Download Geologic Map of the Prisor Hill Quadrangle, Sierra

Geologic Map of the Prisor Hill Quadrangle,
Sierra County, New Mexico
By
William Seager
March, 2005
New Mexico Bureau of Geology and Mineral Resources
Open-file Digital Geologic Map OF-GM 114
Scale 1:24,000
This work was supported by the U.S. Geological Survey, National Cooperative Geologic
Mapping Program (STATEMAP) under USGS Cooperative Agreement and the New
Mexico Bureau of Geology and Mineral Resources.
New Mexico Bureau of Geology and Mineral Resources
801 Leroy Place, Socorro, New Mexico, 87801-4796
The views and conclusions contained in this document are those of the author and
should not be interpreted as necessarily representing the official policies,
either expressed or implied, of the U.S. Government or the State of New Mexico.
Geologic Maps of the Upham Hills and Prisor Hill
Quadrangles, Sierra County, New Mexico
By
William R. Seager
June, 2005
New Mexico Bureau of Geology and Mineral Resources
Open-file Digital Geologic Map OF-GMs
113 and 114
Scale 1:24,000
This work was supported by the U.S. Geological Survey, National Cooperative Geologic
Mapping Program (STATEMAP) under USGS Cooperative Agreement and the New
Mexico Bureau of Geology and Mineral Resources.
New Mexico Bureau of Geology and Mineral Resources
801 Leroy Place, Socorro, New Mexico, 87801-4796
The views and conclusions contained in this document are those of the author and
should not be interpreted as necessarily representing the official policies,
either expressed or implied, of the U.S. Government or the State of New Mexico.
Geology of the Upham Hills and Prisor Hills quadrangles,
Sierra County, New Mexico
INTRODUCTION
The Prisor Hill and Upham Hills 7 ½ minute quadrangles are located in the south-central
part of the Jornada del Muerto, approximately 72 km north-northwest of Las Cruces and
45km) southeast of Truth or Consequences, New Mexico (Fig. 1). Access to the area is
limited to a few graded dirt roads, the most important of which is an occasionally
maintained road that joins Interstate 25 at the Upham interchange, then traverses the
Jornada del Muerto northward to New Mexico highway 51 at Engle. This road skirts the
western boundaries of both the Upham Hills and Prisor Hill quadrangles. Maintained
county roads branch from the Upham-Engle road at Aleman Draw and at Rincon Arroyo,
providing access to ranches in the Aleman Draw, Prisor Hill and Flat Lake areas. Entry into
Prisor Hill, Upham Hills, Point of Rocks Hills and the broad expanses of desert floor
between these uplands is furnished by a secondary system of ranch roads of variable
quality. All of the roads in the quadrangles can become impassable or nearly so following
heavy rains.
Figure 1— Location map, Prisor Hill and Upham Hills quadrangles.
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The two quadrangles occupy the central, topographically lowest part of the Jornada del
Muerto, an area near 4400-4600 ft elevation, where the distal fringes of east-sloping
piedmonts from the Caballo Mountains and west-sloping piedmonts of the San Andres
Mountains join. The piedmont slopes are basically bedrock pediments, and the alluvial fans,
eolian and arroyo deposits that mantle them comprise only a thin veneer of sediment. In this
regard, the central Jornada del Muerto is unlike any of the deep, sediment filled basins of
the Rio Grande rift. Located at the toes of the fans and pediment surfaces, Jornada Draw
(Fig. 2), a south-flowing, axial ephemeral stream, delivers runoff from the western
piedmont slopes of the San Andres Mountains and east-central slopes of the Caballo
Mountains to Flat Lake playa. At an elevation of 4,350 ft, the playa represents local base
level for the entire map area, except for the southwestern corner of the Upham Hills
quadrangle, where Rincon arroyo flows to the Rio Grande near Rincon, NM.
Figure 2— Jornada Draw crossing broad alluvial plain just north of Point of Rocks Hills. View looks
northward. Uvas Basaltic Andesite in foreground on Point of Rocks hill.
