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Geological Society of America Bulletin
Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension
in Sonora, Mexico
Fred W. McDowell, Jaime Roldán-Quintana and Ricardo Amaya-Martínez
Geological Society of America Bulletin 1997;109;1349-1360
doi: 10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2
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Notes
Geological Society of America
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Interrelationship of sedimentary and volcanic deposits associated with
Tertiary extension in Sonora, Mexico
Fred W. McDowell*
Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712
Jaime Rold‡n-Quintana
Instituto de Geolog’a, Universidad Nacional Aut—noma de MŽxico, Apartado Postal 1039,
Hermosillo, Sonora 83000, MŽxico
Ricardo Amaya-Mart’nez
Departamento de Geolog’a, Universidad de Sonora, Hermosillo, Sonora 83000, MŽxico
ABSTRACT
Clastic sedimentary deposits and associated volcanic rocks record
the progress of Tertiary extension in the Mexican state of Sonora. These
deposits accumulated within basins located throughout the eastern
two-thirds of the state. The rocks are slightly indurated conglomerates
and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks
provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined
in an east-west belt across south-central Sonora.
The volcanic rocks occur in three distinct associations. At the base
and interbedded within the lower portions of the sedimentary sections
are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with
these lava flows, and we conclude that the magmatism was triggered by
early faulting along the basin margins. A second association present
near the base of the sections comprises massive lava domes and flows of
intermediate composition that contain distinctive phenocrysts of dark
brown amphibole. The domes apparently formed when viscous lava
welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a
thick lava flow that is conformably interbedded with, and sheds clasts
laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the
coarsest and thickest lower portions of the sedimentary sections.
K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation
by the major river of Sonora has exposed a full section of the volcanic
and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma,
and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic
rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections.
The mafic lava flows, dominantly basaltic andesite, are similar in
major-element composition to mafic lava flows that cap felsic sections
within the Sierra Madre Occidental volcanic field of western Mexico
*E-mail: [email protected]
and the mid-Tertiary Datil-Mogollon volcanic field in southwestern
New Mexico. They are higher in silica and lower in total alkalies than
younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in
the Datil-Mogollon field. They are similarly distinct from basaltic dikes
and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state.
A convergent-margin magmatic arc was active in the present-day
Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate
that extension continued throughout the region after 10 Ma with little
coeval volcanism.
INTRODUCTION
Much of western North America has been affected by extension since middle Tertiary time. Regional descriptions of the history and geometry of this
extension have focused primarily on the Great Basin, where two phases have
been recognized (Zoback et al., 1981; Eaton, 1982). The first phase began as
early as 35 Ma (Gans et al., 1989), and was characterized by relatively high
strain rates that produced extreme rotations along relatively closely spaced
listric fault systems. In much of the region, the extension direction was westsouthwestÐeast-northeast. During late Miocene time (after ca. 10 Ma),
extension in the Great Basin was characterized by lesser degrees of block rotation along more widely spaced brittle normal faults. Locally, strain rates
were much lower, and the direction of extension changed to northwest-southeast. The latter phase produced the pattern of elongate ranges and basins that
characterizes the Basin and Range physiographic province.
Virtually the entire state of Sonora lies within the Basin and Range
province, which continues to the south at least into central Mexico (Henry
and Aranda-G—mez, 1992; Stewart and Rold‡n-Quintana, 1994). The modern topography of Sonora consists of linear ranges oriented north-northwest
and separated by broad alluvial valleys. In the western portion of the state,
the ranges have been largely submerged and the bounding faults mostly
concealed by a thick veneer of young alluvium. As a consequence, the distribution of young basins does not correspond well to the original Neogene
topography. In contrast, in central and eastern Sonora a deeper level of
erosion has enhanced the topographic contrast between north- to northnorthwestÐoriented ranges and valleys, and has exposed some of the rangebounding faults.
GSA Bulletin, October 1997; v. 109; no. 10; p. 1349Ñ1360; 6 figures; 2 tables.
1349
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MCDOWELL ET AL.
1350
Gans (1997) has focused on the Santa RosaÐSan Nicol‡s area (Fig. 2) in the
eastern portion of our study area, where we have made only reconnaissance
examinations. Gans concluded that extension was in progress there by 27
Ma and was largely completed by 20 Ma.
BAUCARIT AND LISTA BLANCA FORMATIONS
Although our emphasis is on the genetic significance of the stratigraphic
sections, the rock units correspond in part to two formations that have been
described for many years within the literature of Sonoran geology. Dumble
(1900) was the first to describe the occurrence of clastic sediments filling
linear basins in Sonora. King (1939) applied the name of B‡ucarit Formation to these rocks to designate a sequence of Òslightly indurated, wellbedded sandstones, conglomerates, and some clays. In the lower part of the
formation, where fully developed, there are one or more basalt flows,
interbedded with basalt agglomerate.Ó Clasts include virtually all older
rocks that are locally exposed in the adjacent highlands. The type locality
110°
111°
S
112°
109°
ARIZONA
31°
SONORA
T
M
CHIHUAHUA
Stewart and Rold‡n-Quintana (1994) compiled the distribution of stratal
tilts and major normal faults for the state of Sonora. In addition to delineating the 10Ð15 km width of ranges and basins, the compilation showed that
rotation of strata was generally consistent within three specific domains.
These domains are separated by synformal or antiformal boundaries and
accommodation zones. The area discussed in this paper includes their eastern
domain, in which dips are generally east to east-northeast from 60¡ to less
than 5¡, their central domain, in which dips are mostly west to southwest
from 55¡ to less than 5¡, and the antiformal boundary between the domains.
For the most part the tilted strata are middle Tertiary to Neogene age. Rocks
as young as 10 Ma cap tilted blocks, but the most extreme stratal tilts occur
in rocks older than 10Ð11 Ma (Stewart and Rold‡n-Quintana, 1994).
Another extensional event that affected the western margin of Sonora
during Neogene time is the development of the Gulf of California extensional province (Stock and Hodges, 1989). The most prominent aspect of
this province is the oblique extension that occurred during the main phase of
opening of the Gulf, from 5.5 Ma to the present. An earlier interval of more
orthogonal opening began as early as 15 Ma in some areas of the Gulf, as
indicated by differential rotation of dated volcanic horizons, and by incursion of marine sediments into the developing proto-Gulf of California
(Smith, 1991). Gulf extension may have begun as an extensional domain
within the Basin and Range before evolving into a distinct tectonic province
(Henry, 1989).
In northern Sonora the oldest deformation related to Tertiary extension
has been reported from metamorphic core complexes (Nourse et al., 1994).
These are found in the areas of Magdalena, Tubutama, Sas‡be, Mazat‡n,
Puerto del Sol, and the northern portion of the Aconchi batholith (Fig. 1).
Timing of mid-crustal extension within the Sonoran core complexes is from
25 to 18 Ma (Nourse et al., 1994). Detachment faults produced large extensional basins that contain Tertiary sediments with volcanic interbeds. In the
Magdalena basin, a sequence of latite flows at the base of the sedimentary
section has K-Ar ages of 27 and 22 Ma (Miranda-Gasca and DeJong, 1992).
In the Tubutama basin, a 22 Ma latite is at the base of a section of Miocene
lake deposits that are covered in angular unconformity by a sequence of red
conglomerates. The conglomerates are overlain by a 7 Ma basalt flow
(G—mez-Caballero et al., 1981).
The objective of this paper is to present new information about Neogene
basins formed by extensional faulting in Sonora. This extension produced
grabens and half-grabens that accumulated sedimentary and volcanic fill.
Because these rocks constitute the primary record of extension in Sonora,
we describe their field relations, emphasizing the age and nature of the volcanic rocks. The study covers the entire known east-west extent of the clastic sedimentary deposits at lat 28¡ to 29¡N (Fig. 2). This area is well to the
east of the Gulf of California vicinity, and is south of known exposures of
metamorphic core complexes. The study is reconnaissance in nature and
attempts to examine the rock record in the context of its relationship to
Neogene extension. The primary focus is on the well-exposed clastic
deposits within the basin of the Rio Yaqui, SonoraÕs major permanent river
(Figs. 2 and 3). Also examined in some detail are exposures in the area of
Suaqui Grande, 30 km to the west of the Rio Yaqui. Results of reconnaissance examinations of scattered exposures both to the east in the YŽcora
area and in the Arroyo Los Pilares, and to the west near the town of La
Colorada in the northern Sierra Bacatete, and at Cerro Lista Blanca (Fig. 2)
are also included.
