accumulations of strata than predicted by loaddriven flexural models (DeCelles and Currie 1996; DeCelles and Giles 1996; Currie
1998). Similarly, some foredeep depozones also exhibit unusual basin amplitudes
unexplainable by simple flexure beneath observed topographic and basin fill loads
(DeCelles and Giles 1996). These inconsistencies may represent the effects of dynamic
subsidence resulting from viscous-coupling of continental lithosphere with mantle
material entrained by a subducting oceanic slab (Mitrovica et al. 1989; Gurnis 1992;
DeCelles and Currie 1996; DeCelles and Giles 1996). The effects of dynamic subsidence
can generate kilometer-scale subsidence at wavelengths extending 1000 km or more from
the trench (Mitrovica et al. 1989), potentially generating constructive or destructive
interference between load-driven flexural waveforms and dynamic subsidence-related
flexural waves (DeCelles and Giles 1996).
2.2.2 Foreland Basin Recognition Criteria
With the competing influences of load-driven and dynamic flexural subsidence,
the presence of orogenic belt thrust and reverse faults and potential forebulge normal
faulting, recognition of foreland basin systems from ancient stratigraphic deposits can be
challenging. A first order glance shows that foreland basin strata form a doubly tapered
wedge-shaped geometry in cross-section (Fig. 2-2) (DeCelles and Giles 1996). Foreland
basin depozones migrate with the fold-thrust belt, successively replacing one another in
an upwards-coarsening sequence that potentially records the passage of each stratigraphic
22
depocenter (DeCelles and Currie 1996; DeCelles and Giles 1996; Horton and DeCelles
1997).
Straddling the fold-thrust belt and extending cratonward to the frontal tip of thrust
belt deformation, the wedge-top depozone is characterized by an abundance of fault- and
fold-related growth structures and unconformities indicating deposition within the
deformation front (DeCelles and Giles 1996; Horton and DeCelles 1997). Wedge-top
sediments commonly contain coarse-grained, texturally and compositionally immature
sediment derived directly from the thrust belt (DeCelles and Giles 1996). Subaerial
wedge-top strata include coarse-grained alluvial and fluvial sediments while subaqueous
strata consist of fine-grained shelf and lacustrine sediments and mass flow deposits
(DeCelles and Giles 1996).
Cratonward of the wedge-top depozone, the foredeep depozone accumulates
sediment shed from the thrust front and, more rarely, sediment eroded from the magmatic
arc, forebulge and/or craton (Dickinson 1974; DeCelles and Giles 1996). Foredeep
provenance typically depends upon the pre-orogenic history of strata involved in
orogenic folding and thrusting (Dickinson 1974; Dickinson and Suczek 1979). Thickest
stratal accumulations occur adjacent to the wedge-top depozone, commonly reaching 2-8
km in depth and tapering towards the forebulge 100-300 km away (Flemings and Jordan
1989; DeCelles and Giles 1996). Subaerial foredeep depocenters often contain fluvial
megafans showing transverse paleoflow and axial fluvial systems fed by tributaries from
the thrust front and craton (Cant and Stockmal 1989; Flemings and Jordan 1989;
DeCelles and Giles 1996; Horton and DeCelles 1997). Deposition within subaqueous
foredeep depozones consists of shallow water (< 200m) siliciclastic and carbonate shelf
23
deposits (DeCelles and Giles 1996; Gupta and Allen 2000). Where both subaqueous and
subaerial settings coexist, the transitional zone commonly contains large delta deposits
(DeCelles and Giles 1996).
At the cratonward edge of the foredeep, the forebulge depozone consists of a
positive amplitude feature 60-470 km wide and 10s to 100s of meters high that separates
the foredeep from the back-bulge depozone (DeCelles and Giles 1996). The topographic
expression of the forebulge may be suppressed by burial and flexural loading beneath
sediment shed from the thrust belt (Flemings and Jordan 1989). The forebulge depozone
typically represents a zone of limited or condensed deposition commonly accompanied
by paleosol development, or an area of non-deposition resulting in an unconformity
(Jacobi 1981; Cant and Stockmal 1989; Flemings and Jordan 1990; DeCelles and Giles
1996; Currie 1998). Stratal thinning, onlap and the presence of local normal-fault bound
depositional centers also characterize forebulge depozones (DeCelles and Giles 1996;
Hudson 2000). In subaqueous environments, carbonate ramps and reefs may form on the
forebulge (DeCelles and Giles 1996; Ver Straeten and Brett 2000).
Subaqueous carbonate ramps and reefs may also extend into the back-bulge
depocenter in addition to other shallow marine sediments (DeCelles and Currie 1996;
DeCelles and Giles 1996; Ver Straeten and Brett 2000). Subaerial back-bulge deposits
largely consist of fine-grained sediment transported from the fold-thrust belt but cratonic
and forebulge sources may be important sediment contributors as well (DeCelles and
Giles 1996). Provenance work by Currie (1998) shows back-bulge sediments plotting
within the continental block and recycled orogen provenance fields of Dickinson and
Suczek (1979) suggesting orogenic belt and cratonic sediment contributions. Although
24
fine-grained fold-thrust belt strata tend to dominate back-bulge deposition, coarse-grained
deposits adjacent to the flank of the uplifted forebulge deposystem may also be present
(DeCelles and Giles 1996; Currie 1998). Back-bulge deposits reach thickness greater than
250 meters, contradicting flexural model predictions (DeCelles and Currie 1996; Horton
and DeCelles 1997; Currie 1998) although additional accommodation space may result
from dynamic subsidence (Mitrovicia et al. 1989; Gurnis 1992). These deposits typically
accumulate within elongate, closed basins (DeCelles and Giles 1996; Horton and
DeCelles 1997).
25
3. EL MOLINO FORMATION
3.1 General Description
The upper succession El Molino Formation contains a complex association of
Maastrichtian-Paleocene mudrocks, carbonate strata, sandstones and minor evaporite
rocks. Basal El Molino Formation rocks are identified by the first calcareous sandstone or
carbonate beds overlying the Chaunaca, Torotoro or La Puerta (and equivalent)
Formations or older rocks (Lohmann and Branisa 1962; Fiedler and Mertmann 1997;
Sempere et al. 1997). The lower contact between the El Molino and the Cretaceous
Chaunaca/Torotoro Formations is conformable whereas unconformable (locally angular
with <15°) contacts divide El Molino Formation strata from underlying La Puerta
Formation and Paleozoic strata.
For the purpose of regional correlation of El Molino Formation strata, gastropod
fossils proved extremely useful. The gastropods used for correlation are <5 cm long, <2
cm wide and have a spiral-shaped conical morphology (Figs. 3-1, 3-2). For a more
detailed listing of El Molino Formation fossil content and related paleoenvironmental
interpretations see Gayet et al. (1991, 1993, 2001) and Marshall et al. (1997).
At a scale of hundreds of meters, El Molino Formation rocks form two
lithostratigraphic sequences. The first lithostratigraphic sequence (e.g., Torotoro, Ichilula
sections, Appendix 1) consists of three lithostratigraphic members: 1) a lower member
containing calcareous sandstone, carbonate and mudrock beds that forms the lower ~50%
of a given stratigraphic section; 2) comprising ~35% of the stratigraphic section, the
middle member is dominated by mudrock; and 3) an upper member consisting of
26
Figure 3-1. Photograph showing silicified gastropod fossils used for regional stratigraphic
correlation within the El Molino Formation. Photograph taken at Incapampa locality.
27
Figure 3-2. Photograph showing a close-view of gastropod fossils used for regional stratigraphic correlation within the El Molino Formation. Photograph taken at Incapampa locality.
28
interbedded mudrock and carbonate strata that usually makes up the remaining ~15% of
the section. This first lithostratigraphic sequence has been recognized and reported by
previous workers and, with the exception of stratigraphic sections circa Lake Titicaca,
commonly outcrops on the Altiplano and the western Eastern Cordillera (e.g., Marshall et
al. 1997; Sempere et al. 1997). The second sequence (e.g., Jesus de Machaca, Incapampa
sections, Appendix 1) contains a lower member characterized by calcareous sandstone,
carbonate and mudrock beds (similar to the lower member of the first sequence) and
transitions up section to an upper member consisting of sandstone and mudrock strata.
The lower member typically comprises ~25% of the section. This second
lithostratigraphic sequence occurs at sections near Lake Titicaca and in the eastern
Eastern Cordillera.
Both sequences form transitional contacts with the overlying Santa Lucia
Formation. This contact consists of a subtle color change from reddish- purple El Molino
Formation mudrock to brick red Santa Lucia mudrock. Sempere et al. (1997) suggest that
the transitional nature of the boundary results from changing climatic conditions.
3.1.1 Regional Correlations
The El Molino Formation has been correlated with the Bolivian Eslabon-Flora
and Cajones Formations, Peruvian Vilquechico Formation, northwest Argentinian
Yacoraite, Lecho, Tunal and Olmedo Formations, and the north Chilean Estratos de
Quebrada Blanca de Poquis and Pajonales Formations (Cherroni 1977; Sempere et al.
1997). The presence of similar fossil faunas (e.g., the fish species Gasteroclupea
branisae and Pucapristis branisi) support correlations between El Molino Formation
29
strata and the Eslabon-Flora, Cajones, Vilquechico and Yacoraite Formations (Cherroni
1977; Salfity and Marquillas 1981; Jaillard et al. 1993).
3.1.2 Age Control
The El Molino Formation represents the best-dated stratigraphic sequence in the
entire Upper Triassic?-Paleocene Puca Group. Isotopic dates, magnetostratigraphy and
fossil data (Fig. 1-3) place El Molino Formation deposition between Maastrichtian and
Paleocene time. Two Altiplano tuffs from the basal 25m of the El Molino section yielded
40
Ar/39Ar isotopic dates of 71.59 ± 0.25 Ma and 72.57 ± 0.15 Ma (Sempere et al. 1997).
With these dates as a baseline, subsequent magnetostratigraphy places El Molino
deposition between chrons 32n and 26r (~73-60Ma) (Sempere et al. 1997). Fossil fauna
collected south of Cochabamba further support these age data with 9 fossil taxa restricted
exclusively to the Maastrichtian and 6 taxa limited to Maastrichtian-Paleocene time
(Gayet et al. 2001).
3.1.3 Depositional Environment-Previous Work
The depositional setting of the El Molino Formation has been addressed in
numerous studies (e.g., Gayet et al. 1993, 2001; Rouchy et al. 1993; Sempere 1994, 1995;
Camoin et al. 1997; Sempere et al. 1997; Deconinck et al. 2000). These studies generated
two competing hypotheses suggesting that El Molino Formation deposition occurred in
either a non-marine lacustrine system with periodic outflow to marine waters (Rouchy et
al. 1993; Camoin et al. 1997; Deconinck et al. 2000) or a lacustrine system that
experienced periodic marine incursions (Gayet et al. 1993, 2001; Sempere 1994, 1995;
Sempere et al. 1997).
30
The data used to support an exclusively non-marine hypothesis include: 1) δ18O
(PDB) values of –14.2 to 2.8‰, and δ13C (PDB) values of –12.9 to 2.8‰ (Camoin et al.