Three groups of hills and ridges surmount the vast desert surface of the Jornada del Muerto
in the map area: Prisor Hill, Upham Hills (Fig.3), and Point of Rocks Hills. None of the
hills stand much above 180m above the surrounding lowlands, and with few exceptions, are
somewhat rounded and subdued in their form, owing in part to the armor- like apron of
colluvium that mantles lower slopes, merging downward into small alluvial fans or
pediment veneers. All of the hills are a product of normal faulting, although in each case the
uplands are on the downthrown side of important normal faults. This rather unfamiliar
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relationship results from the superior durability and resistance to weathering of hangingwall rocks relative to footwall rocks. However, movement on a normal fault in the Point of
Rocks Hills has elevated one footwall block there to an elevation of 5,172 ft, the highest
point in the two quadrangles.
Figure 3— Upham Hills in middle distance with alluvial plain of Jornada Draw below. Uvas Basaltic
Andesite on northeasternmost Point of Rocks hill in foreground. View looks northeast.
The relatively flat, mostly undrained expanse of sand-covered desert south of Point of
Rocks is the La Mesa surface. Underlain by stage IV soil carbonate, the La Mesa surface
represents the constructional top of “ancestral Rio Grande” fluvial sands and gravel
deposited by the river when it flowed northeastward from the Hatch-Rincon area to the
central axis of the Jornada del Muerto, and then southward toward the eastern side of the
Dona Ana Mountains and to the Mesilla Valley. Above the western shore of Flat Lake
playa, the deposit is truncated by the Jornada Draw fault scarp, and locally within this
escarpment the ancient river deposits are exposed. The entire map area is nearly treeless;
only an isolated Juniper in upland areas offers a contrast to the vast stretches of desert
dominated by mesquite and creosote. A variety of grasses have developed on finer- grained
parts of distal alluvial fans, in and near modern drainageways, and on parts of the alluvial
plains adjacent to Jornada Draw. Other parts of the same alluvial plains, as well as much of
Flat Lake playa, are barren (Fig. 2). Few studies of the geology of the south- central part of
the Jornada del Muerto have been published. The earliest geologic maps by Darton (1928)
and Dane and Bachman (1965) reveal little detail. A more recent geologic map (125,000)
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by Seager et al. (1987) provides more stratigraphic information, but fails to identify the
important Jornada Draw fault zone, as well as certain surficial deposits. Geologic maps
(1:24,000) of the adjacent Alivio, Upham, and Cutter quadrangles are in press (Seager, in
press; Seager and Mack, in press,). Discussions of surficial deposits and Tertiary rock units
in “Geology of the Caballo Mountains” (Seager and Mack, 2003) were taken in part from
studies of these adjacent quadrangles; these discussions also apply to the geology of the
Prisor Hill and Upham Hills quadrangles.
I thank Greg Mack and Curtis Monger for their assistance in identifying soils and for
helpful discussions about the geology of the area. I am also grateful to J.R. Hennessey for
drafting the “Correlation of Units” chart, and to Barbara Nolen and John Kennedy for
obtaining photographs of the area for me. The New Mexico Bureau of Geology and Mineral
Resources, Peter Scholle, Director, provided funds to cover travel expenses for this project.
STRATIGRAPHY
Stratigraphic units exposed in the Prisor Hill and Upham Hills quadrangles can be divided
into 5 groups: early Tertiary “Laramide” basin fill; middle Tertiary volcanic rocks; early
Miocene paleocanyon fill; Plio-Pleistocene Camp Rice Formation; and late Pleistocene and
Holocene surficial deposits. Except for the thin ash-flow tuff units in the middle Tertiary
Bell Top Formation, complete sections of mapped rock units are not exposed in the study
area. Thicknesses described in the following sections, or shown on geologic cross-sections,
are taken from exposures in neighboring quadrangles or from data from the Exxon Prisor
Hill No 1 oil test, located a few km northeast of the Upham Hills quad (Fig.1).