In addition to our field studies we have included the work of CochemŽ
and Demant (1991) in the Tep—ca area and in reconnaissance farther to the
east; the study of Bockoven (1980) near YŽcora; and that of AmayaMart’nez et al. (1992) at Cerro Lista Blanca (Fig. 2). Bartolini et al. (1994)
examined volcanic and sedimentary rocks related to extension in Sonora
covering an area much larger than that of this report. A concurrent study by
45
A
30°
PS
MZ
H
IT
29°
Fig. 2
16
GU
LF
OF
Y
28°
G
CA
LI
FO
RN
IA
Baucarit
*
A
0
50 km
O
AL
N
SI
Figure 1. Map of Sonora, showing the distribution of Neogene continental clastic basins (shaded). Locations of Highways 45 and 16 are
shown. Abbreviations: HÑHermosillo; GÑGuaymas; YÑYŽcora;
ITÑIsla Tibur—n. Italicized bold letters mark locations of metamorphic core complexes (SÑSasabe; TÑTubutama; MÑMagdalena; AÑ
Aconchi; PSÑPuerto del Sol; MZÑMazat‡n). An asterisk is placed at
the type locality of the B‡ucarit Formation. The rectangle encloses the
area of Figure 2.
Geological Society of America Bulletin, October 1997
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TERTIARY EXTENSION IN SONORA, MEXICO
Hermosillo
110°
111°
109°
29°
16
R. Yaqui
La
DII Colorada
RIO YAQUI
BASIN
45
30
WESTERN
LOCALITIES
DIII
*
28,29
AIII
Co. Lista 24
Blanca
DIV
BI
DI
BII
DVI
DV
11
*
*
SG
BIII BIV
*
SUAQUI AI
GRANDE
AREA
AV
Sierra
Bacatete
Sierra
Libre
5
AIV
Onavas
CI Tepoca
San 16
Nicolas
14
AII
CII
CIII
Yecora
*
15
EASTERN
LOCALITIES
Sierra
Santa
Ursula
28°
0
50 km
Guaymas
Figure 2. Location map for part of south-central Sonora showing geographic features mentioned in the text. Shaded areas indicate major Neogene clastic sedimentary deposits; asterisks show known outcrops of distinctive amphibole-bearing intermediate-composition igneous rock (see
text). Letters paired with Roman numerals (e.g., A IV) indicate locations of stratigraphic columns displayed in Figure 4, AÐD. Numbered dots are
locations for dated samples not indicated in Figure 4. SGÑSuaqui Grande. A dot-dashed line shows the border between the states of Sonora and
Chihuahua. The solid lines are the locations of Highways 16 and 45, and the dotted line locates the Rio Yaqui and its reservoir system.
designated by King (1939) for the B‡ucarit Formation is the Arroyo Cedros,
near the town of B‡ucarit (Fig. 1) about 100 km to the south of the area covered in this paper.
A subdivision of the B‡ucarit Formation into an upper clastic and lower
volcanic member was originally applied by King (1939). Lava flows and
volcanic breccias of basaltic to andesitic composition are prominent within
the lower member of the B‡ucarit Formation, particularly within the eastern part of its known distribution. Typically, the volcanic rocks are conformable, either immediately beneath or intercalated within the oldest sedimentary layers. The lava flows are characterized by brecciated bases, and
by altered olivine phenocrysts. Some also contain augite and hornblende as
phenocryst phases.
Although King (1939) interpreted the contacts between B‡ucarit Formation and older rocks either as unconformities or as thrusts, we have observed
only normal fault contacts or unconformities. The sedimentary rocks have a
patchy distribution and probably did not form continuous cover throughout
their area of deposition. The lack of paleocurrent studies and minimal information about internal stratigraphy preclude reliable reconstruction of
original depocenters for the B‡ucarit Formation. These depocenters probably were closed continental basins, within which the coarsest strata were locally derived and accumulated in alluvial fans or talus deposits. The centers
of deposition correspond only partly with present-day basins.
Clastic sedimentary deposits similar to those of the B‡ucarit Formation
have now been recognized throughout much of the province of parallel
ranges and valleys in Sonora (Fig. 1). Where present as tilted strata in uplifted blocks, the significance of the deposits is obvious. Elsewhere, it may
be difficult to discern them from younger Pliocene to modern unconsolidated alluvial deposits. Their only distinguishable characteristic is com-
monly a greater degree of induration than is observed within the younger
deposits. The identification is especially difficult in western Sonora, where
little exhumation has occurred. As far as we are aware, no sedimentary
deposits that can be related to those of the B‡ucarit Formation have been
recognized to the west of the area shown in Figure 2.
Age information contained within the B‡ucarit Formation is significant
with respect to the history of extension in Sonora. Extremely limited vertebrate remains suggest a Neogene or Quaternary age for the upper portion of
the B‡ucarit Formation (King, 1939). Palynologic study of fine-grained
sediments within the upper part of the B‡ucarit Formation to the northeast
of Hermosillo has yielded estimates ranging from Pliocene to Pleistocene
age (Martinez-Hern‡ndez, 1984). Obviously, the time interval represented
by deposition of the B‡ucarit Formation is poorly defined.
Lista Blanca division was first designated by Dumble (1900) for a conglomerate and volcanic sequence at Cerro Lista Blanca (Fig. 2), to which
he assigned a tentative Triassic age. Later, King (1939) attributed a
Cretaceous age to the section. Morales et al. (1990) established a Tertiary
age by showing that volcanic rocks at Cerro Lista Blanca lie conformably
above conglomerates that they reassigned to the B‡ucarit Formation.
They also reported a K-Ar age of 10.4 ± 0.2 Ma for a latite flow within the
Lista Blanca section. Also at Cerro Lista Blanca, Amaya-Mart’nez et al.
(1992) described a 48 m thick section of rhyolite lava flows and tuffs,
ignimbrite and tuffaceous sandstone, all conformably above conglomerates of the B‡ucarit Formation. The sequence dips uniformly 25¡ to the
southwest. We concur with the consensus of these later workers that the
conglomerates originally included by Dumble (1900) and King (1939)
within the Lista Blanca Formation are instead probably correlative with
the B‡ucarit Formation.
Geological Society of America Bulletin, October 1997
1351
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MCDOWELL ET AL.
109°30'
28°43' 109°37'
pre-N
pre-N
Nsl
III
60
65°
6
Nsu
Qal
pre-N
Qal
GEOLOGY AND GEOCHRONOLOGY OF SPECIFIC
LOCALITIES
Nsl
EXPLANATION
Qal
Introduction
To
24
Nsu
younger deposits
T-Q
Nsl
pre-N
Ns–Neogene clastic rocks
18
Nsu–upper clastic rocks
Nsl–lower clastic rocks
Qal
pre-N–older rocks
pre-N
29
IV
35
7
Sample location with age
Nsl
Nsu
10
Highway 16
Qal
Rio Yaqui
0
30
20
Nsl
Normal fault with direction of dip
3 km
T-Q
Nsl
N
On
70°
Nsu
25
I
T-Q
Nsl
10
Nsl
II
15
Nsu
7 33.5 Ma
Ns
LD
Ns
V
pre-N
pre-N
Qal
28°24'
109°29'
Figure 3. Generalized geology of Rio Yaqui basin. For clarity, a
stipple has been added to the outcrop area of the upper clastic rocks
(Nsu). Roman numerals mark the locations of columns in Figure 4A.
TOÑTonich’; ONÑOnavas; LDÑLa Dura.
The lava flows within the Lista Blanca Formation are likely to be close
to their source, although no vents have yet been recognized. One major
source is probably located within the area of Cerro Lista BlancaÐnorthern
Sierra del Bacatete (Fig. 2), where the section is thickest and lava units are
the most prevalent. No caldera source for the ignimbrite components of the
Lista Blanca Formation has yet been recognized. It is possible that the ignimbrites could have originated in western Sonora, where thicker Neogene
volcanic exposures have been described from the GuaymasÐSierra Santa
Ursula areas (Mora-Alvarez, 1992, 1993; Sheridan et al., 1979). A thick section of felsic volcanic rocks is also present in the Sierra Libre (Fig. 2).
1352
It is obvious that the formal definitions and the lateral extents of both the
B‡ucarit and Lista Blanca Formations remain only vaguely determined.
Based in considerable part on reviewersÕ responses, we have abandoned
attempts to assign the sedimentary and volcanic rocks that we describe to
these formations. In order not to detract from our emphasis on the genetic
relation of these deposits to extensional faulting, we have avoided using the
terms B‡ucarit and Lista Blanca Formations throughout the remainder of this
paper. However, these names have been applied in previous studies within
much of the area that we describe. When the nature and extent of the these formations is better understood, their formal redefinition should be undertaken.