1997); 2) δ18O and δ13C covariance (Camoin et al. 1997); and 3) dominance of terrestrial
and freshwater fossil fauna (Rouchy et al. 1993; Camoin et al. 1997). While the preferred
explanation for the stable isotope (δ18O and δ13C) data is that of non-marine deposition
(Keith and Weber 1967; Platt 1989; Talbot 1990; Talbot and Kelts 1990), a marine
depositional setting cannot be ruled out if based only on these data. Average δ18O values
from Cretaceous (n=39) and Tertiary (n=63) marine limestones range from –4.49±1.85 to
–2.99±2.37 while average δ13C values are from 0.25±3.44‰ to -1.51±2.77‰ (Keith and
Weber 1967). El Molino Formation values for δ18O and δ13C are –14.2 to 2.8‰ and –
12.9 to 2.8‰, respectively, thus failing to exclude a marine depositional setting.
Covariant δ18O and δ13C values are characteristic of hydrologically-closed lacustrine
basin systems (Talbot 1990; Talbot and Kelts 1990). However, isotopic covariance has
also been identified in restricted marine settings (Ingram et al. 1996). This result also
demonstrates that δ18O and δ13C covariance fails to exclude marine deposition.
In support of a non-marine origin is the presence of non-marine fauna within El
Molino Formation rocks, but this result still fails to exclude periodic marine conditions.
Of the numerous faunal genera identified in the El Molino Formation, most are
freshwater, brackish water or marine fauna capable of entering non-marine environments
(Gayet et al. 1993, 2001; Rouchy et al. 1993; Camoin et al. 1997). However, at least one
exclusively marine genus (Aulopiformes Enchodus) exists within El Molino Formation
strata (Gayet et al. 2001). The presence of Aulopiformes Enchodus and the predominance
of marine taxa within the El Molino Formation indicate episodic marine deposition
31
(Gayet et al. 1993, 2001; Sempere et al. 1997). Although many of the marine taxa present
could enter the El Molino Formation depositional system during periods of lacustrine
outflow (Rouchy et al. 1993; Camoin et al. 1997), evidence supporting the existence of
Aulopiformes Enchodus within non-marine waters would be required to fully debunk a
marine-influenced depositional system hypothesis. Regardless of actual depositional
conditions, the presence of marine fauna indicates that the El Molino Formation
depositional system was at or near sea level.
The work cited above (Rouchy et al. 1993; Camoin et al. 1997; Sempere et al.
1997; Deconinck et al. 2000; Gayet et al. 2001) demonstrates the difficulties encountered
when attempting to distinguish shallow marine from lacustrine deposits in the El Molino
Formation. In order to simplify facies analysis for the El Molino Formation and to reflect
the dominant lacustrine depositional signature (e.g., Rouchy et al. 1993; Camoin et al.
1997; Deconinck et al. 2000) lacustrine facies terminology has been adopted.
3.2 Depositional Environment-This Work
Observations from 16 regionally-distributed measured stratigraphic sections (Fig.
3-3) suggests that El Molino Formation strata consists of five facies associations (FA): 1)
open water facies; 2) nearshore facies; 3) beach, bar and shoal facies; 4) floodplain
facies; and 5) fluvial facies.
3.2.1 FA1 Open Water Facies
3.2.1.1 Description
Black, gray, brown and green 0.05-17m thick shale bedsets alternate with thickly
laminated (3-10 mm thick) 0.02-0.14m thick yellow, gray and green carbonate mudstone
bedsets (Figs. 3-4). Shale and carbonate mudstone beds have planar bounding surfaces
32
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
Camargo
Bolivia
El Puente
Tupiza
Tarija
22
22
Major Towns
0
100 km
Measured Section
Political Boundary
Figure 3-3. Map showing the location of measured stratigraphic sections,
and major towns.
33
5
Fl
Fl
Sh with Sr
Fl
Sh with Sr
Fl
Fl
4
meters
3
Fl
2
Fl
1
Fl
0
M
SS
Cl
Figure 3-4. Graphic representation of FA1: Open water facies association. Lithologic
legend and guide to lithology codes are shown in Figures 3-5 and Table 3-1.
34
Massive Sandstone
Tubular, vertical burrows
Horizontally-laminated
sandstone
Soft Sediment Deformation
Flat Stromatolites
Massive Mudstone
Chert Nodules
Carbonate
Calcarerous Nodules
Sandy or Silty
Carbonate
Bivalves
Oolitic Carbonate
Gastropods
Mostly covered
siltstone interval
Vertebrate Fossils
Covered Interval
Ostracods
Trough cross-stratified
sandstone
Sandy, pebble to small
boulder conglomerate
Fossil Hash
n = 20 Arrow indicates paleocurrent
direction.
n = number of measurements
Intraclasts
Bioturbation
Cruciform trace fossils
0
M SS CL
Root traces
conglomerate
mudstone
sandstone
Figure 3-5. Graphic legend showing lithologic symbols and conventions used in
stratigraphic columns within this study. Note that sandstone and mudrock lithologies are further described by lithologic codes found in Table 3-1.
35
36
Ripple cross-laminated
fine- to coarse-grained
sandstone
Horizontally laminated
sandstone
Trough cross-stratified
fine- to very coarse
sandstone
Massive fine- to very
coarse-grained sandstone
Sr
P
Sm
St
Sh
Paleosol
Suspension fallout or weak traction transport followed
by dessication and shrinkage.
Massive mudrock with
mud cracks
Fm
Pedogenic alteration of post-depositional mudrock and
sandstone by eluviation, illuviation, roots, and bioturbation.
Original depositional process unknown due to destruction
of primary sedimentary structures.
Lower flow regime migration of 3-D dunes.
Upper flow regime plane bed migration.
Migration of lower flow regime current ripples (unidirectional
or bidirectional).
Migration of lower flow regime current ripples (unidirectional
of bidirectional.
Fr
Horizontally laminated
clay to very fine-grained
material
Ripple cross-stratified
mudrock.
Process
Suspension fallout or weak traction transport of clay, mud, silt
and very fine-grained material followed by subsequent destruction of primary sedimentary structures.
Suspension fallout.
Fl
Lithofacies Code
Lithofacies
Fsm
Massive mud- to very
fine-grained material
Table 3-1. Lithofacies codes used in stratigraphic columns throughout the text (Modified from Miall 1977).
Note that lithofacies code Fr is used here to distinguish ripple cross-lamination in mudrocks rather than
Miall's (1977) original usage to denote presence of root casts and mottling).
and appear laterally continuous over 100s of meters. Black and brown shale strata often
exude a musty, organic aroma.
Shale and carbonate mudstone strata frequently interbed with tabular 0.4-1.2 m
vertical sequences of normally graded very fine sandstone to siltstone. Vertical sequences
consist of: 1) a planar non- to weakly-erosive basal contact; 2) horizontally-laminated
very fine sandstone with rare 0.05-0.3m convolute bedding; and 3) ripple cross-laminated
very fine sandstone and siltstone. At one location, basal sandstone beds contain mudrock
intraclasts.
Offshore lacustrine facies (FA1) rocks infrequently occur within the basin. Where
present, FA1 strata outcrop within the basal half of a given El Molino Formation
stratigraphic section. The greatest concentration of FA1 strata occurs at the Ichilula and
Maragua localities with subordinate outcroppings at Challa Mayu, Vilcapuyo and
Camargo (Fig. 3-6). FA1 stratigraphic packages rarely reach thickness greater than 7m
and pass from or into FA2 nearshore facies strata.
3.2.1.2 Interpretation
Thin, flat lamination suggests deposition of clastic and carbonate mud by
suspension settling (Zhang et al. 1998). Carbonate material is derived from three possible
sources: 1) primary inorganic precipitation of CaCO3; 2) biogenically-produced CaCO3
shells or structural components; and 3) allochthonous carbonate clasts derived from the
drainage basin or cannibalized from carbonate deposits within the depositional basin
(Kelts and Hsu 1978; Dean and Fouch 1983; Eugster and Kelts 1983). Carbonate material
derived from source (3) requires the erosion and transportation of drainage system
carbonate rocks (Kelts and Hsu 1978). While potential carbonate source rocks existed
37
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
22
Bolivia
El Puente
Tupiza
Major City
Measured Section
Political Boundary
Camargo
Tarija
Facies Distribution
0
22
100 km
Figure 3-6. Regional distribution of FA1: Open water facies association.
38
within the depositional basin (e.g., Permo-Carboniferous Copacabana Formation), the
paucity of marlstone beds suggests that chemically precipitated and/or microfossilderived material dominated carbonate mud input. Chemically precipitated and
microfossil-derived carbonate sediment requires quiet water depositional conditions
devoid of significant clastic input (Tucker and Wright 1995).
Interstratified shale and laminated carbonate mudstone commonly represent
lacustrine deposition (Eugster and Kelts 1983). The irregular stacking distribution of
siliciclastic and carbonate laminae, and the dominance of siliciclastic laminae in FA1,
suggest long-term variability in sedimentation rather than seasonal variability. Since
siliciclastic input dilutes carbonate sedimentation (Tucker and Wright 1995), carbonate
production likely occurred during highstands when the depositional area was far from
inflow point sources. Laminae preservation suggests deposition within an environment
prohibitive to bioturbation, pedogenesis, intra-sediment growth of saline minerals or
other processes capable of destroying primary sedimentary structures. Depositional
environments that meet this criteria include: 1) shallow, hypersaline lakes; and 2) anoxic
bottom water conditions (Hsu and Kelt 1978; Shinn 1983; Soreghan 1998; Del Papa
1999; Lukasik 2000). The absence of mud cracks, evaporite accumulations and red,
oxidized strata excludes a shallow, hypersaline lake interpretation. The black, gray,
brown and green coloration and organic aroma of shale strata suggests deposition within
an anoxic environment.
Very fine-grained 0.4-1.2 m thick sandstone and siltstone beds in FA1 represent
intermittent increases in the energy available for sediment transport. Planar basal contacts
and rare intraclasts in overlying sandstones indicate deposition by non-erosive to weakly
39
erosive processes. Horizontal lamination indicates bedload transport in plane beds while
overlying ripple cross-laminated strata represent mixed bedload and suspended load
transport in current ripples. The sedimentary structures and grain-size distribution of
these beds suggests deposition during fine-grained storm events or low-density turbidity
flows (Middleton and Hampton 1976; Cheel and Leckie 1992). Storm events introduce
coarser-grained sediment into fine-grained open water environments by lowering wave
base and thereby increasing the energy available for offshore sediment transport at a
given depth. Fine-grained storm bed strata ideally consist of an erosive basal contact that
may exhibit sole marks followed by deposition of intraclast-rich horizontally-laminated
sandstone, an overlying hummocky cross-stratified layer and cap of ripple cross-stratified
sediment (Cheel and Leckie 1992). Turbidity flows can represent the remobilization of
basin margin deposits triggered by a destabilizing event and transported basinward by
turbulent flow (Boggs 1995). Low-density turbidity flows ideally include the following
sequence of sedimentary structures: 1) an erosive base containing flute casts and tool
marks; 2) horizontally-laminated sandstone; 3) small-scale cross-bedded sandstone or
siltstone; 4) ripple cross-stratified sandstone or mudrock (Middleton and Hampton 1976).
Sedimentary structures and grain-size distributions present in FA1 sandstone and siltstone
rocks are consistent with either interpretation.
In conclusion, an anoxic to low oxygen depositional interpretation within a
meromictic lacustrine environment for FA1 strata is supported by the following
observations: 1) interstratified black, brown and green shale and laminated carbonate
mudstone; 2) the lack of body or trace fossils; and 3) absence of evaporitic minerals
and/or mud cracks.