In this section we describe the stratigraphic and timing relationships that
we have examined within the study area. Field observations from specific
localities are portrayed on the generalized stratigraphic columns of Figure
4 (AÐD). These are grouped by subareas that are located in Figure 2. The
discussion below emphasizes only general relationships. In order to establish the timing of clastic sedimentation within Neogene extension-related
basins, and by inference the time of extension, we have obtained K-Ar ages
for 30 volcanic rock samples. Many of these are tied stratigraphically into
the generalized columns of Figure 4. Locations for the others are given on
Figures 2 and 3. All of the data are in Table 1, where the samples are organized according to the major geographic subdivisions, and by stratigraphic
position within those subdivisions. Samples are numbered consecutively to
coordinate them with the figures and text di scussion. Potassium was determined by flame photometry, using lithium as an internal standard and
sodium as a buffer. Radiogenic argon was analyzed by isotope dilution
using on-line extraction and purification with a 3-in gas-source mass spectrometer operated under computer control. Further details were given in
McDowell and Mauger (1994).
In the previous section two specific associations for the volcanic rocks
were given: mafic lava flows that occur either at the base or within the basal
parts of the clastic sedimentary sections, and rhyolitic ignimbrites and lava
flows that cap the coarser and thicker parts of the sections. In addition, we
have recognized a third distinctive and widespread igneous rock type associated with the lower parts of the clastic sections. This rock type is a porphyritic lava flow of intermediate composition that contains striking phenocrysts of euhedral dark brown amphibole, with opaque rims of variable
thickness. Andesine and, in some cases, clinopyroxene also are present as
phenocrysts. The matrix, either glassy or microlitic, is dominated by plagioclase. The typical field occurrence is massive domes for which external
contacts are rarely exposed. Less commonly the rock occurs as tabular lava
flows. Locally, the lava flows and domes underlie clastic sedimentary
layers. Individual flows and domes are laterally discontinuous, indicative of
a relatively high initial viscosity. Our interpretation is that this amphibolebearing magma welled up at locations along developing faults at graben
margins. We regard this rock type as an important marker for the progress of
extensional faulting in this area of the Sonoran Basin and Range, and have
indicated its known outcrops in Figure 2.
Rio Yaqui Basin
The Rio Yaqui basin is located in eastern Sonora about 180 km southeast
of the city of Hermosillo (Fig. 2). The present course of the Rio Yaqui follows a Neogene paleobasin that is from 6 to 7.5 km wide and more than 35
km long (Fig. 3). The valley is one of the few localities in which both the up-
Geological Society of America Bulletin, October 1997
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A
III
Arroyo
el Salto
I
Arroyo
el Obispo
9
10
well consolidated conglomerates
and sandstones in 3-6 m-thick
layers; section includes 5-10 m-thick
rhyolitic tuffs and a 280 m-thick
sequence of basaltic andesite flows
and breccias
S.M.O. volcanic rocks
25.3±1.6 Ma (lava dome)
III
Arroyo Molina
rhyolitic ignimbrites, lava
flows, and breccias
clastic sediments, conformably
above mafic flows
section dips 50°W
3 27.5±1.1 Ma (mafic flow)
unconformity
unconformity
S.M.O. volcanic rocks
K volcanic and
intrusive rocks
unconformity
27.1±0.9 Ma (ignim.)
S.M.O. volcanic rocks
II
Arroyo
las Palomas
12.3±0.3 Ma
conformable contact
conglomerates
section dips 25°SE
mafic lava flows
amphibole-bearing intermediate composition
lava dome
K-T intrusive and volcanic rocks
20.2±0.4 Ma (mafic flow)
unconf.
6
12
section dips 50-62°E
12.0±0.8 Ma
rhyolitic lava flows and
ignimbrites
unconformity
polymictic conglomerates
with unsorted, subrounded
clasts; no mafic flows
section dips 40°NE
12.5±0.3 Ma (ignim.)
well consolidated, pebble-boulder
conglomerate in 0.5-1 m-thick
strata with local sandy intervals;
four intervals of mafic flows and
breccias containing plag. and oliv.
phenocrysts
section dips 20-30°E
2
II
Rancho
el Pozo
rhyolitic ignimbrites and
lava flows
cobble-boulder conglomerate,
unsorted, subrounded clasts
section dips 6°E
unconformity
poorly consolidated, thin bedded
boulder conglomerate, some
sandy horizons
1
B
I
Cerro Vigia
4 km north of
Suaqui Grande
chaotic mixture of mafic flows and
breccias with pockets of clastic
sediments
section dips 30-35°W
normal fault contact (fault dips W)
8
rhyolitic volcanic rocks
unconformities
sandstones with subordinate conglomerates
with basaltic andesite lavas
section dips 65°E
IV
Arroyo
Cajon Bonito
K-T intrusive rocks
IV
Arroyo
Taraicito
amphibole-bearing
intermediate lava flow
and breccia
K volcanic rocks
conglomerates and
lava flows of basaltic
andesite
section dips 40°E
200 m
63.6±1.5 Ma
K-T intrusive and volcanic rocks
V
La Dura
0
coarse clastic sediments
4
200 m
20.5±1.1 Ma (lava dome)
intermediate composition dome
intrudes lower section, has
brecciated carapace that has shed
blocks into clastic section; upper
sediments lap onto dome
I
Cerro
Lista Blanca
0
Cocheme and Demant, 1991
Gans (1997)
III
Rancho
El Churi
II
Yecora
Bockoven, 1980
14,17,16.7,17.4
locally derived andesitic
lava flows, flatlying
14.4±0.3 Ma
23.3±1.1 Ma
section dips about 20°W
200 m
0
Paleozoic rocks
IV
Pozo de Leyva
basaltic lava flow
20
8.9±0.4 Ma
conformable contact
air fall tuffs, ignimbrites
K-T intrusive rocks
mafic lava flows
conformable contact
conglomerates
?
VI
E. Sierra
el Bacatete
V
N. Sierra
el Bacatete
25
Bockoven, 1980
23.2±0.8 Ma
12.7±0.3 Ma
12.3±0.6 Ma
conformable contact
coarse clastic sediments
dip is about 20°W
unconformity
18.4±0.4 Ma
19
III
Arroyo los Pilares
18
rhyolitic ignimbrite
26
S.M.O. volcanic rocks basaltic andesite lava flows,
interbedded with rhyolite lava
flows and ignimbrites
thin tuff
clastic sediments
section dips 15°W
lower mafic lava flows
clastic sediments
section dips 5°W
(?) conformable contact
mafic lava flows with
interbedded rhyolitic
ignimbrites (SMO volc. rocks)
rhy. ignimbrite
27
conformable contact
coarse clastic sediments
dip is about 20°W
unconformity
coarse clastic sediments,
generally flat lying,with no
interbedded volcanic rocks
unconformity
17
13
10.4 Ma
21 11.5±0.3 Ma
22 12.8±0.6 Ma
23 12.2±0.3 Ma
II
NW La
Colorada
rhyolite ignimbrites and
lava flows
coarse clastic sediments
dip is about 30°W
C
I
Tepoca
D
rhyolitic ignimbrite
spherulitic lava flow
intermediate lava flow
conformable contact
conglomerates
11.5±0.9 Ma
rhyolitic ignimbrites
lithic tuffs
section dips about 20°W
conformable contact
conglomerates
unconformity
K-T intrusive rocks
?
200 m
0
Figure 4. Grouping of stratigraphic columns by areas. (A) Rio Yaqui basin. (B) Suaqui Grande area. (C) Eastern localities. (D) Western localities.
Stratigraphic positions of K-Ar dated samples are shown with ages and circled numbers keyed to Table 1. Ages in brackets are from Bartolini et al.
(1994) and Gans (1997). SMOÑSierra Madre Occidental.
Geological Society of America Bulletin, October 1997
1353
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MCDOWELL ET AL.
per and lower parts of the clastic section are well exposed (Rold‡n-Quintana
and McDowell, 1992). Within the Rio Yaqui basin five sections were studied in detail (Fig. 4A), but intense faulting allowed reliable thickness determinations for only two of them. Eight K-Ar ages provide chronologic control
on clastic sedimentation within the Rio Yaqui basin (Table 1, samples 1Ð8):
none of these ages are from north of the Highway 16 bridge (Fig. 3).
The clastic sediments were deposited unconformably on early Paleozoic
sedimentary rocks, on granitoid or related volcanic rocks of the Late
CretaceousÐearly Tertiary batholith (sample 8; 64 Ma; Fig. 4A, column IV),
or on mid-Tertiary volcanic rocks that presumably are outliers of the Sierra
Madre Occidental volcanic field. Ages of 27.1 Ma (sample 6; Fig. 4A,
column I) and 33.5 Ma (7; Fig. 3) are consistent with ages for the felsic
components of that field from the YŽcora area and from localities farther to
the east (Bockoven, 1980; McDowell et al., 1990; McDowell, 1993;
McDowell and Mauger, 1994). They provide an older time limit for deposition of the clastic sediments within the basin.
The lower part of the clastic section crops out in a continuous belt for
about 20 km along the western margin of the Rio Yaqui basin (Fig. 3),
where it dips east 10¡ to 30 ¡ toward the center of the basin. In the southern part of the basin, discontinuous outcrops are also present along the eastern margin. The lower section is composed of clastic and volcanic rocks in
about equal proportions. The clastic rocks include conglomerates and
sandstones, and subordinate siltstones. The volcanic rocks are lava flows
and breccias, dominantly of basaltic andesite and andesite, but some basalt
may be present. The lava flows have well-developed breccias at their bases
and otherwise are vesicular with phenocrysts of plagioclase and olivine,
mostly altered to iddingsite. Some lava flows also contain augite as
phenocrysts. In many localities the mafic lava flows compose the lowest
part of the section. The overlying section is typically conformable and consists of interbedded lava flows and clastic layers. We can recognize no distinction between the basal and intercalated lava flows, and we assign both
to the clastic-volcanic section.
In the Arroyo el Obispo, west of the Rio Yaqui (Figs. 3 and 4A, column I),
a 500 m section dips 20¡ to 30¡ to the east and comprises mafic lava flows
and breccias alternating with conglomerate and sandstone. The upper contact is placed at the base of a prominent rhyolitic ignimbrite dated as 12.5 Ma
(sample 1, Fig. 4A, column I). This is the youngest igneous rock present in
this part of the Rio Yaqui basin. A K-Ar age of 20.2 Ma (sample 2, Fig. 4A,
column I) was obtained for a sample from the lowest of four intervals of
mafic lava flows. This interval has some sedimentary strata below it.
Thin exposures of the lower part of the section are present on the east side
of the Rio Yaqui basin. At Arroyo las Palomas (Fig. 4A, column II), mafic
lava flows (27.5 Ma; sample 3) lie directly on a pluton of the Sonoran
batholith; there is no basal sedimentary interval. Layers of clastic sediment
are present above the lava flows. At Arroyo Cajon Bonito (Fig. 4A, column
IV), undated mafic lava flows are chaotically mixed with pockets of clastic
sedimentary rocks.
In the northern part of the Rio Yaqui basin, in Arroyo el Salto (Fig. 3), the
lower part of the clastic section is about 650 m thick and rests unconformably on a (Tertiary?) amphibole-bearing lava flow of intermediate
TABLE 1. K-AR AGES
Index
Sample
Rio Yaqui Basin
Capping rhyolites
1
SO 31(R)
Ignimbrite
Interbedded mafic flows
2
SO 33
3
SO 48
Basaltic andesite
Amphibole-bearing lava-dome
4
SO 82
5
SO 4
Pre-Neogene rocks
6
SO 32(R)
Ignimbrite
7
SO 101
tuff
8
SO 57
Andesite flow
Suaqui Grande area
Capping rhyolites
9
SO 76
Ignimbrite
10
SO 75
Ignimbrite
Interbedded mafic flow
11
SO 28
Andesite
Amphibole-bearing lava-dome
12
SO 106
SO 106
1354
Mineral
K
(%)
40*Ar
(%)
40*Ar × 10–6
(scc/gm)
3146.40N
639.45E
Alkali feldspar
4.465
4.573
73
77
2.215
2.181
12.5 ± 0.3
3146.20N
639.45E
3144.30N
643.70E
Whole rock
1.396
1.395
1.344
1.330
73
70
68
71
1.121
1.083
1.400
1.479
20.2 ± 0.4
3138.05N
637.65E
Amphibole
0.7160
Amphibole
0.5481
0.5566
0.6013
0.5751
0.5145
0.5441
0.5537
20.5 ± 1.1
3157.25N
623.05E
55
36
57
53
60
3146.75N
636.60E
3141.01N
641.92E
3156.35N
645.05E
Alkali feldspar
0.5477
0.5436
0.6308
0.6373
1.406
1.393
25
22
50
52
89
90
0.5731
0.5853
0.8417
0.8215
3.468
3.574
27.1 ± 0.9
3135.45N
594.20E
3135.25N
593.95E
Alkali feldspar
4.176
4.170
3.091
63
73
71
67
2.034
1.863
1.493
1.481
12.0 ± 0.8
3143.95N
615.00E
Whole rock
1.739
1.703
78
68
1.545
1.616
23.5 ± 0.8
3145.53N
605.61E
Amphibole
0.6110
0.6269
Plagioclase
0.2900
0.2841
0.7699§
0.6401
0.5850
0.5318§
0.3443
0.3012
25.3 ± 1.6
3145.53N
605.61E
26
27
54
19
27
28
UTM coordinates
(km)
Whole rock
Plagioclase
Whole rock
Alkali feldspar
Geological Society of America Bulletin, October 1997
Age† ± 1s
(Ma)
27.5 ± 1.1
25.4 ± 0.5
33.5 ± 0.8
63.6 ± 1.5
12.3 ± 0.3
28.7 ± 2.7
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TERTIARY EXTENSION IN SONORA, MEXICO
TABLE 1. (Continued)
Index
Sample
UTM coordinates
(km)
Eastern localities
Capping andesite flow
13
SO 14
SO 14
Basal mafic flows
14
15
16
17
SO 40
Basaltic andesite
SO 41
Basaltic andesite
SO 19
3 21 3
Basaltic Andesite
3148.39N
669.42E
3148.39N
669.42E
3135.10N
670.05E
3123.00N
665.25E
3147.85N
697.05E
3138.3N
709.8E
Pre-Neogene rocks
18
3 21 2
Rhyolite flow
3 21 2
Rhyolite flow
19
3 20 4
Ignimbrite
3138.8N
710.5E
3138.8N
710.5E
3139.8N
724.0E
Western localities
Capping volcanic rocks
20
SO 60
Basalt flow
21
LB-2
Rhyolite? flow
22
SO 110
Rhyolite flow
23
SO 108
Rhyolite ignimbrite
24
SO 67
Rhyolite Ignimbrite
3144.50N
556.55E
3148N
562E
3148.18N
562.15E
3148.95N
562.95E
3161.35N
573.65E
25
26
27
28
SO 61
Rhyolite? flow
SO 69
Rhyolite flow
SO 70
Rhyolite ignimbrite
SO 9
Rhyolite ignimbrite
3139.95N
559.35E
3177.40N
545.25E
3186.80N
539.25E
3173.45N
574.15E
29
SO 111
3173.08N
Rhyolite ignimbrite 574.49E
Amphibole-bearing lava flow
30
SO 68
3181.70N
541.05E
K
(%)
40*Ar
(%)
40*Ar × 10–6
(scc/gm)
Age† ± 1s
(Ma)
2.517
2.569
0.2015
0.1983
67
66
13
20
1.449
1.406
0.1357
0.1326
14.4 ± 0.3
1.614
1.609
0.1607
0.1561
1.501
1.482
1.669
1.680
69
67
16
17
77
64
40
52
1.058
1.090
0.1731
0.1706
1.320
1.345
1.174
1.226
6.882
6.776
0.6349
0.6360
0.6282
0.6953
0.6697
0.6634
66
69
55
54
52
56
6.345
6.490
0.5646
0.5479
0.5962
0.6129
Whole rock
0.3292
Plagioclase
0.6615
0.6575
0.4375
0.4371
5.546
5.497
4.094
3.680
4.116
7.198
7.176
4.558
4.532
3.345
3.263
4.502
4.498
25
30
41
32
21
24
80
82
11
14
0.1096
0.1175
0.2927
0.3001
0.2167
0.2186
2.638
2.605
1.798
2.000
3.057
3.395
2.227
2.260
1.629
1.526
3.856
4.221
3.846
2.054
2.136
0.6140
0.6103
Mineral
Whole rock
Plagioclase
Whole rock
Plagioclase
Whole rock
Whole rock
Biotite
Plagioclase
Plagioclase
Plagioclase
Alkali feldspar
Alkali feldspar
Alkali feldspar
Alkali feldspar
4.269
4.235
80
85
65
71
13
13
86
84
81
60
64
Plagioclase
0.6520
0.6503
26
51
Alkali feldspar
Alkali feldspar
Alkali feldspar
17.2 ± 1.6
17.1 ± 0.4
27.7 ± 1.4
22.8 ± 0.5
18.4 ± 0.4
24.0 ± 0.4
22.4 ± 0.5
23.3 ± 1.1
8.9 ± 0.4
11.5 ± 0.3
12.8 ± 0.6
12.2 ± 0.3
12.3 ± 1.2
11.5 ± 0.9
12.7 ± 0.3
12.3 ± 0.6
22.6 ± 1.2
12.6 ± 0.4
24.0 ± 0.6
*Radiogenic argon (scc/gm= cm3 at STP per gram of sample).