40
3.2.2 FA2: Nearshore Facies
3.2.2.1 Description
The nearshore facies (Fig. 3-7) consists of intercalated carbonate and clastic
strata. Carbonate strata include 0.05-1.2m thick massive, yellow to gray mudstone and
wackestone beds with polygonal mud cracks (Fig. 3-8), horizontal trace fossils, rare
dinosaur trackways (Fig. 3-9) and chert nodules. Wackestone beds contain articulated
gastropod and bivalve body fossils, casts and molds. Opaline or sparry minerals
commonly replace gastropod body fossils.
Low relief, smooth 0.1-0.5m stromatolite boundstone strata (Fig. 3-10) commonly
occur within or cap massive carbonate mudstone units. Rare mammilated stromatolite
boundstone beds occur at the Challa Mayu locality where bed thickness ranges from 0.10.3m. Low relief, smooth stromatolite boundstone beds are laterally extensive for 10’s of
meters while mammilated stromatolitic boundstone rocks extend laterally for 1-3m before
passing into carbonate or clastic rocks.
Clastic strata consist of red to reddish purple, 0.1-40m thick, massive mudstones
and 0.1-25m thick horizontally-laminated and ripple cross-laminated mudstones and
siltstones. Calcareous, clayey mudstone beds contain symmetrical or nearly symmetrical
map view ripples that commonly cap carbonate rocks. In addition, mudstone and siltstone
beds include mud cracks and vertical, horizontal and cruciform trace fossils.
FA2 rocks represent the dominant facies association in the study area (Fig. 3-11),
reaching continuous stratigraphic thickness greater than120m (e.g., El Puente, Appendix
1) and comprising >80% of a given section. FA2 rocks pass into FA1, FA3 or, more
rarely FA4 rocks.
41
5
Fsm
4
Fm
Fsm with Fm, Fr
3
meters
Fsm
Fm
Fl with Fr
2
Fl
Fl
Fsm
1
Fm
0
M
SS
Cl
Figure 3-7. Graphic representation of FA2: Nearshore lacustrine facies association.
Lithologic codes and legend show in Figure 3-5 and Table 3-1.
42
Figure 3-8. Mud cracks present in the top of a massive limestone bed from facies association 2: Nearshore facies. Photograph is from the Maragua section.
43
Figure 3-9. Photograph showing a theropod trackway in El Molino Formation FA2 rocks
near Sucre. The theropod trackway is bracketed by the dashed blue lines.
44
Figure 3-10. Mammilated stromatolites within FA2 nearshore facies strata at Challa Mayu.
45
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
22
Bolivia
El Puente
Tupiza
Major City
Measured Section
Political Boundary
Camargo
Tarija
Facies Distribution
0
22
100 km
Figure 3-11. Geographic distribution of FA2 nearshore lacustrine facies.
46
3.2.2.2 Interpretation
Massive carbonate sediment accumulation in FA2 strata results from the chemical
precipitation of carbonate material and/or suspension fallout of biotic shells and biogenic
structural parts. Carbonate wackestone beds indicate deposition within relatively quiet
waters that lacked sufficient energy to winnow out carbonate mud material. Preservation
of articulated gastropod and bivalve material within these beds suggests minimal
transport and erosion indicating low energy deposition, probably in or very near the
oxygenated, shallow nearshore life environment of the fauna (Surdam and Stanley 1979;
Fouch and Dean 1982; Pratt and James 1986; Castle 1990; Soreghan 1998).
Stromatolitic boundstone beds represent the preserved remains of microbial algal
mats and trapped sediment (Pratt and James 1986; Tucker and Wright 1990). The primary
control on stromatolite development is invertebrate grazing (Kendall and Skipwith 1968;
Playford and Cockbain 1976). Conditions prohibitive to invertebrate grazing include
hypersaline conditions and sites prone to subaerial exposure (Birke 1974; Playford and
Cockbain 1976). Although stromatolites can exist at depths greater than 60m (Gow
1981), flat, smooth stromatolites commonly occupy stable, shallow (>-12m) low-energy
shorelines while more highly ornamented (e.g., mammilated) stromatolites exist between
–2 to 0m along higher energy shorelines (Hoffman 1976; Casanova 1986).
Laminated, ripple cross-laminated and massive mudstones and siltstones represent
sediment deposition by suspension fallout and the migration of current ripples during
bedload transport. The presence of mud cracks within these strata suggests periodic
subaerial exposure. While desiccation processes during intervals of subaerial exposure
produce mud cracks, mud cracks also result from subaqueous shrinkage processes
47
(synaeresis cracks) and syndepositional burial stress (diastasis cracks), making an
interpretation for periodic subaerial exposure based just on mud cracks somewhat
tenuous (Donovan and Foster 1972; Cowan and James 1992; Paik and Kim 1998).
However, the association of these rocks with FA4 floodplain facies strata (discussed
below), and the rarity of subaqueous shrinkage cracks (Astin and Rogers 1991) and
documented diastasis cracks supports an interpretation for episodic subaerial exposure.
Destruction of primary sedimentary structures in massive clastic and carbonate
beds results from bioturbation, mud-cracking, intra-sediment growth of saline minerals
and/or pedogenesis (Eugster and Kelts 1983; Smoot and Castens-Seidell 1983; Platt
1989). Destruction of FA2 primary sedimentary structures probably resulted from a
combination of two processes: 1) bioturbation; and 2) mud-cracking. Numerous
horizontal and vertical trace fossils present in siliciclastic and carbonate strata indicate
the presence of burrowing fauna capable of disrupting primary sedimentary structures.
Abundant mud cracks preserved in massive FA2 rocks and in juxtaposed laminated and
ripple cross-laminated mudrocks suggest depositional conditions amenable to mud crack
development. Although the destruction of FA2 primary sedimentary structures by the
growth of intra-sediment saline minerals or pedogenesis remains a possible mechanism
for sediment homogenization, evaporite minerals and/or evaporite mineral casts as well
as pedogenic features such as hackly texture, mottling, root traces, distinct color or
textural horizons and nodular carbonate accumulations (Retallack 1988; Kraus and Aslan
1999; Mack et al. 2000) are absent.
In summary, FA2 strata are interpreted as low energy, nearshore deposits based
on the presence of: 1) thick, massive carbonate mudstone and wackestone deposits; 2)
48
articulated bivalve and gastropod fossils; 3) ubiquitous low relief, flat stromatolites; and
4) desiccation mud cracks.
3.2.3 FA3 Beach, Bar and Shoal Facies
3.2.3.1 Description:
FA3 strata (Fig. 3-12) consist of 0.01-4.3m peloidal, oolitic, fossiliferous and
quartzose packstone, grainstone and very fine- to coarse-grained sandstone beds. These
rocks contain concave-up or planar basal contacts, trough cross-bedding, horizontal
lamination and massive sedimentary structures (Figs. 3-13, 3-14). Less commonly the
packstone, grainstone and sandstone units contain ripple cross-stratified caps. In rare
instances, trough cross-bedded or horizontally-laminated packstone, grainstone and
sandstone beds contain carbonate or mud intraclasts. Fossil material consists of < 0.01m
unidentifiable shell fragments and/or 0.01-0.04 m large bivalve fragments. Opal or sparite
commonly replaces original fossil matter. Packstone and grainstone beds commonly
consist of very well-sorted peloids or ooids. More rarely, peloidal and oolitic packstone
and grainstone beds will contain sand-sized clastic material and fossil fragments. Thin
0.01-0.15m thick grainstone and packstone beds commonly cap stromatolitic
boundstones. Sandstone strata range from poorly-sorted sandstones containing angular
ostracod, unidentified fossil, peloidal, oolitic and clastic fragments to quartz-rich, wellrounded, very well-sorted sandstones. Additionally, fish bone fragments occur in poorlysorted sandstones from Vilcapuyo while pebble-sized clasts are present in several Vila
Vila and Incapampa sandstone beds. At Incapampa, articulated silica-replaced gastropod
body fossils are present within moderate- to well-sorted sandstone beds. Vertical and
49
5
Sh
Sm
St
4
Sm
meters
3
2
Sm
1
Sh
St
0
M
SS
Cl
Figure 3-12. Graphic representation of FA3 beach, bar and shoal facies association.
Lithofacies codes and legend are shown in Figure 3-5 and Table 3-1.
50
Figure 3-13. Horizontally-laminated and low angle trough cross-stratified peloidal sandstone within FA3 beach, bar and shoal facies at Torotoro.
51
Figure 3-14. Fossiliferous packstone present in El Molino Formation FA3 beach, bar and
shoal facies strata at Challa Mayu.
52
cruciform trace fossils occur within massive sandstones at Incapampa, Vila Vila and
Suticolla.
Figure 3-15 shows the distribution of FA3 packstone, grainstone and sandstone
beds. The greatest thickness of FA3 strata occurs in stratigraphic sections along the
eastern boundary of the area of investigation (Torotoro, Vila Vila, Incapampa, Camargo
and El Puente sections; see Appendix 1). Beds of 1-4.3m thick oolitic and peloidal
grainstones and packstones dominate FA3 rocks at Torotoro, El Puente and Camargo
while sections further east consist largely of sandstone beds. FA3 rocks from the eastern
boundary of the area of interest grade upwards into FA4 and FA5 strata. Although FA3
strata appear broadly distributed, most FA3 occurrences consist of 0.01-0.15m thick
grainstone and packstone beds that cap FA2 carbonate mudstone, wackestone and
stromatolitic boundstone beds.
3.2.3.2 Interpretation
Ooid and peloid formation can occur in pedogenic, fluvial, cave, lagoonal,
shallow marine and lacustrine environments (Tucker and Wright 1990), however, ooids
are most commonly known from agitated shallow marine (e.g., Bahama Banks; Cambrian
Port au Port Group) and saline lacustrine waters (e.g., Great Salt Lake; Green River
Formation) (Eugster and Hardie 1975; Hardie et al. 1978; Surdam and Stanley 1979;
Dean and Fouch 1983; Eugster and Kelts 1983; Smoot 1983; Pratt and James 1986;
Chow and James 1987; Castle 1990). The association of these rocks with gastropod,
bivalve and fish fossils and fragments suggests a lagoonal, shallow marine or saline
lacustrine setting for El Molino Formation ooids and peloids. Trough cross-bedded and
horizontally-laminated, peloidal and oolitic packstone, grainstone and sandstone beds
53
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Suticolla
Cochabamba
Oruro
18
Torotoro
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu
Ichilula
Incapampa
Potosi
Chile
22
Bolivia
El Puente
Tupiza
Major City
Measured Section
Political Boundary
Camargo
Tarija
Facies Distribution
0
22
100 km
Figure 3-15. Regional distribution of FA3: beach, bar and shoal facies association.
Although FA3 strata appear to have significant geographic distribution, primary FA3
facies concentrations occur in El Puente, Camargo, Incapampa and Vila Vila. Other
Fa3 sites largely consist of thin (< 0.3 m) peloidal or fossiliferous beds interbedded
with FA2 strata.
54
suggest bedload transportation in lower flow regime 3-D dunes or upper flow regime
plane beds (Soreghan 1998). Concave-up basal contacts and associated intraclasts
represent erosional processes during bed migration. Similar processes likely governed
deposition of massive, well-sorted packstone, grainstone and sandstone beds but grainsize homogeneity may have masked associated sedimentary structures. Rocks
characterized by good sorting, coarse grain size, abundance of peloids/ooids and an
absence of mud matrix suggest continuous reworking by wave action (Castle 1990;
Soreghan 1998; Lukasik 2000), precluding deposition within lagoonal conditions.