†Decay constants: λ = 4.963 × 10-10yr-1; λ
-10 -1 40
-4
β
ε+ε′ = 0.581 × 10 yr ; K/K = 1.167 × 10 .
§Value not used in age calculation.
composition. Four separate intervals of basaltic andesite lava flows and
breccias, along with felsic tuff horizons, are interbedded with conglomerates, sandstones, and siltstones (Fig. 4A, column III). Felsic tuffs are more
prominent there than in the Arroyo el Obispo section. The Arroyo el Salto
section is tilted eastward at dips of 50¡ to 62¡ (Fig. 3), probably resulting
from local rotation.
To the southwest of La Dura (Figs. 3 and 4A, column V), a 20.5 Ma
amphibole-bearing intermediate-composition dome (sample 4) displays a
significant interrelationship with the clastic section. The outcrop can be
traced continuously from a massive lava dome into a brecciated carapace
with a steep frontal flank, and finally into a volcanic breccia from which a
tongue of reworked clasts extends as a lens into the sedimentary section.
The contemporaneity of the volcanic and sedimentary units is unmistakable
at this locality, and we suggest that such a relationship is generally accurate.
Another amphibole-bearing intermediate-composition lava dome from a
locality about 10 km to the northwest of Arroyo el Obispo has been dated as
25.4 Ma (sample 5; Fig. 2).
The upper part of the clastic section occupies the largest area of the Rio
Yaqui basin, extending about 16 km in a north-south direction (Fig. 3). In
the central part of the basin, the rocks form prominent mesas with general
tilts of 6¡ to 10¡ to the east. In general the tilt of the beds decreases gradually from the lower to the upper portions of the Neogene sections. Recent
Geological Society of America Bulletin, October 1997
1355
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MCDOWELL ET AL.
fluvial erosion by the Rio Yaqui has produced very steep cliffs and pillars
tens of meters high. Rock types include sandstone and poorly to wellcemented conglomerates, but no igneous rocks. There are excellent exposures near the town of T—nichi, in the northern part of the basin, and where
the bridge of Highway 16 crosses the Rio Yaqui (Fig. 3). Near the highway
bridge, Barrera-Moreno and Dom’nguez-Perla (1987) measured a thickness of about 150 m for a section consisting of sandstones in beds 0.5 m
thick, and pebble to cobble conglomerates, either as massive layers or as
beds from 0.5 to 10 m thick. The conglomerates are unsorted and polymictic, and include angular to subangular clasts of granitoid, basalt, andesite,
rhyolitic tuff, chert, and limestone in an arkosic matrix. The base of this
section is not exposed, and the thickness could be several hundred meters.
At Arroyo el Obispo, a 200-m-thick section of poorly consolidated thinbedded pebble to cobble conglomerate with some sandy horizons (Fig. 4A,
column I) is generally conformable on a 12.5 Ma ignimbrite horizon. In the
northern Rio Yaqui basin at Arroyo el Salto (Fig. 4A, column III), 40 m of
conglomerates tilted 6¡ to the east rest in angular unconformity on the older
clastic rocks, which dip 60¡E.
Suaqui Grande Area
Near Suaqui Grande extension-related deposits were examined at four
localities (Figs. 2 and 4B). The deposits unconformably overlie Upper
Cretaceous volcanic and volcaniclastic rocks, granitoids, or middle Tertiary volcanic rocks. Thickness ranges from about 75 m at Arroyo Molina
(Fig. 4B, column III) to greater than 250 m at Arroyo Taraicito (column
IV), and as much as 300 m at Cerro Vigia (column I). The basal units are
either sedimentary rocks or basaltic andesite lava flows. The lava flows
are absent from the section at Cerro Vigia. The clastic rocks generally dip
toward the northeast from 40¡ at Cerro Vigia (column I) to 65¡ at Rancho
el Pozo (column II). All of the sections examined are capped by a considerable thickness (as much as 200 m) of rhyolitic ignimbrites and lava
flows. These units generally dip more gently to the east and are in angular
unconformity with the units beneath them. At Rancho el Pozo (column II),
however, the relationship is conformable. Other significant outcrops of
the clastic sedimentary rocks are present 12 km south of Suaqui Grande,
along the main arroyo. These were not examined in detail.
Four K-Ar ages were obtained for volcanic rocks associated with these
deposits (Table 1, samples 9Ð12; Fig. 4B). A mafic flow 7 km northeast of
Suaqui Grande is dated as 23.5 Ma (sample 11, Fig. 2). Although no full
section was examined at this locality, the sample is from near the base of a
clastic section. A sample of amphibole-bearing intermediate-composition
lava from a dome at Cerro Vigia (Fig. 4B, column I), 4 km to the north of
Suaqui Grande is 25.3 Ma (sample 12; using the more precise amphibole
age). To the west of Suaqui Grande, at Rancho el Pozo (Fig. 4B; column II),
two units of the overlying rhyolitic section were dated: the lower unit is 12.3
Ma (sample 10) and the upper is 12.0 Ma (sample 9).
Localities to the East of the Rio Yaqui Basin
On his reconnaissance map, King (1939) showed major exposures of
Tertiary continental clastic deposits in areas to the east of the Rio Yaqui
basin, extending into the main part of the Sierra Madre Occidental volcanic
field (Fig. 2 and 4C). He considered these deposits to belong to the B‡ucarit
Formation. Some of these areas were described by Bockoven (1980), by
CochemŽ and Demant (1991), and Gans (1997), and were examined by us
only briefly. In the Sierra Madre, these clastic deposits have a tuffaceous
matrix, and angular clasts of various volcanic rocks. We suspect that in
some cases these may be caldera fill rather than related to extension. K-Ar
1356
and Ar-Ar ages for volcanic rocks in close stratigraphic proximity to these
clastic layers have been published by Bockoven (1980), CochemŽ and
Demant (1991), Bartolini et al. (1994), and Gans (1997). Due to the lack of
capping felsic rocks from areas to the east of the Rio Yaqui basin, opportunities to obtain younger time limits to deposition of these sedimentary rocks
are limited (Fig. 4C).
At Tep—ca, 25 km to the east of the Rio Yaqui, CochemŽ and Demant
(1991) described a 250-m-thick section of locally derived flat-lying andesitic lava flows that overlies approximately 100 m of clastic sedimentary
rocks that dip about 15¡ to the west (Fig. 4C, column I). Additional mafic
volcanic rocks underlie the clastic sedimentary rocks in this area. The sedimentary section contains a minor amount of interbedded mafic lava flows
and reworked tuffs, and is capped by a thin white tuff layer. For the overlying volcanic rocks, Bartolini et al. (1994) cited a whole-rock K-Ar age of
14.0 Ma and a hornblende age from an andesite porphyry of 17.2 Ma, for
samples taken Òto the northwest of Tep—ca,Ó and Gans (1997) gave Ar-Ar
ages of 16.7 and 17.4 Ma for samples near Tep—ca (Fig. 4C; column I, ages
in brackets). Our whole-rock age for one of the flows of this sequence (sample 13) is 14.4 Ma, whereas the plagioclase phenocrysts give 17.2 Ma. We
regard the whole-rock age as more reliable owing to its higher potassium
content (Table 1). From approximately 15 km to the northeast of Tep—ca,
Gans (1997) obtained three whole-rock Ar-Ar ages (22.6, 26.4, and 26.4
Ma) for lava flows that underlie clastic sedimentary deposits. In this area,
the entire section dips about 55¡ to the northeast.
Farther to the east there are no lava flows above the conglomerate layers.
In the YŽcora valley (Fig. 4C, column II), gently dipping to flat-lying conglomerates are the youngest rocks exposed, except for minor young fluvial
deposits (CochemŽ and Demant, 1991; Bockoven, 1980; McDowell, 1993).
The Neogene section lies in angular unconformity on a westward-dipping
section of basaltic andesite lava flows interlayered with rhyolitic flows and
tuffs. To the west of the YŽcora valley, mafic lava flows that underlie clastic
sedimentary rocks have whole-rock ages of 27.0 Ma (Bartolini et al., 1994)
and 22.8 Ma (sample 16). Along the eastern valley margin, the underlying
volcanic rocks have ages of 18.4 Ma and 23.2 Ma (mean of biotite and
plagioclase ages) (Fig. 4C, column II; samples 17 and 18). Note that preliminary age results for samples 17, 18, and 19 were given in Bockoven (1980).