In contrast, poorly sorted rocks with angular and sub-angular clasts and easily
eroded fossil fragments did not undergo continuous reworking by wave action. Instead,
the sand-sized sediment fraction likely represents intervals of rapid, high-energy
deposition while finer-grained material accumulated under relatively quiet water
conditions. Poorly sorted rocks are interpreted as backshore washover deposits.
FA3 sandstone, packstone and grainstone beds are interpreted as beach, bar or
shoal deposits. This interpretation is based on a combination of the following evidence:
1) sediment grain size; 2) sedimentary structures indicative of traction transport; 3)
presence of gastropod, bivalve, fish bones and unidentifiable fossil material; 4)
calcareous nature of sediments; and 5) presence of ooids and peloids.
3.2.4 FA4: Floodplain Facies
3.2.4.1 Description
FA4 facies rocks (Fig. 3-16) consist of 0.4-30 m thick laminated and massive red
to reddish-brown mudstone and siltstone beds and 3-9.5 m thick massive medium- to
coarse-grained sandstone beds. Mudstone and siltstone strata contain infrequent ripple
55
5
Fl
Fsm
4
Sm
Fr
Fm
P
meters
3
P
Fsm
Fl
2
Sm
1
Sm
Fl
0
M
SS
Cl
Figure 3-16. Graphic representation of FA4 floodplain facies association. Figure 3-5
and Table 3-1 show lithofacies legend and facies codes used in this figure.
56
cross-lamination, uncommon trace fossils and mud cracks. More rarely, massive
mudstone and siltstone beds include hackly texture, mottling, root casts, distinct lateral
horizons (Fig. 3-17) characterized by color and/or texture and carbonate accumulations
that consist of extremely calcareous matrix material and/or 0.05-0.2m diameter
amalgamated carbonate nodules (Fig. 3-18). Of these features, carbonate accumulation is
most common, forming 0.1-1.5m thick carbonate-rich horizons with gradational lower
contacts and mildly gradational to abrupt upper contacts.
The only outcrops of FA4 facies sandstone occur at Incapampa where massive
sandstone beds contain significant carbonate matrix accumulations and carbonate
nodules. Carbonate nodules range in size from <0.01 to 0.08 m and commonly form
irregular sub-vertical columns. These columns occasionally amalgamate into <1m thick
horizons with gradational or abrupt contacts.
FA4 massive sandstones occur between FA3 strata and FA5 strata. FA4 strata
interbed with FA2, FA3 and FA5 strata and typically outcrop in the upper half or third of
a given El Molino Formation stratigraphic section. FA4 strata were present at all
measured sections excluding those at the Challa Mayu, Vilcapuyo and El Puente
localities. Figure 3-19 shows the geographic distribution of FA4 strata.
3.2.4.2 Interpretation
Laminated and massive mudstone and siltstone beds represent sediment deposited
during suspension fallout and current migration (Zhang et al. 1998), and are interpreted
as floodplain deposits. Sediment homogenization by bioturbation, desiccation processes
and/or pedogenesis act to destroy primary sedimentary structures within massive
mudstone and siltstone strata (Eugster and Kelts 1983; Smoot and Castens-Seidell 1983;
57
Figure 3-17. El Molino Formation FA4 floodplain facies strata at Incapampa displaying
horizontal banding commonly associated with pedogenesis. Soil horizon bands are further
emphasized by blue dashed lines. Person is ~ 1.8 m tall.
58
Figure 3-18. Photograph of amalgamated carbonate nodules within FA4 floodplain
facies massive sandstone at Incapampa.
59
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
22
Bolivia
El Puente
Tupiza
Major City
Measured Section
Political Boundary
Camargo
Tarija
Facies Distribution
0
22
100 km
Figure 3-19. Regional distribution of FA4 floodplain facies association strata. Note that
facies distribution shows broad depositional extent but main FA4 facies concentrations
occur at Jesus de Machaca, Caluyo, Chuvata and Incapampa.
60
Platt 1989). Destruction of primary sedimentary structures by bioturbation is supported
by the presence of rare trace fossils and root traces. Sediment homogenization by
desiccation processes is indicated by mud cracks. Evidence for pedogenesis includes
hackly texture, mottling, root traces, distinct color and/or textural horizons and carbonate
accumulations (Retallack 1988; Kraus and Aslan 1999; Mack et al. 2000). Although not
prevalent in FA4 rocks, hackly texture is interpreted as remnant soil structure resulting
from repeated shrinking and swelling of soil clays while mottling results from
reduction/oxidation reactions during soil formation (Fanning and Fanning 1989). Color
and/or textural horizons represent zones of soil eluviation and illuviation.
Carbonate nodules and amalgamated nodule accumulations within massive
mudstone, siltstone and sandstone strata are interpreted as zones of carbonate illuviation
(Bk horizon) within relict soils. Authigenic carbonate forms in pedogenic environments,
at or near the water table, near springs or in zones of shallow infiltration and runoff
(Mack et al. 2000). These authors provide the following criteria for distinguishing
pedogenic carbonate from other forms of authigenic carbonate: 1) presence of welldeveloped paleosol horizons; 2) gradational or erosional upper contact and gradational
lower contact; and 3) vertical orientation of carbonate accumulations. Carbonate nodules
and accumulations in FA4 meet criteria (2) and (3), supporting a pedogenic origin.
Pedogenic carbonate accumulations commonly form in arid to sub-humid climates where
precipitation <100cm/year (Cerling 1984; Cerling and Quade 1993; Pendall et al. 1994;
Mack et al. 2000).
While paleosols commonly occur in floodplain environments, laminated
mudstone and siltstone as well as massive mudstone, siltstone and sandstone strata
61
support interpretations for several additional depositional settings. In order to
differentiate FA4 strata from FA2, FA3 or FA5 rocks, the following generalizations were
made. FA4 floodplain rocks interbedded with FA2 nearshore or FA3 beach, bar and shoal
strata were distinguished exclusively by the presence of pedogenic features. Pedogenic
features were also used to differentiate between FA4 and FA5 massive sandstone beds.
Lastly, laminated and massive mudstone and siltstone associated with FA5 fluvial facies
deposits were interpreted as FA4 floodplain strata with or without evidence for
pedogenesis.
3.2.5 FA5: Fluvial Facies
3.2.5.1 Description
The fluvial facies (Fig. 3-20) consist of 0.1-12 m thick, laterally extensive
(>60m), very fine- to coarse-grained, trough cross-stratified, horizontally-laminated,
ripple cross-laminated and massive sandstone beds. FA5 sandstone strata can be divided
into two subsets based on sand body geometry and the nature of the basal contact: 1)
horizontally-laminated and trough cross-bedded fine- to coarse-grained sandstone beds
with lenticular sand body geometry and sharp, erosive basal contacts (Fig. 3-21); or 2)
horizontally-laminated, trough cross-stratified or ripple cross-laminated, very fine- to
medium-grained sandstone units with tabular sand body geometry and weakly- to nonerosive basal contacts (Fig. 3-22).
Lens-shaped sandstone beds form 3-12 m thick, stacked multistory sand bodies.
Occasionally, laminated or massive 0.02-0.2 m thick mudstone and siltstone beds drape
sandstone bedsets. Many trough cross-beds contain granule- to pebble-sized mudstone
intraclasts that delineate the lower bounding surface. Individual sandstone beds display
62
5
Sh
St
4
St
3
meters
Sh
Sr
Sh
St
Sh
2
St
Sh
Sh
St
Sh
St
1
0
M
SS
Cl
Figure 3-20. Graphic representation of FA5 fluvial facies association. Figure 3-5 and
Table 3-1 show lithology legends and facies codes.
63
Figure 3-21. Lenticular sandstone bed from FA5 fluvial facies at Incapampa. Note the erosional, concave up sandstone base truncating underlying mudrock. Jacob staff is 1.6 m tall.
64
Figure 3-22. Horizontally-laminated sandstone within an FA5 fluvial facies tabular sandstone.
65
normal grading or lack grading while grain-size at the bedset scale may vary
considerably. At the composite bedset scale, grain-size commonly fines upward as
poorly- to moderately-sorted, 0.1-0.6 m thick trough cross-beds are replaced by
moderately- to well-sorted, horizontally-laminated and ripple cross-laminated strata.
Tabular sand bodies consist of 0.1-3.5 m thick sand bodies, dominated by
horizontally-laminated strata with rare trough cross-beds and/or ripple cross-laminated
caps. Less frequently, tabular sandstone beds show massive sedimentary structures that
typically cap sandstone beds. Tabular sand bodies display normal grading and consist of
moderately- to well-sorted sediment. FA4 floodplain facies strata bracket tabular
sandstone beds.
Where present, fluvial facies strata occupy the upper half to third of El Molino
Formation strata and interbed with FA4 floodplain facies strata. While individual sand
bodies typically fine upwards, grain size actually increases up stratigraphic section. FA5
sandstone units outcrop at Jesus de Machaca, Caluyo, Chuvata and Incapampa (Fig. 323).
3.2.5.2 Interpretation
FA5 horizontally-laminated, trough cross-stratified and ripple cross-laminated
sandstone beds represent deposition by traction currents. Horizontally-laminated
sandstone beds represent sediment transported within plane beds while trough crossstratified and ripple cross-laminated sandstone strata indicate the migration of lower flow
regime dunes and current ripples (Miall 1978).
Multi-story, stacked, lenticular-shaped sand bodies are interpreted as fluvial
channel sandstone beds. The presence of abundant mudstone intraclasts within trough
66
66
Lago
Titicaca
Chuvata
Caluyo
La Paz
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
18
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
22
Bolivia
El Puente
Tupiza
Major City
Measured Section
Political Boundary
Camargo
Tarija
Facies Distribution
0
22
100 km
Figure 3-23. Regional distribution of FA5 fluvial facies association strata.
67
cross-beds suggests the erosion and reworking of muddy sediments within or adjacent to
the fluvial channel (Cant 1982). Grain-size variation and the draping of trough cross-beds
by mudstone or siltstone indicate inconsistent channel flow velocities and potentially
represent brief periods of channel abandonment and subaerial exposure. Fining upward
grain-size trends and bedform transitions from horizontally-laminated to ripple crosslaminated sandstone beds suggest an overall reduction in flow velocity and the
progressive abandonment of the fluvial channel (Miall 1978,1996).
Sandstone strata showing tabular geometry and non- to weakly-erosive basal
bounding surfaces are interpreted as unconfined sheet flood beds. The absence or near
absence of erosive bounding surfaces suggests rapid dissipation of flow energy.
Decreasing flow energy is also suggested by bedform transitions from horizontallylaminated beds and rare trough cross beds to ripple cross-laminated strata (Miall 1978,
1996). The prevalence of massive sandstone structure at the top of these sand bodies may
have resulted from post-depositional pedogenesis, desiccation processes or bioturbation
(Eugster and Kelts 1983; Smoot and Castens-Seidell 1983; Platt 1989).
The increase in sandstone outcrop frequency and upward increase in grain-size
suggests increased fluvial flow energy and/or reduced transport distance from drainage
basin source rocks.