The easternmost locality of clastic sedimentary deposits is at Arroyo los
Pilares (Figs. 2 and 4C, column III), about 15 km to the east of YŽcora
(Fig. 2). The section is gently dipping, has a minimum thickness of a few
tens of meters, and includes no mafic volcanic rocks. The Neogene section
is generally conformable with underlying basaltic andesite lava flows, and
is interbedded with rhyolite lava flows and tuffs. One K-Ar age for a rhyolite tuff is 23.3 Ma (sample 19).
During reconnaissance into unmapped areas to the south of Highway 16,
southwest of the town of San Nicol‡s (Fig. 2), we noted extensive exposures
of coarse clastic rocks interlayered with mafic lava flows. The total thickness of these sections is unknown. Just to the south of San Nicol‡s, these
sedimentary rocks were deposited on plutons of the Sonoran batholith and
on a felsic tuff with a K-Ar age of 54 Ma. About 13 km due south of Tep—ca,
a whole-rock age of 17.1 Ma was obtained for a basaltic andesite lava flow
(sample 14) (Fig. 2). Gans (1997) determined a whole-rock Ar-Ar age of
18.0 Ma from the same locality. He placed this flow within the clastic sedimentary section. Bartolini et al. (1994) gave groundmass K-Ar ages of 19.5
and 16.3 Ma for mafic lava flows from nearby localities. Gans (1997)
determined ages of 22.3 and 26.1 Ma for mafic lava flows a few kilometers
to the east, at or near the base of the sedimentary section. About 14 km farther to the south, plagioclase separated from a mafic lava of unknown stratigraphic position gave a plagioclase K-Ar age of 27.7 Ma (sample 15).
Thirty-five km to the south of Tep—ca, near the town of Nuri, an estimated
Geological Society of America Bulletin, October 1997
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TERTIARY EXTENSION IN SONORA, MEXICO
minimum thickness of 2000 m of coarse conglomerates are interbedded
with abundant mafic lava flows.
Western Localities
To the west of Suaqui Grande (Fig. 2), basin and range structure is less
well exposed, in part due to cover by younger clastic deposits. Exposures of
Neogene conglomerates are generally thinner and lack interbedded volcanic
rocks. The sections dip gently (up to 20¡) to the west. No mafic lava flows
have been noted intercalated within clastic sedimentary rocks from any
areas to the west of the Suaqui Grande area. However, at many localities the
base of the section is not exposed. Where the lower contact is visible, the
conglomerates lie directly and unconformably on Late CretaceousÐearly
Tertiary granitoids or on Mesozoic and Paleozoic sedimentary units. The
distinction of the Neogene rocks from younger clastic deposits is based
primarily on the observation that they are capped conformably by felsic
volcanic rocks.
Six sections have been examined in this portion of the study area (Figs. 2
and 4D). At Cerro Lista Blanca (Figs. 2 and 4D, column I), Bartolini et al.
(1991) reported a thickness of more than 100 m for clastic sedimentary
rocks, but elsewhere the exposed thickness is significantly less. The clastic
sections are everywhere overlain conformably by an interval of rhyolitic
ignimbrites and lava flows that is about 50 m thick at Cerro Lista Blanca
(Amaya-Mart’nez et al., 1992).
There is only one K-Ar age from this western area that provides an
older time limit for deposition of clastic sediments. A porphyritic lava
flow-breccia sampled 4 km to the south of La Colorada (Fig. 2; sample
30) is 24.0 Ma. This brecciated flow underlies clastic sedimentary rocks
and is petrographically similar to the amphibole-bearing intermediate
composition rock that forms prominent domes and lava flows in the Rio
Yaqui basin and Suaqui Grande areas. It is unknown whether additional
sedimentary layers are present beneath the flow at this locality.
The remaining K-Ar ages from the western localities are for rocks of the
overlying rhyolitic volcanic section, and thus provide younger limits to the
time of clastic sedimentation. These ages are very uniform throughout the
western area. At Cerro Lista Blanca (Fig. 4D; column I), we obtained ages for
the lowest (12.2 Ma; sample 23) and highest (12.8 Ma; sample 22) units in the
rhyolitic section. Although the K-Ar ages do not agree with stratigraphic
order, their analytical uncertainties overlap (Table 1). A third age of 11.5 ± 0.3
Ma (sample 21) is for a sample lithologically similar to the youngest unit, but
of unknown stratigraphic position. Bartolini et al. (1994) published a younger
K-Ar age of 10.4 ± 0.4 Ma from the Cerro Lista Blanca section.
At Pozo de Leyva (Fig. 4D, column IV), 10 km to the west of Cerro
Lista Blanca, a capping basaltic lava flow is 8.9 Ma (sample 20). This
flow conformably overlies the westernmost documented exposure of
Neogene clastic deposits. From the thick section of rhyolites overlying
clastic sediments in the northern Sierra el Bacatete (Fig. 4D, column V),
sample 25 is 11.5 Ma. Northwest of the town of La Colorada (Figs. 2 and
4D; column II) a rhyolitic ignimbrite that overlies the sedimentary rocks
is 12.3 Ma (sample 27). A published K-Ar age from the same general
location is 11.6 Ma (Bartolini et al., 1994). At Rancho el Churi (Fig. 4D,
column III), about 5 km south of La Colorada, a 20-m-thick rhyolitic ignimbrite is 12.7 Ma (sample 26) and is conformably above an 80-m-thick
section of clastic sedimentary rocks.
Capping felsic rocks have been sampled for dating at scattered localities
between Suaqui Grande and Cerro Lista Blanca (Figs. 2 and 4D). Some of
these are small outcrops that overlie clastic sedimentary rocks. At one location, an ignimbrite gave an age of 12.3 Ma (Fig. 2; sample 24). At prominent ignimbrite exposures along Highway 16, one sample gave an age of
22.6 Ma (Fig. 2; sample 28), a value that is probably too old. A resampling
of the same unit from a nearby location gave a more reasonable age of 12.6
Ma (sample 29). We have no explanation for the older age, and believe it to
be spurious. Bartolini et al. (1994) reported a K-Ar feldspar age of 12.8 Ma
from a nearby locality.
GEOCHEMISTRY
Six new major-element analyses have been obtained for samples of mafic
lava flows from within the study area. K-Ar ages for the analyzed flows
range from 8.9 to 27.7 Ma. No analyses are available for the amphibolebearing lava flows and domes of intermediate composition that are associated with the lower parts of the clastic sections. Table 2 contains the original oxide values prior to any normalizations for volatiles and adjustments
for iron oxidation. The analyses were recalculated volatile free and adjusted
for Fe+3/Fe+2 = 0.15 for plotting in Figures 5 and 6. On a plot of total alkalies against silica (Fig. 5), these compositions (solid squares) are basaltic andesite to andesite, with one exception (most mafic solid circle), which is
basalt. Using the classification of Le Maitre (1989), the compositions straddle the boundaries between subalkaline (basaltic andesite and andesite) and
alkaline (trachybasaltic andesite and trachyandesite) fields.
In addition to our single analyses from the Tep—ca area, CochemŽ and
Demant (1991) published seven major-element analyses for lava flows from
the Tep—ca section. All of these flows clearly overlie a significant thickness
of clastic sedimentary rocks. These analyses are plotted as open squares in
Figures 5 and 6. All of the analyzed Tep—ca flows are andesitic in composition. Also plotted in Figures 5 and 6 are major-element analyses for 12 samples of mafic lava flows that occur above rhyolitic volcanic units of the Sierra
Madre Occidental volcanic field from near YŽcora (Bockoven, 1980Ñfilled
diamonds; CochemŽ and Demant, 1991Ñopen diamonds). These flows underlie clastic sedimentary rocks. Both on the plot of total alkalies-silica and
on a plot of MgO against SiO2 (Fig. 6), it is apparent that there is complete
overlap of compositions for mafic-intermediate lava flows that cap felsic sections in the Sierra Madre field, and those that underlie or are interbedded with
the oldest clastic sediments in our study area. In contrast, there is a distinct
difference between these compositions and those of Neogene mafic lava
flows from volcanic sequences in western Sonora, adjacent to the Gulf of
California. Those analyses have been summarized by Mora-Alvarez (1992)
and are plotted as solid circles in Figures 5 and 6. The following section provides further discussion, and extends these comparisons to include regional
suites of analyzed mafic-intermediate igneous rocks that are plotted as fields
in Figure 5.