3.3 Discussion
The El Molino Formation consists of open water (FA1), nearshore (FA2), beach,
bar and shoal (FA3), floodplain (FA4) and fluvial (FA5) facies strata. These strata are
interpreted as deposits within a dynamic, dominantly lacustrine depositional system with
basin margin fluvial/floodplain strata becoming important towards the end of El Molino
68
Formation deposition. The majority of lacustrine deposition occurred under
hydrologically-closed basin conditions. Hydrologically-closed lacustrine basins are
characterized by rapid and frequent facies shifts and the presence of evaporites (Hardie et
al. 1978; Eugster and Kelts 1983). While determining the actual timing of El Molino
Formation facies shifts is impossible with the given data, an inference supporting rapid
and frequent facies changes can be made based on the frequency of lithofacies variation
within a given stratigraphic section. In addition, the presence of mud cracks within FA2
nearshore facies deposits and FA2 associations with pedogenic features of FA4
floodplain facies strata suggests episodic subaerial exposure of nearshore deposits.
Gypsum deposits at Agua Salud also support a hydrologically-closed basin interpretation.
Hydrologically-closed lacustrine settings can be further divided into perennial and
ephemeral saline lakes (Eugster and Hardie 1983). Perennial saline lakes (e.g. Great Salt
Lake) hold water year round. Alternately, ephemeral saline lakes (e.g. Death Valley) only
contain water during periods of increased precipitation and/or reduced evaporation, and
are commonly characterized by evaporite accumulations indicative of hypersalinity and
evidence for subaerial exposure. Although periodic subaerial exposure of lacustrine strata
and limited evaporite deposition is consistent with an ephemeral saline lake environment,
the presence of ubiquitous gastropods and bivalves indicates that El Molino Formation
lake waters were not hypersaline, instead suggesting a perennial saline lake environment.
The presence of marine fossils indicates that the El Molino Formation was
periodically a shallow marine (Gayet et al. 1991, 1993, 2001; Sempere et al. 1997) or an
open lacustrine system (Rouchy et al. 1993; Camoin et al. 1997). As suggested by
Sempere et al. (1997), a shallow marine depositional environment is circumstantially
69
supported by contemporaneous global high-stands during deposition of Lower and Upper
El Molino Formation carbonates. The Upper Zuni cycle 4.5 and Tejas cycle 1.2 global
high-stands occur during deposition of significant lower and upper El Molino Formation
carbonate beds attributed to a shallow marine environment (Haq et al. 1987; Sempere et
al. 1997). However, the Tejas cycle 1.1 high-stand, which shows a higher amplitude than
the Tejas cycle 1.2, corresponds with a phase of mudrock deposition attributed to Middle
El Molino Formation non-marine deposition (Haq et al. 1987; Rouchy et al. 1993;
Camoin et al. 1997; Sempere et al. 1997). This calls into question the inference relating
Upper El Molino Formation deposition to a marine transgression.
The fluvial/floodplain strata present near Lake Titicaca represent Middle to Upper
El Molino Formation deposits. The coarsening-upward trend seen in Lake Titicaca
deposits suggests progradation of fluvial sediments into the lacustrine system. This trend
is more compatible with fluvial inflow into the El Molino Formation depositional basin
than outflow. A coarsening-upward trend also occurs at Incapampa where beach, bar and
shoal facies are replaced up section by fluvial/floodplain strata, again supporting the
progradation of fluvial sediment into the El Molino Formation depositional basin. Based
on stratigraphic stacking patterns present at Jesus de Machaca, Caluyo, Chuvata,
Incapampa and Vila Vila, the Upper El Molino Formation basin was characterized by
progradation of basin perimeter sediments into the basin. This observation makes an
Upper El Molino Formation marine transgression unlikely.
The sedimentary facies present in the El Molino Formation provide some
evidence for determining tectonic setting. Lacustrine depositional environments exist in
syn-rift and post-rift settings as well as foreland basin wedge-top, foredeep and back-
70
bulge depocenters (Stanley and Collinson 1979; Surdam and Stanley 1979; Sclater and
Christie 1980; Ingersoll 1988; Cohen 1990; McHargue et al. 1992; Friedmann and
Burbank 1995; DeCelles and Giles 1996; Currie 1998; Crossley and Cripps 1999). The
absence of El Molino Formation alluvial fan facies strata and growth strata, indicating
syndepositional normal or thrust faulting, eliminates syn-rift or foreland basin wedge-top
depositional environments. Remaining depositional environments include foreland basin
foredeep and back-bulge depocenters in addition to post-rift settings.
71
4. PALEOCURRENT DIRECTIONS AND PROVENANCE
4.1 Methods and Background Data
Paleocurrent directions were measured and calculated for nineteen stations from
four measured stratigraphic section localities. Data include 185 measurements taken from
sandstone trough cross-bed axes and trough cross-bed limbs. Paleocurrent direction for
trough cross-bedded sandstone limbs was calculated using DeCelles et al. (1983) method
I. Paleocurrent direction determination using DeCelles et al. (1983) method I is
accomplished by measuring the strike and dip of trough cross-bed 3-D limb exposures.
Trough cross-bed limb measurements are divided into right and left hand populations
based on their relative orientation and subsequently plotted as poles on a stereographic
Schmidt Net. An average point is selected from each population and subsequently fitted
to a great circle. The pole to that great circle represents the trend and plunge of the
average trough axis for an outcrop which can be used as a paleocurrent direction indicator
once corrected for stratigraphic strike and dip (DeCelles et al. 1983).
Twenty-three sandstone thin-section samples and eight clast counts were collected
from medium-grained to pebbly sandstones for provenance analysis. Due to the limited
distribution of sandstone strata throughout the El Molino Formation depositional system,
sandstone provenance data were only collected from the Incapampa, Jesus de Machaca,
Suticolla, Vila Vila and Vilcapuyo stratigraphic sections. Sandstone samples collected
from Vilcapuyo show clast compositions consisting of >80% autochthonous oolitic,
peloidal and fossil fragment material and, therefore, were not used for subsequent
provenance studies.
72
Sandstone thin-section samples were stained for calcium and alkali feldspars
using the procedure of Houghton (1980). At least 400 grains were identified from each
thin-section following the Gazzi-Dickinson method (Dickinson 1970; Ingersoll et al.
1984). The Gazzi-Dickinson method of thin section point counting is designed to
maximize source rock data important to tectonic investigations. For this purpose, sandsized crystals and grains within larger fragments are counted the same as crystals and
grains not included within larger fragments (e.g. a sand-sized quartz crystal within a
volcaniclastic fragment is counted as a quartz crystal and not as a volcanic lithic
fragment). This effectively eliminates compositional variation based on differences in
grain size, thereby reducing the effects of transport history on source rock provenance
determination (Ingersoll et al. 1984). Table 4-1 summarizes grain parameters and
recalculated parameters used during thin-section point-counting and subsequent analyses.
Table 4-2 shows raw thin-section point-count data.
Clast counts consist of ~100 clast identifications per count. Clasts counts were
conducted by first designating a ~0.5m square box on the outcrop. Lithologies for
granule-sized or greater clasts within the box were then identified and marked until the
count reached 100. Clast lithologies present in the El Molino Formation include quartzrich beige sandstones and quartzites, greenish-gray calcium carbonates, dolomitic
conglomerates, chert and quartz clasts, and reddish-beige, reddish-purple, alligator green,
white, blue-black, brown-black and gray quartzites. Quartz-rich beige sandstones
represent Cretaceous La Puerta Formation rocks. La Puerta Formation and equivalent
strata display broad distribution throughout the Eastern Cordillera and eastern Altiplano.
However, these rocks are absent from the Jesus de Machaca locality, indicating non-
73
Table 4-1. Table showing parameters and recalculated parameters for thin section
point counts using the Gazzi-Dickinson method (modified from Ingersoll et al. 1984).
Counted Parameters
Recalculated Parameters
Qp = polycrystalline quartz (including chert)
Q = Qp + Qm
Qm = monocrystalline quartz
F=P+K
P = plagioclase feldspar
L = Lv + Lm + Ls
K = potassium feldspar
Lv = volcanic or hypabyssal lithic fragments
Lm = metamorphic lithic fragments
Ls = sedimentary lithic fragments
M = phyllosilicates
D = dense minerals
Misc. = miscellaneous and unidentified
74
75
Qm
225
268
160
193
191
178
240
225
323
374
338
321
376
257
358
331
317
Sample Name
JM-2
JM-4
JM-5
JM-6
JM-7
JM-8
JM-9
JM-10
SU-EM-02
SU-EM-15
SU-EM-48
SU-EM-79
SU-EM-98
IN-EM-162
IN-EM-170
IN-EM-270
IN-EM-325
136
19
49
66
20
7
17
15
12
5
8
23
10
14
16
23
37
Qp
393
377
380
383
343
381
355
336
388
230
276
183
203
205
194
263
262
Q
3
20
15
4
35
8
35
60
7
129
112
155
165
171
160
107
71
P
1
3
4
13
15
2
6
2
2
23
4
40
26
20
37
23
64
K
4
23
19
17
50
10
41
62
9
152
116
195
191
191
197
130
135
F
1
Lv
1
1
1
1
1
1
2
2
2
4
5
4
2
2
4
5
4
1
L
2
8
Lm
2
8
Ls
1
1
2
1
3
2
1
3
1
1
2
3
1
1
1
2
D
1
1
M
1
1
1
1
1
3
1
3
4
AA
Table 4-2. Raw point count data used in provenance analysis and in related figures. Sample name prefixes indicate
sample locality as follows: JM = Jesus de Machaca; SU = Suticolla; and IN = Incapampa. Middle code indicates
formation name: EM = El Molino.
deposition or deposition and subsequent erosion prior to El Molino Formation deposition.
Greenish-gray calcium carbonate clasts appear identical to CaCO3 nodules present within
underlying, pedogenically-modified La Puerta Formation rocks. Although an exact
identification of dolomitic conglomerate clasts cannot be made since no known
conglomerate underlying the El Molino Formation contains dolomite, the best
interpretation places these clasts within the basal La Puerta Formation. Basal La Puerta
Formation strata include conglomeratic beds in several locations and overlie PermoCarboniferous carbonates. Quartz clasts represent vein quartz present within igneous
intrusions and flows associated with the La Puerta Formation. Both clast count localities
contain igneous intrusions and/or flows within underlying La Puerta Formation strata.
Permo-Carboniferous carbonates represent the likely source for chert clasts (Horton et al.
2001b; Sempere et al. 2002) while reddish-beige, reddish-purple, alligator green, white,
blue-black, brown-black and gray quartzites represent Paleozoic strata. Table 4-3
summarizes lithologies present within El Molino Formation strata with associated raw
clast count data.
4.2 Description
A regional overview of detrital composition shows diminishing amounts of
feldspar from north to south. The Jesus de Machaca section displays the highest feldspar
content of the three thin-section sample localities while Incapampa shows the lowest
feldspar content (Figs. 4-1, 4-2). Feldspar from El Molino Formation sandstone samples
consists primarily of plagioclase. Northwest- and north-derived sandstones from the Jesus
de Machaca section (Fig. 4-1) show quartzo-feldspathic composition (Fig. 4-2).
Monocrystalline quartz (Qm) dominates the quartz (Q) fraction while plagioclase (P)
76
77
0
1
2
1
Dolomitic Conglomerate, Cretaceous?
La Puerta Fm
Vein Quartz, Cretaceous? Igneous
Flows and Intrusions
Chert, Permo-Carboniferous
Ordivician to Silurian Quartzites
12
0
2
0
0
54
11
29
0
0
7
0
93
0
0
8
0
92
0
0
0
0
85
0
16
101
CaCO3 nodules, Cretaceous? La
Puerta Fm
Dolomitic Conglomerate, Cretaceous?
La Puerta Fm
Vein Quartz, Cretaceous? Igneous
Flows and Intrusions
Chert, Permo-Carboniferous
Ordivician to Silurian Quartzites
N=
100
2
20
2
0
11
100
19
0
2
0
8
100
22
54
10
7
0
100
19
60
4
0
0
N=
100
100
101
100
100
Clast Type
Incapampa -24m Incapampa 12.8 m Incapampa 197m Incapampa 280m Incapampa 290m
Beige SS/Quartzite, Cretaceous?
La Puerta Fm
0
67
71
7
23
0
CaCO3 nodules, Cretaceous? La
Puerta Fm
Table 4-3. Raw El Molino and La Puerta Formation clast count data. Numbers next to locality refers to stratigraphic level where El
Molino Formation base equals 0 m. Negative numbers represent La Puerta Formation rocks underlying El Molino Formation strata.
Clast Type
Vila Vila 30m Vila Vila 50m Vila Vila 90m Incapampa -50m Incapampa -28m
Beige SS/Quartzite, Cretaceous?
La Puerta Fm
96
86
7
0
0
66
62
Vila Vila Clast Counts
Lake Titicaca
Chuvata
Caluyo
90 m
La Paz
50 m
Jesus de
Machaca
Agua Salud
Cochabamba
Suticolla
Torotoro
Oruro
18
30m
Incapampa Clast Counts
San Pedro
de Huayalloco Lago
Maragua
Poopo
Vilcapuyo
Vila Vila
Andamarca
Sucre
Challa Mayu Incapampa
Ichilula
Potosi
Chile
Bolivia
-24 m
290 m
-28 m
280 m
-50 m
197 m
18
Camargo
El Puente
Tarija
Tupiza
22
13 m
22
Measured Section
Political Boundary
Major Town
0
100 km
Argentina
Legend for Clast Count Graphs
Cretaceous La Puerta Formation
Permian Chert
Cretaceous? CaCO3 Nodules
Paleozoic Quartzites
Cretaceous? Conglomerates
Vein Quartz (age unknown)
Figure 4-1. Map showing paleocurrent directions and clast count data. Paleocurrent data
consist of measurements taken from the axes and limbs of trough cross-bedded sandstone
beds. Trough cross-bed paleocurrent data were extracted using the DeCelles et al. (1983).
method I. DeCelles et al. (1983) method 1 data were calculated from 20 ± 4 individual
measurements subequally divided between right and left hand limbs. Stratigraphic level
and number of measurements per station can be found in Appendix 1. Clast count data
consists of 100 ±11 counts per outcrop. Clast count meter levels are in reference to the
base of the El Molino Formation (i.e. base of El Molino Formation equals 0).
78
Continental Block
Provenances
Qm
Jesus de Machaca
Q
Recycled Orogen
Provenances
50
Recycled Orogen
Provenances
Continental Block
Provenances
50
50
50
Qm
F
F
L
50
Magmatic Arc
Provenances
Increasing Ratio of
Plutonic to Volcanic
Sources in
Magmatic Arc
Provenances
50
50 Increasing Maturity from
Magmatic Arc
Provenances
Continental Block Provenances
P
Q
L
50
K
50
Qm
Suticolla
Continental Block
Provenances
Continental Block
Provenances
Recycled Orogen
Provenances
50
Recycled Orogen
Provenances
50
50
50
Qm
F
F
L
50
Magmatic Arc
Provenances
Increasing Ratio of
Plutonic to Volcanic
Sources in
Magmatic Arc
Provenances
50
P
Q
50
Magmatic Arc
Provenances
Increasing Maturity from
Continental Block Provenances
K
50
L
50
Qm
Incapampa
Continental Block
Provenances
50
Recycled Orogen
Provenances
Continental Block
Provenances
Recycled Orogen
Provenances
50
50
50
Qm
F
50
L
F
Magmatic Arc
Provenances
Increasing Ratio of
Plutonic to Volcanic
Sources in
Magmatic Arc
Provenances
P
50
L
Magmatic Arc
Provenances
50
50
Increasing Maturity from
Continental Block Provenances
K
50
Figure 4-2. Ternary diagrams illustrating the composition of El Molino Formation sandstone
beds at Incapampa and Suticolla. Dashed lines delineate the provenance fields discussed in
the text. Provenance fields taken from Dickinson (1979). Q = Quartz; Qm = Monocrystalline
Quartz; F = Feldspar; L = Total Lithic Fragments; P = Plagioclase; K = Potassium Feldspar.
79
makes up the bulk of the feldspar fraction (F). Rare lithic fragments (L) consist of
sedimentary lithoclasts (Ls). Sandstone samples from lowermost and uppermost Jesus de
Machaca El Molino Formation section show a high quartz fraction compared to feldspar
while samples from middle section rocks display subequal quartz and feldspar fractions
(Fig. 4-2).
Quartzose sandstones (Fig. 4-2) from the Suticolla section exhibit east-northeast
to west-southwest paleoflow (Fig. 4-1). Polycrystalline quartz represents < 6% of the
total quartz fraction while the proportionally low feldspar fraction consists mostly of
plagioclase with very minor potassium feldspar (Fig. 4-2). Lithic grains are insignificant
at Suticolla, consisting of < 1% sedimentary lithic clasts (Ls).
Vila Vila pebbly sandstones lacked preserved sedimentary structures necessary
for paleocurrent determination. Clast counts from pebbly sandstones indicate initially
high Cretaceous(?) La Puerta Formation sandstone and quartzite clast content and
gradually transitions to Cretaceous(?) vein quartz, Permo-Carboniferous chert and
Paleozoic quartzites (Fig. 4-1).
Further south at Incapampa, clast counts from the La Puerta Formation are
dominated by Cretaceous(?) vein quartz with subordinate amounts of Paleozoic quartzites
(Fig. 4-1). Unconformably overlying the La Puerta Formation, El Molino Formation clast
counts display high initial proportions of La Puerta Formation clasts and passes up
section into a Permo-Carboniferous chert-dominated composition. Paleocurrent
measurements indicate paleoflow to the northwest and west (Fig. 4-1).
El Molino Formation thin-section samples at Incampampa consist of quartzdominated sandstones largely composed of monocrystalline quartz (Fig. 4-2).
80
Polycrystalline quartz grains from the lowermost Incapampa section represent ~35% of
the quartz fraction but immediately diminishes to < 18%. Feldspar at Incapampa accounts
for < 6% of total grains with plagioclase dominating the feldspar fraction in all but the
uppermost sample.
4.3Interpretation
Paleoflow data (Fig. 4-1) differs throughout the basin indicating variable sandstone
source areas. However, all sandstone thin-section point counts plot within the continental
block provenance field of Dickinson and Suczek (1979). Paleocurrent data at Jesus de
Machaca suggests a northern or northwestern sandstone source area. Abundant
monocrystalline quartz represents the erosion of Paleozoic quartzites whereas Jesus de
Machaca plagioclase suggests the erosion and transport of magmatic arc source rocks.
The concentration of increased plagioclase within middle El Molino Formation suggests
temporary modifications within the drainage system to include more magmatic arc source
rock area.
Paleoflow at Suticolla indicates sandstone source rocks to the northeast and east.
The quartzose nature of these rocks suggests erosion of quartz-rich Mesozoic/Paleozoic
sedimentary and metasedimentary rocks. Subordinate feldspar may represent recycled
detritus present within source rocks or distal detritus from the magmatic arc. The lack of
volcanic lithic fragments rules out a proximal igneous source for sandstone feldspar.
Paleocurrent indicators were absent at the Vila Vila locality. However, clast count
data (Fig. 4-1) show an unroofing sequence that begins with the erosion of La Puerta
Formation strata. Clast count data from higher stratigraphic levels indicates that La
Puerta Formation erosion became less important as older Paleozoic strata became
81
available for erosion and transport. Since Mesozoic and Paleozoic strata underlie El
Molino Formation rocks north, south and west of Vila Vila, the clast source area must lie
east.
Incapampa paleoflow indicates an eastern or southeastern detrital source area.
Clast count data (Fig. 4-1) shows initial erosion of Cretaceous strata and subsequent
unroofing of Permo-Carboniferous chert with subordinate Cretaceous and Paleozoic
constituents. The dominance of monocrystalline and polycrystalline quartz in thin-section
samples is consistent with the interpretation suggested by clast count data.
4.4 Discussion
Limited provenance and paleocurrent data suggest that the El Molino Formation
drainage systems flowed towards a central depositional basin. The absence of
paleocurrent data for western and southern El Molino Formation rocks allows the
possibility of southern or western outlets to the sea. However, the presence of high
topography associated with the magmatic arc to the west reduces the probability of a
western outlet. Paleocurrent flow toward a central basin supports syn-rift, post-rift and
foreland basin depositional settings.
Clast counts from pebbly sandstones deposited within this basin show an
uncomplicated top-to-bottom unroofing sequence consistent with erosion by stratigraphic
downcutting. This simple unroofing sequence supports deposition within a syn-rift, postrift or foreland basin tectonic setting. However, deposition within syn-rift and foreland
basin settings could only occur during periods of relative tectonic quiescence. The
initiation of new syndepositional normal or thrust faulting would disrupt the unroofing
sequence by uplifting new strata for erosion and transport. Assuming that syn-rift and
82
foreland basin foredeep deposits are zones affected by fault activity, clast count data best
support deposition within a foreland basin back bulge or post-rift basin.
Thin-section provenance data support syn-rift or post-rift tectonic settings.
Dickinson and Suczek (1979) show that sandstone thin-section point counts plotting
within continental block provenance reflect sediment from broad uplifted cratonic or
locally uplifted normal fault-bounded basement source areas. Quartzose sandstone
deposits from Torotoro and Incapampa indicate the presence of quartz-rich source rocks
or an extensive, well-developed drainage system. The abundance of quartzose clasts
within pebbly sandstone clast counts indicates the presence of quartz-rich source rocks
and supports deposition within syn-rift, post-rift and foreland basins.
However, rocks near Lake Titicaca indicate sediment deposition within syn-rift or
post-rift basins. South- and southeast-directed paleoflow of plagioclase-laden sediment
from Lake Titicaca (Fig. 4-1) suggests erosion and transport of magmatic arc sediment.
The presence of magmatic arc sediment argues against deposition within a foreland basin
system since the associated fold-thrust belt would bar sediment transport from a western
magmatic arc. However, the possibility of foreland basin deposition remains if Central
Andean fold-thrust belt development proves to be diachronous. If a fold-thrust belt
northwest of the modern Lake Titicaca failed to develop before or during El Molino
Formation deposition, the transport of magmatic arc sediment could flow into a foreland
basin system from the northwest.
In conclusion, paleocurrent and provenance data suggest deposition during
periods of tectonic quiescence. Although deposition within a syn-rift basin or the wedgetop or foredeep depozones of foreland basin systems cannot be excluded, an
83
interpretation suggesting post-rift thermal sag or back-bulge basin deposition best fits the
data presented.