TABLE 2. WHOLE-ROCK CHEMICAL ANALYSES
SO 60
SO 28
SO 48
Location: Pozo Leyva Suaqui Grande Rio Yaqui
8.9 Ma
23.5 Ma
27.5 Ma
Age:
SO 14
Tepoca
14.4 Ma
SO 40
SO 41
Nuri
Nuri
17.1 Ma 27.7 Ma
47.79
57.41
50.68
60.17
53.30
55.08
SiO2
2.10
0.74
1.58
0.76
1.29
1.09
TiO2
15.75
17.08
16.88
16.76
16.96
17.11
Al2O3
6.50
5.69
8.91
2.47
4.88
6.81
Fe2O3
FeO
4.96
0.40
0.83
2.35
2.95
0.50
MnO
0.19
0.09
0.14
0.08
0.13
0.12
MgO
6.85
2.99
4.16
2.18
4.60
3.38
CaO
10.04
6.44
8.24
5.08
7.30
6.51
3.30
3.80
3.50
3.57
3.67
3.58
Na2O
0.37
2.04
1.56
2.97
1.96
2.26
K2O
0.32
0.26
0.99
0.27
0.53
0.42
P2O5
0.50
0.75
0.92
0.32
0.59
0.90
H201.28
0.91
0.92
1.33
0.76
0.98
H2O+
0.15
0.02
0.04
0.07
0.03
0.02
CO2
TOTAL
100.10
98.62
99.35
98.38
98.95
98.76
Notes: All analyses in percent. SO 60 = Olivine basalt; SO 28 = Fine-grained andesite; SO 48 = Olivine basaltic andesite; SO 14 = Pyroxene andesite; SO 40 =
Olivine-pyroxene basaltic andesite; SO 41 = Olivine-pyroxene basaltic andesite.
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MCDOWELL ET AL.
DISCUSSION
10
TA
8
%K2O+Na2O
TBA
TB
6
4
BA
B
A
2
0
45
49
53
61
57
65
%SiO2
Figure 5. Plot of total alkalies against silica, after International Union
of Geological Sciences (IUGS) (Le Maitre, 1989) for mafic and intermediate composition Neogene volcanic rocks from south-central Sonora.
Labeled fields are: BÑbasalt; BAÑbasaltic andesite; AÑandesite;
TBÑtrachybasalt; TBAÑtrachybasaltic andesite; TAÑtrachyandesite. Solid symbols are used for analyses from this report (Table 2),
Bockoven (1980), and Mora-Alvarez (1992). Open symbols are data
from CochemŽ and Demant (1991). Squares represent volcanic rocks
associated with clastic sedimentary sections, diamonds are used for capping mafic lavas in the western Sierra Madre Occidental volcanic field,
and dots are samples from the Circum-Gulf volcanic province (see text).
Various lines enclose data from surrounding regions discussed in the
text as follows: solid lineÑmid-Tertiary (20-30 Ma) mafic and intermediate rocks from the Mogollon-Datil field of southwest New Mexico
(Davis and Hawkesworth, 1993); dash-dot lineÑSCORBA (Cameron
et al., 1989); dotted lineÑNeogene (< 20 Ma) mafic rocks from the
Mogollon-Datil field of southwest New Mexico (Davis and
Hawkesworth, 1995); and dashed lineÑalkalic basalts related to extensional faulting in the Nazas area, Durango (Aguirre-D’az and McDowell, 1993), and Trans-Pecos Texas (Henry and Price, 1986).
8
%MgO
6
4
2
0
45
50
55
60
65
%SiO2
Figure 6. Plot of MgO against SiO2. Sources of data and use of symbols
are the same as for Figure 5.
1358
The association of volcanic strata with coarse clastic deposits that accumulated in developing extensional basins provides a means to date the
progress of extension in south-central Sonora. In the Rio Yaqui basin, where
the thickest and most complete sections of these deposits are found, they are
punctuated by the appearance of prominent rhyolite ignimbrites. These rhyolites are dated as 12.5 Ma, and they cover the coarsest and thickest lower
two-thirds of the clastic strata. Although most of the basin fill is thus older
than 12.5 Ma, the presence of additional clastic material above the ignimbrites and the 25¡ E tilt of the upper section demonstrate that basin-andrange style brittle extension continued in the late Miocene. In the northern
Rio Yaqui basin, where the rhyolite strata are absent, there is a distinct unconformity within the clastic section at about the same stratigraphic level. It
is not clear whether these observations reflect two discrete episodes or continuous extension.
In areas to the west of the Rio Yaqui basin, the rhyolites become thicker
and individual units more numerous. Our K-Ar ages for these volcanic
rocks have a narrow range, from 12.8 to 11.5 Ma for 11 samples taken
over an area of 100 × 50 km. Three published K-Ar ages (Bartolini et al.,
1994) from within the same area are generally consistent, but have a
longer range, from 12.8 to 10.4 Ma. For the thickest section of these rocks
at Cerro Lista Blanca, available K-Ar ages range from 12.8 to 10.4 Ma.
These results indicate that deposition of the greatest thickness of clastic
sediments associated with extension was completed in the region before
12.8 Ma, but that some deposition and tilting continued to occur, at least
in some areas. Neither the K-Ar ages nor the continuity of outcrop allow
specific correlation of these rhyolitic units across the study area. Highprecision Ar-Ar dating, perhaps along with systematic paleomagnetic
measurements, will be needed to fully exploit the potential of these rocks
as time and stratigraphic markers.
In western Sonora, to the west of the area shown in Figure 2, volcanic
rocks equivalent in stratigraphic position and age become thicker and more
diverse. In contrast, clastic sedimentary deposits are not evident to the west,
although in many places the base of the volcanic section is not exposed. In
western Sonora, McDowell et al. (1993) assigned volcanic rocks of this
general age and character to the circum-Gulf volcanic province. Within the
Sierra Santa Ursula and surrounding ranges (Fig. 1), an age range of 23 to
11.5 Ma has been established for the felsic volcanism of this province
(Mora-Alvarez, 1993).
The area of Tep—ca, about 40 km to the west of YŽcora, provides the
easternmost locality where volcanic rocks overlie extension-related clastic
sedimentary strata (CochemŽ and Demant, 1991). These rocks, andesite
rather than rhyolite in composition, have K-Ar and Ar-Ar ages between 17.5
and 14.5 Ma (CochemŽ and Demant, 1991; Bartolini et al., 1994; Gans,
1997; Table 1). Deposition of clastic sedimentary rocks was completed in
the Tep—ca area before about 17 Ma.
Dating of volcanic rocks associated with the lower parts of the clastic
sections indicates when extension began in south-central Sonora. One complication arises from the ambiguity in stratigraphic position and in composition (see the following) between basaltic andesite lava flows that directly
underlie all clastic strata and those that cap the rhyolitic sections of the
Sierra Madre Occidental volcanic field. Where such lava flows are directly
and conformably overlain by sedimentary strata, and where the overlying
section contains interbedded lavas of similar aspect, we have assigned them
to the clastic section. Our K-Ar dating of these rocks indicates that formation of structural basins and accumulation of coarse clastic sediments began
locally as early as 27 Ma (Fig. 4). The distribution of these ages, however,
suggests that each basin had a distinct history of development. We find no
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TERTIARY EXTENSION IN SONORA, MEXICO
systematic geographic pattern to that development.
The most comprehensive age controls are from the southern portion of the
Rio Yaqui basin. There, distal units from the Sierra Madre Occidental volcanic
field that underlie clastic sediments have ages of 33 and 27 Ma. At Arroyo las
Palomas (Fig. 4A; column II) a basal mafic lava flow that we have assigned
to the sedimentary section is dated as 27 Ma. Thus, the beginning of basin
evolution at this locality is possibly 27 Ma. Elsewhere in the basin, mafic
flows in a similar stratigraphic position are dated as 20 Ma. Further dating is
warranted for the northern section at Arroyo el Salto (Fig. 4A; column III),
where numerous volcanic layers bounding and within the clastic section have
been documented. To the west near Suaqui Grande a mafic lava flow, probably at the base of the clastic section, is dated as 23.5 Ma.
From areas to the south of Tep—ca, available K-Ar and Ar-Ar ages for
mafic flows are between 16 and 28 Ma (Table 1; Bartolini et al., 1994; Gans,
1997). Apparently, flows 19 Ma and younger are intercalated within clastic
sedimentary sections, whereas those 22 Ma and older are in a basal or nearbasal position. East of Tep—ca, extensional basins were superposed on the
western margin of the Sierra Madre Occidental volcanic field, and only
older limits to the beginning of extension can be dated. The youngest volcanism in this part of the Sierra is generally about 24 Ma, somewhat
younger than that within other parts of the field (McDowell et al., 1990).
East of YŽcora (Fig. 4C; column II), this volcanism is characterized by an
intercalation of felsic ignimbrites and lava flows with massive lava flows of
basaltic andesite that are as young as 18 Ma (Bockoven, 1980; CochemŽ
and Demant, 1991; McDowell, 1993). At Arroyo los Pilares (Fig. 4C;
column III), a thin section of clastic sedimentary rocks overlies all volcanic
rocks. Although no dated locality is nearby, a general older limit for deposition of these sediments is 24 Ma.
Another opportunity to establish timing for the beginning of extension is
from dating of restricted but massive amphibole-bearing domes and lava
flows of intermediate composition that are closely related to faulting at the
margins of the basins. These occur as discontinuous masses along or near
the margins where the greatest thicknesses of clastic sedimentary rocks are
found. Thus, they appear to mark the active side of half grabens. K-Ar ages
from three localities are 25.3, 25.4, and 20.5 Ma. Near La Colorada, in the
western portion of the area shown in Figure 2, a similar, intermediatecomposition flow breccia is 24.0 Ma.
Limited age results from northern Sonora are consistent with the timing
we have determined for deposition of clastic sediments. Bartolini et al.
(1994) showed that volcanic layers that predate sedimentary deposits are as
young as 24 Ma. Most of their dated volcanic units intercalated with sedimentary layers are 21 to 23 Ma, but one age as young as 13 Ma was quoted.
Their ages for overlying volcanic rocks are much younger at 5.5 and 7 Ma.
Bartolini et al. (1994) also summarized K-Ar ages for volcanic rocks from
coastal Sonora, including many published by Gastil and Krummenacher
(1977). Most of those rocks appear to belong to the Miocene convergentmargin arc that existed in the Gulf region from 24 to about 11.5 Ma (Stock
and Hodges, 1989; Sawlan, 1991; Mart’n-Barajas et al., 1994). No continental clastic deposits are known to be stratigraphically associated with
these rocks. Neogene sedimentary rocks on Isla Tibur—n (Fig. 1; Neuhaus et
al., 1988) are partly marine and related to the initial opening of the Gulf of
California rift system.
Neither ages nor major-element compositions provide a basis for distinguishing basaltic andesite lava flows that occur within or near the base of
the clastic sequences from those that occur as capping lava flows in the
Sierra Madre Occidental volcanic field. On a diagram of total alkalies versus SiO2 (Fig. 5), these compositions completely overlap. Also shown in
Figure 5 are fields that encompass analyses for 30Ð20 Ma mafic to intermediate lavas of the Mogollon-Datil volcanic field in southwestern New
Mexico (Davis and Hawkesworth, 1993), and for capping basaltic andesite
lava flows (Southern Cordilleran basaltic andesites [SCORBA]) from a regional study across the Sierra Madre Occidental volcanic field (Cameron et
al., 1989). It is evident that the compositions of all of these groups are similar with respect to alkali and silica contents. The only significant difference
might be that the SCORBA suite occupies a more restricted compositional
range. Wark (1991) showed that, at the T—mochic Volcanic Center in the
central portion of the Sierra Madre Occidental, capping basaltic andesite
lavas equivalent to SCORBA have chemical and isotopic compositions that
support a genetic relationship to the underlying felsic volcanic section, and
that they therefore probably represent the final stage of volcanism in the
Sierra Madre Occidental volcanic field.
Another group of analyzed rocks plotted in Figures 5 and 6 are of mafic
lava flows associated with thick volcanic sequences adjacent to the Gulf of
California (the Circum-Gulf volcanic province of McDowell et al., 1993).
These data were summarized and discussed by Mora-Alvarez (1993). The
stratigraphic setting of the Circum-Gulf lavas is similar to that for mafic
lavas in the Sierra Madre Occidental volcanic field, that is, they occur
directly above felsic lavas and tuffs. However, they are significantly
younger, having K-Ar ages from 11 to 8.5 Ma (Mora-Alvarez, 1993). As is
clear from Figures 5 and 6, they have lower concentrations of alkalies and
are higher in MgO at equivalent SiO2 content. One analysis reported in
Table 2 (sample SO 60) is from the westernmost locality described in this
study (Pozo de Leyva, Fig. 4D, column IV). It is a true basalt, unlike the
basaltic andesites of the remaining analyzed samples. We place the Pozo de
Leyva lava flow within the Circum-Gulf group, because of its similarity in
age, although it has lower silica content than others of that group (Fig. 5).
Other analyses plotted as fields in Figure 5 include post-20 Ma mafic
rocks from the Datil-Mogollon province (Davis and Hawkesworth, 1995),
and 20Ð24 Ma mafic lava flows from Trans-Pecos Texas (Henry and Price,
1986), and from Nazas, Durango (Aguirre-D’az and McDowell, 1993). The
latter two groups are clearly associated with brittle extension in their
respective areas. They are alkali basalts and tend to have higher alkali contents than the Neogene basaltic andesites from Sonora (Fig. 5).
CONCLUSIONS
(1) The early stage of Tertiary extension in south-central Sonora is
marked by deposition of clastic sediments in discontinuous linear grabens
and half-grabens. These basins are particularly well exposed throughout the
eastern two-thirds of the state. In their easternmost occurrence these tectonic
basins were superimposed on the western margin of the Sierra Madre Occidental volcanic field.
(2) Potassium-argon dates of mafic flows conformably beneath the sedimentary column are as old as 27 Ma, and may reflect the earliest time of
Tertiary extension in south-central Sonora. Dated mafic flows within the
basal portion of the clastic sedimentary section, as well as amphibolebearing intermediate-composition domes and lava flows that occur along
the basin margins, indicate that extension was underway in most basins by
24 Ma and across the entire area by 20 Ma. This timing is generally coincident with the development of metamorphic core complexes in northern
Sonora and parts of Arizona, Nevada, and Utah (Nourse et al., 1994, and
references cited therein). It also coincides with the development of a 24 to
11.5 Ma magmatic arc in western Sonora and Baja California, in the area of
the present-day Gulf of California. Early extension in central Sonora was
geometrically in a back-arc position.
(3) The massive lower portions of the clastic sedimentary sections typically are from 100 to 500 m thick. Throughout much of the region these sections are capped, generally conformably, by felsic volcanic rocks having KAr ages of 12.8 to 11.5 Ma. This narrow age range provides a convenient
time and stratigraphic marker within the period of extension. It does not nec-
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MCDOWELL ET AL.
essarily indicate that there were two discrete periods of extension. Continued
sedimentation and rotation of beds younger than 11.5 Ma show that extension continued in late Miocene time. During this time extension was coeval
with initial evolution of the Gulf of California extensional province.
(4) The composition of mafic-intermediate lava flows at and near the
base of the clastic sedimentary sections is similar to that of the capping
basaltic andesite lava flows from the Sierra Madre Occidental volcanic
field. This resemblance includes the regional suite known as the Southern
Cordilleran basaltic andesites (SCORBA), and mafic to intermediate lava
flows dated as 20 to 30 Ma from the Datil-Mogollon volcanic field of southwestern New Mexico. All of these compositions are distinct from those of
11 to 8.5 Ma capping mafic lavas in the Gulf of California extensional
province, and mafic lava flows younger than 20 Ma from the Datil-Mogollon field. They are also compositionally distinct from 20 to 24 Ma alkali
basalts both from Trans-Pecos Texas and from the Nazas area of northern
Durango. The Texas and Nazas rocks are true alkali basalts (Fig. 5) that are
clearly associated with extensional fault systems.
ACKNOWLEDGMENTS
This study was supported by National Science Foundation grants EAR9204635 and EAR-8720380 to McDowell, and by CONACYT Project 1204T9203 to Rold‡n-Quintana and Amaya-Mart’nez. Sample LB-2 was collected
by Edward Erlich, and samples 3 20 4, 3 21 2, and 3 21 3 were collected by
Neil Bockoven. Gabriela Mora-Alvarez, Gerardo Aguirre-D’az, Andrew
Bowen, and Tony Faucett helped with mineral separations and with the K-Ar
analyses. An earlier version of this manuscript was critiqued thoroughly by
Christopher Henry and John Stewart. Bulletin editor Myron Best and
reviewers John Dilles and Phillip Gans provided careful and thoughtful reviews that guided the final revisions of this manuscript, as did Chris Henry in
an additional review. Gans also provided a preprint of his manuscript in press.
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MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 1, 1996
REVISED MANUSCRIPT RECEIVED MARCH 5, 1997
MANUSCRIPT ACCEPTED APRIL 2, 1997
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Geological Society of America Bulletin, October 1997