84
5. CONCLUSIONS
The El Molino Formation represents a complex sequence of carbonates, mudrocks
and sandstones that form two lithostratigraphic sequences. The first lithostratigraphic
sequence occupies the central Altiplano and western Eastern Cordillera and consists of
three members:
•
A lower member comprising ~50% of a stratigraphic section that includes
carbonates, carbonate sands and mudrocks;
•
A mudrock-rich middle member that makes up ~35% of the section; and
•
An upper member consisting of carbonate and mudrock.
The second lithostratigraphic sequence dominates El Molino Formation stratigraphy near
Lake Titicaca and in the eastern Eastern Cordillera and contains two members:
•
A lower member consisting of carbonate sands, carbonates and mudrocks that
comprises ~25% of a given stratigraphic section; and
•
An upper member containing sandstones and mudrocks that coarsens upward
within a given section.
Within these lithostratigraphic sequences, five facies associations have been
identified and their geographic distribution mapped (Fig. 5-1):
•
An open water facies association (FA1) comprised of shales and carbonate
mudstones showing very limited distribution confined to the east-central
Altiplano and west-central Eastern Cordillera;
•
A nearshore facies association (FA2) containing mudrocks, and massive
carbonate mudstones and wackestones that represent the most common facies
association present in the El Molino Formation;
85
66
66
Lago
Titicaca
Lake
Titicaca
FA1
La Paz
FA2
Cochabamba
Cochabamba
Oruro
Oruro
Oruro
Lago
Poopo
Lago
Poopo
18
Lago
Poopo
Sucre
Sucre
Potosi
Sucre
Potosi
Potosi
Bolivia
Tupiza
Tarija
Chile
Tarija
Chile
Chile
Bolivia
Tupiza
FA3
La Paz
La Paz
Cochabamba
18
66
Lake
Titicaca
Bolivia
Tupiza
22
Tarija
22
Lake
Titicaca
Lake
Titicaca
FA4
FA5
La Paz
La Paz
Cochabamba
Cochabamba
Oruro
Oruro
18
Lago
Poopo
18
Lago
Poopo
Sucre
Sucre
Potosi
Potosi
Tupiza Tarija
Chile
Chile
Bolivia
22
Bolivia
Tupiza Tarija
22
FA1. Description: Black, gray, brown and green shale and carbonate mudstone laminae.
Interpretation: Open water facies.
FA2. Description: Massive carbonate mudstone and wackestone; stromatolitic boundstones;
Massive, horizontally-laminated and ripple cross-laminated mudstone. Interpretation: Nearshore
facies.
FA3. Description: Peloidal, oolitic, fossiliferous and quartzose packstone, grainstone and very
fine to coarse-grained sandstone beds; Trough cross-bedded, horizontally laminated and massive sedimentary structures. Interpretation: Beach, bar and shoal facies.
FA4.. Description: Laminated and massive mudstone and medium- to coarse-grained sandstone
beds; Massive mudstone and sandstone strata with carbonate nodules, hackly texture, mottling,
mud cracks, root casts and distinct horizons characterized by color and/or texture. Interpretation:
Floodplain facies.
FA5. Description: Horizontally-laminated and trough cross-bedded fine- to coarse-grained sandstone beds with sharp, erosive basal contacts and multistory geometry; Horizontally-laminated,
trough cross-bedded or ripple cross laminated very fine- to medium- grained sandstone beds with
planar basal contacts. Interpretation: Fluvial facies.
Figure 5-1. Summary diagram showing the distribution of facies present within the El Molino Formation and briefly describing the lithologies and interpretations for facies association.
86
•
A beach, bar, and shoal facies association (FA3) consisting of peloidal,
oolitic, fossiliferous and quartzose grainstones, packstones and sandstones that
show broad geographic distribution but are mainly concentrated in the eastern
Eastern Cordillera;
•
A floodplain facies association (FA4) containing mudrocks and massive
sandstones that also show broad geographic distribution but primarily occur
near Lake Titicaca and in the eastern Eastern Cordillera; and
•
A fluvial facies association (FA5) consisting of sandstones that are limited to
sites near Lake Titicaca as well as Suticolla and Incapampa section localities.
The tendency of these facies associations to overlap one another in map view (Fig. 5-1)
and to superimpose one another vertically supports an interpretation for El Molino
Formation deposition predominantly within a hydrologically-closed lacustrine system.
Based on the abundance of fossil material and the paucity of evaporite deposits, perennial
lacustrine conditions are favored over an ephemeral lacustrine setting. However, rare
evaporite deposits (e.g. Agua Salud) indicate that ephemeral lacustrine conditions
periodically existed in some, if not all, parts of the basin. Marine and brackish water
fossils collected by previous workers also indicate that the El Molino Formation
depositional basin either experienced a shallow marine incursion(s) or was briefly a
hydrologically-open brackish-water lacustrine system. Based on comparisons between
the global sea level curve, age data and El Molino Formation lithologies, these conditions
likely existed during deposition of lithostratigraphic sequences 1 and 2 lower members.
Subsequent to these conditions, strata from the lithostratigraphic sequence 2 upper
member prograded into the El Molino Formation depositional basin.
87
With the exception of sediment deposited near Lake Titicaca, these progradational
strata show quartz-rich sandstone provenance that probably reflects the erosion and
recycling of Cretaceous(?) sandstone and Paleozoic quartzite sediment. Strata near Lake
Titicaca show quartzo-feldspathic composition interpreted to represent sediment transport
from source areas proximal to magmatic arc rocks. Despite the provenance differences
between strata deposited near Lake Titicaca and eastern Eastern Cordillera strata, all
sandstone thin section point count data plot within the continental block provenance field
of Dickinson (1979) suggesting syn-rift, post-rift thermal sag and foreland basin backbulge depositional conditions. Clast count data show a relatively simple unroofing
sequence indicative of the tectonic quiescence associated with post-rift thermal sag and
foreland basin back-bulge settings.
Post-rift thermal sag and/or foreland basin back-bulge tectonic settings, in
addition to syn-rift basins, and foreland basin wedge-top and foredeep depozones can all
support lacustrine conditions. However, in light of the provenance data and due to the
absence of faulting or fault-related growth strata, El Molino Formation strata are
interpreted as post-rift thermal sag and/or foreland basin back-bulge deposits.
88
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and Welsink, H. J. eds., Petroleum Basins of South America: American
Association of Petroleum Geologists Memoir 62, 305-324.
Zhang, G., Buatois, L. A., Mangano, M. G. and Acenolaza, F. G., 1998, Sedimentary
facies and environmental ichnology of a ?Permian playa-lake complex in
western Argentina. Paleogeography, Paleoclimatology, Paleoecology, 138,
221-243.
100
APPENDIX A: STRATIGRAPHIC SECTIONS
180
170
Sm
Sm
Sh
Sm
Sm
Sh
Sm
Sm
Sh
Sh
Sh
160
150
140
130
Sh
Sh
Sm with Sh
120
Sh
Sh
110
Sh
90
Fl
70
Sr
70
Sm
60
meters
50
50
40
40
Fl
Sm
Sh
Sh
Sm
Sm
Sm with Sr
Sh
Sm with Sr
Sh
Sm with Sr
Sh with Sr
Fl
St
n = 20
Sm
St
Sh with Sm, Sr
Fm
Sm
Sm
St
Sm
St with Sr
St
n = 22
St
80
80
60
Sm
90
30
30
Fl
20
Fl
10
Fl
20
10
0
M SS CL
Agua Salud
P
40
30
Fr
20
Fr
Fl
Fl
Fl
Fl
10
Fl with Fr
St
Fsm
0
50
meters
meters
100
St
0
M SS CL
Caluyo
101
M SS CL
Camargo
200
400
190
390
Fr
P
P
180
170
380
Fl with Fr, Fm
Fl with Fr, Fm
370
P
Fr
Fr
160
150
360
P
P with Fr
P
Fr
Fr
350
140
Fr
Fsm
Fr
Fsm
Fr
Fsm
Fr
P
Fr
Fl
Fsm
340
Fsm
130
330
120
Fl
320
110
Fl
Sh
Fl
310
Fsm
meters
90
300
Fsm
P
P
290
Fsm
80
70
280
Fl
Fl
Fl
270
Fsm
60
260
Fl
Fl
50
250
40
240
Fsm
Fl
30
20
10
Fl
Fl with Fr
Fl with Sh
Sm
St
Fr
Fr
Fr
Fsm
Fr
0
230
Fl
Fm
Fr
Fsm
220
210
Fsm
Fm
Fsm
200
M SS CL
M SS CL
Andamarca
102
420
meters
meters
Fr
100
410
400
Fsm
Fl with Fr
M SS CL
200
400
Sh
190
Sh
St
390
St with Sm
Sr
180
380
170
370
St
Fsm
160
360
Sh
150
350
Sh
Sh
140
St
Sm
340
Sm with Sh
120
Sh
Sm
St
110
Sh
Sh
100
St
Sm with Sh
330
Sm
320
310
meters
500
300
Fsm
90
290
490
Sm
Sm
80
280
480
Fsm
70
270
60
St
Sh
Sm
St
50
Sh
40
Sm
Sm
Sh
St
Sh
30
Sh
St
Sh
20
Sm
10
Sh
Sm
0
M SS CL
St with Sh
470
Sm
460
260
250
Sm
Sm
240
Sm
230
Sm with Sh
meters
meters
130
450
440
430
220
420
210
410
200
400
Sm
Sm
Sm
Sm
Sh
St
Sh
Sm
Sm
M SS CL
Chuvata
103
M SS CL
200
190
180
170
170
160
160
Fl
150
150
Fr
140
140
130
130
120
120
Fr
110
100
meters
90
100
90
Fl
80
80
Fl
70
70
Fl
60
60
Fl
50
Fl
40
50
Fm
Fl
Fl
Fm
Fl
Fl
Fl
Fm
Fl
Fl
Fl
250
40
240
30
230
Fl
30
Sh
Fl
Fl
20
Sr
Fl
Sr
Sh
10
0
meters
meters
110
20
220
10
210
0
200
M SS CL
M SS CL
Challa Mayu
M SS CL
El Puente
104
Fl
Fr
Fl with Fr
390
380
180
370
Fr
Fl with Fr
Fl with Fr
Fl
140
360
Fsm
350
Fr
340
Fr
100
90
80
70
60
50
Fl with Fr
Fr with Fm
Fr with Fm
Fl
Fr
Fr
Fr
Fl
Fr
Fl
Fr
Fl
Fr
meters
meters
110
Fl
Fr
Fr
Fr
Fl
0
550
Fsm
540
Fr
Fsm
Fr
Fsm
320
Fr
520
310
Fr
Fsm
Sr
510
Fr
300
Fsm
500
Fr
700
490
Fsm
690
480
Fr
Fl
290
280
270
260
250
Fl with Fm
230
Sr
Sr
Fl with Fr
Fl
P
220
Fl with Fr
200
M SS CL
Fr
Fr with Fm
530
Fl
10
560
Fr
Fsm
240
30
20
570
Fr
Fr with Fm
Fr
330
Fl with Fr
40
Fr
Fr
Fsm
Fl
120
590
Fr
Fm
130
Fr
580
Fsm
170
150
Fsm
600
Fr
Fl
160
Fr
210
FsmFsm with Fr
Fr
Fsm with Fr
Fr
Fsm with Fr
P with Fm
Fsm
Fr
Fsm with Fr
P
Fsm
Sh with Sr
Fsm
St
Fl with Fr
Sr
Sh with Sr
Fr
Fr
Fsm
Fsm with Fr
Fl
Fl with Fr
Sh with Sr
M SS CL
470
670
460
660
450
105
650
Fsm
Sr
440
640
430
630
420
620
Fr
Fm
Fr
610
Sr
410
Fsm
400
600
M SS CL
Ichilula
680
meters
190
400
Fl
meters
200
Fsm
Fr
Fl
M SS CL
200
190
Sm
Sm
180
St
170
n=10
Sm
160
Sm
Sm
150
Sm
140
St
n=9
n=6
St
130
Sm
120
Sm
110
St
100
St
n=10
300
n=4
St
90
St
80
70
290
n=15 n=6
St
50
40
n=13
260
St
250
Sm
St
240
n=5
St
St
30
n=8
230
Sm
20
P
P
P
270
Sh
Sm
Sh
Sm
St
St
St n=22
Sm
Sm
Sm
60
Sm
Sm
Sm
280
St
meters
meters
St
220
Sm
Sm
10
210
Sm
0
St
Sh
St
Sm
St
200
M SS CL
M SS CL
Incapampa
106
200
400
Sh
190
390
Fsm
Sh
180
Fsm
380
St
Fsm
n=20
Fsm
170
370
Fsm
Sh
St
St
160
150
360
St n=20
Fsm
St
Fsm with Sm
350
140
St
Fsm with Sm
340
130
St
330
Fsm with Sm
Fsm with Sm
St
120
n=20
320
Sm
310
meters
100
300
Sm
90
290
80
280
Fsm with Sm
Fr
60
50
Fsm with Sm
270
70
Fr
Sh
St
Fr
260
460
St
450
St
Fsm with Sm
240
n=20
Fsm with Sm
440
n=20
230
20
220
420
210
410
200
400
Sh
Sh
St
430
30
10
St
Fsm with Sm
250
St
40
470
meters
meters
110
St
St
St
0
M SS CL
M SS CL
Jesus de Machaca
107
M SS CL
n=20
170
160
150
140
Fm
Fsm
Fl
Fsm
Fl
130
Fl
120
Fl with Fm
110
Fl
170
160
Fl
160
150
Fsm
Sh
150
140
Fsm
P
Fsm
P
Fsm
P
Sm
Fsm
Sm
140
130
120
110
Sm
Sm
Fsm
130
120
110
Fsm
Fsm
Fl
Fl
Fsm
80
Fsm
70
60
Fsm
Fl
Fsm
Fl
Fsm
Sh, Sr
Fl
50
40
30
Fm
Fsm
Sr
Fsm
90
Fsm
St
80
St
70
Fsm
Sh
Sm
P
70
60
Fsm
60
n = 18
40
St
Fsm
20
20
10
10
Fsm
Sh
P
Sm
Sh
Sm
0
0
M SS CL
Julo Grande
90
Sm
80
Fsm
Sm
St
n = 18
Fsm
Sm with Sh
P
Sh
Sh with Sm
50
30
Fsm
Fsm
meters
90
100
100
meters
Fl
Fsm
meters
100
50
Sm
Sm
40
Sm
30
Sm
n = 17
Sm
20
Sr
St
10
Sh
0
Sm
M SS CL
M SS CL
Suticolla
Vila Vila
108
200
400
Fsm
Fsm
190
180
390
Fsm
Fsm
Fsm
380
Fr
Fsm
Fl
Fsm
170
160
370
Fsm
Fsm
150
360
Fsm
350
Fsm
Fsm
Fsm with Fm
140
340
Fsm
130
330
120
Fsm
Fsm
meters
100
90
Fsm
Fm
Fl
310
Fl
Fl
300
290
Fl
Fl
80
Fm
Sr
Sr
70
60
280
Fl
Fl
Fl
Fm
270
Fl
Fl
Fl
Fl
Fl
Fsm with Fl
260
Fr
50
40
30
Fr
Fr
Fl
Fsm
20
250
240
230
450
Fl
Fl
Sh with Sr
Fl
Sh with Sr
Fl
220
440
meters
meters
110
320
430
420
Fsm
210
Fl
Fl
Fsm
0
200
M SS CL
Fsm with Fr
Fsm
Fsm
10
Fsm
M SS CL
Maragua
109
410
Fsm with Fr, Fm
Fsm with Fr
400
Fsm
M SS CL
200
Fl
190
P
Fsm
180
P
170
Sr
P
160
P
150
140
130
P
Fsm
Fr
120
100
Fm with Fr
Fm
Fm with Fr
Fm
Fl with Fr
90
Fl with Fr
80
Fl
Fl
70
60
50
40
240
30
230
meters
meters
110
20
10
220
210
Fsm
0
200
M SS CL
Fsm
Fsm
Sh
Fr
P
Fr
Fsm
Fl
M SS CL
San Pedro de Huallaloco
110
200
190
180
Fsm
170
160
150
140
Fl
Sr
Fl
Sh
Fl
130
Fl
Fl
Fl
meters
110
100
90
80
Fl
Sh
Fl
Fl
Fl
50
40
280
Fl
70
60
290
Fl
270
Fr
Sh with Fm
Sh
260
Sh
Fl
Fl
Fl
meters
120
250
Fl
230
Fsm
20
10
0
Fl
240
Fsm
30
Sh
Sh with Sr
Sm
220
Fl
Fsm
210
Fsm
Sm with Sh
200
M SS CL
Sh
Fl
M SS CL
Vilcapuyo
111
Section measured in San
S20°43.518'
W065°12.605' Pedro river valley north
of distillery.
Camargo
112
Chuvata
Challa Mayu
S16°40.237'
Measured section in valley Base of El Molino Fm obscured by intrusion.
W068°39.135' north of Estancia Caluyo. Upper contact conformable with Santa Lucia.
Caluyo
El Molino Formation overlies Chaupiuno Fm,
a ~20m sandstone unit with micrite lenses.
Santa Lucia and Impora Fm overlie El Molino
2km NW of Challa Mayu. El Molino overlies Chaunaca Fm and underS19°10.94'
W066°06.233' Measured in quebrada by lies Santa Lucia Fm.
sharp 90° turn in Challapata-Potosi road.
S16°35.353'
East of Desaguadero.
Base of El Molino Fm is truncated by thrust
W068°59.058' Measured near the pueblo fault. El Molino strata passes upwards into
Asfranal. Indigenous pop- Santa Lucia Fm.
ulation was extremely suspicious and instructed us
to leave. We returned with
appropriate paperwork and
were allowed to work.
Conformably overlies Chaunaca Fm and underlies Santa Lucia Fm.
S18°47.413'
W067°31.709'
Andamarca
Stratigraphic Notes
Near Estancias Quebrada Problematic section. Unconformably overlies
and Agua Salud.
La Puerta? Interpreted as El Molino based on
reddish-purple mudrock color and presence
of calcareous sandstone. Conformably overlies condensed Santa Lucia? and Luribay Fm.
Location Notes
S16°57.8'
W067°35.63'
Coordinates
Agua Salud
Location Name
APPENDIX B: STRATIGRAPHIC SECTION COORDINATES AND NOTES
113
S19°27.56'
W064°50.58'
S16°42.733'
Began section in drainage
W068°46.874' NW of small pueblo to N
of Jesus de Machaca. Section measured ~E and normal to initial drainage.
Incapampa
Jesus de Machaca
Measured section in N.
Incapampa syncline along
foot path S. of Rio Pilcamayo and E. of foot bridge
Ichilula is a small pueblo
in the northwest Rio Mulatos thrust belt between
pueblos Chilla Viento and
Circoya.
S19°24.312'
W067°9.397'
Ichilula
Location Notes
S21°14.215'
Measured section East of
W065°10.788' the pueblo El Puente
starting setion east of
limestone quarry.
Coordinates
El Puente
Location Name
El Molino Fm unconformably overlies La
Puerta Fm. La Puerta Fm. contains numerous
calcareous nodules organized into laterally
extensive horizons and interpreted as sandy
paleosols. El Molino Fm underlies Incapampa
Fm possibly suggesting that Santa Lucia Fm
rocks were included in El Molino Fm data.
However, a facies shift attibutable to Santa
Lucia depostion was not evident.
Much of this section is obscured by cover.
Base of El Molino overlies Paleozoic rocks.
El Molino Fm passes upwards into Santa
Lucia Fm.
El Molino Fm lies between Chaunaca and
Santa Lucia Fm. El Molino and Santa Lucia
Fm contact occurs under cover. Upper El
Molino and most of the Santa Lucia Fm
form a valley around synclinal core.
El Molino Fm forms slightly angular unconformity (<10°) with underlying Chaupiuno
Fm sandstone. Chaupiuno Fm contains secondary calcareous nodules that may be pedogenic in nature. Santa Lucia Fm overlies El
Molino Fm.
Stratigraphic Notes
114
Location Notes
S18°03.352'
Measured section up
W065°47.833' drainage ~100m S of Julo
Grande
Measured section up
drainage N of Vila Vila.
Torotoro
Vila Vila
Measured section along
La Paz-Cochabamba road
W of Suticolla and E of
Pongo.
S19°6.378'
Measured section in valley
W065°25.643' SW of Irupampa. Indigenous population to SW was
hostile, prevented completion of section and
would not return our credentials. Locals did not
speak Spanish.
S18°37.333'
Measured section immediW067°35.931' ately N of town.
Coordinates
Suticolla
San Pedro de Huallaloco
Location Name
Maragua
Problematic section. Believed to be El Molino Fm. Lithology includes calcareous sandstone with brecciated limestone clasts and
pebbles. Overlies La Puerta Fm with interbedded basalt flow similar to Incapampa.
El Molino Fm overlies La Puerta Fm. Santa
Lucia Fm overlies El Molino Fm.
Basal stratigraphic relations difficult to discern. Base of El Molino Fm section may include Torotoro rocks. Base of measured section overlies Paleozoic rocks. Santa Lucia
Fm overlies El Molino Fm.
El Molino Fm overlies Chaunaca Fm and
underlies Santa Lucia Fm. Rocks at San Pedro de Huallaloco are overturned.
Base of El Molino Fm was under cover but
sporadic outcrops and stratigraphy thoughout
the region suggests that the Chaupiuno and
Aroifilla Fms underly El Molino rocks. Santa Lucia strata overlies El Molino Fm, forming a valley within the core of the syncline.
Stratigraphic Notes
115
Vilcapuyo
Location Name
S19°4.012'
W066°30.813'
Coordinates
Section measured about
5km from the end of the
pavement on the Challapata-Potosi road.
Location Notes
El Molino Fm conformably overlies Chaunaca Fm. Santa Lucia Fm overlies El Molino
Fm.
Stratigraphic Notes
VITA
Richard John Fink was born in Atlanta, Georgia, on May 31st, 1966 and raised by
Lieutenant Colonel (United States Army retired) Henry John Fink Jr. and Ilse Rehne
Fink. Richard attended the Miller School of Albemarle from 1977-81and graduated from
Woodbridge Senior High School in Woodbridge, Virginia, in 1984. After several
semesters at George Mason University in Fairfax, Virginia, he enlisted in the United
States Navy. Richard served six years in the Navy, working as a sonar technician aboard
the fast-attack submarine USS Billfish SSN-676. Following the completion of his
enlistment, he moved to Montana where he earned his Bachelor of Science in Earth
Sciences degree at Montana State University in 1999. In 1999, he enrolled into the
Master of Science program at Louisiana State University where he hopes to graduate in
August 2002.
116