Early Tertiary “Laramide” basin fill (Love Ranch Formation)
The Love Ranch Formation is the syn- to post-orogenic basin fill of the Laramide Love
Ranch basin (Kottlowski et al., 1956; Seager et al., 1997). A Paleocene and/or Eocene age
of the formation is indicated by its position between the McRae Formation, which contains
dinosaurs of latest Cretaceous age, and the overlying Palm Park Formation of late Eocene
age. A fining-upward sequence, the formation contains coarse-grained, alluvial fan deposits
in the lower part that grade upward into fluvial conglomerate and sandstone and finally into
fine-grained, alluvial-plain and playa deposits (Seager et al., 1997). Clasts record erosional
“unroofing” of Cretaceous volcanic rocks, Paleozoic limestone, and Precambrian granite
from the Rio Grande uplift, with which the basin is yoked. Thickness of the formation
varies according to tectonic setting, but may approach 1000m or more within the basin
adjacent to the Rio Grande uplift. In the Exxon Prisor well, located near the basin center or
on the distal basin flank, approximately 900m of fine-grained Love Ranch clastics were
penetrated, a thickness that is used for subsurface reconstructions in this paper.
Within the study area, most of the formation is covered by pediment gravels; judging
from the significant thickness, low dips, and repetition of the section by movement on the
Jornada Draw fault zone, the formation has a wide subcrop beneath surficial deposits in the
area north of Yost Draw. Only scattered exposures of the formation are present along and
adjacent to Aleman and Yost Draws and in the low escarpment just southwest of
Yost Draw. These outcrops probably represent no more than 250m of the upper part of the
formation and are interpreted to represent basin-center or distal basin-flank deposits. Love
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Ranch strata in the map area become finer grained upward. Stratigraphically lowest exposed
beds consist of inter-bedded tan or reddish-brown conglomerate and conglomeratic
sandstone, red sandstone and purple to red mudstone. Higher in the section, conglomerate
beds are almost entirely replaced by channels of red to tan sandstone, and the ratio of
mudstone to sandstone increases. In the stratigraphically highest and easternmost outcrops,
reddish mudstone prevails and sandstone beds are either thin or absent. In this setting, rare,
thin (1 m) pisolitic limestone beds occur within the mudstone.
Both conglomerate and sandstone beds are in the form of channels, typically a few meters
thick, traceable along strike for hundreds of meters before pinching out within mudstones.
Conglomerate and conglomeratic sandstone consists largely of well-rounded, grainsupported pebbles, and cobbles mixed with variable amounts of sand. Clasts include a
variety of Paleozoic limestone and sandstone, together with conspicuous Precambrian
granite and less abundant porphyries of intermediate composition that probably were
derived from Cretaceous volcanic rocks. Rarely in the study area, a conglomerate bed
consists of angular to sub angular, boulder-sized clasts supported by a matrix of finergrained sediment. Sandstones are mostly 1 to3m- thick beds of coarse to medium-grained,
pale red or tan sand, much of which is cross-bedded in sets up to 2m thick. Mudstone
deposits are bright red to purple and occasionally contain carbonate nodules and filaments,
typical of calcic soil horizons.
Most of the sandstone and conglomerate beds are fluvial in origin, an interpretation that is
consistent with their channel-form shape, the rounding and good sorting of clasts, and clastsupported texture. The occasional matrix-supported conglomerate, consisting of poorly
sorted, angular boulders, is probably the deposit of a debris flow, which occasionally was
spread from proximal alluvial fan positions into axial or other drainages dominated by
fluvial processes. Mudstones associated with channel-form sandstone and conglomerate
beds probably represent deposition on floodplains, some of which were abandoned for
sufficient lengths of time to develop stage II soil carbonate horizons. Mudstones at the top
of the section, which contain few or no sandstone beds, are interpreted to be alluvial plain
deposits; the limestone beds associated with them may have been precipitated by small,
spring-fed lakes or cienegas.
Middle Tertiary volcanic rocks
Palm Park Formation. The Palm Park Formation (Kelley and Silver, 1952; Seager and
Mack, 2003; McMillan, 2004) overlies the Love Ranch Formation conformably or,
perhaps, on a minor disconformity. Based on radiometric ages ranging from 46.3-37.6
Ma from the Palm Park and correlative formations, the Palm Park Formation is late
Eocene in age (McMillan, 2003). It crops out widely across south-central New Mexico,
where the formation averages approximately 600m in thickness. Consisting largely of
lahar deposits, with lesser volumes of intrusive rocks, lava and ash-flow tuff, all of
andesitic composition, the formation is considered to represent volcano slope and intravolcano lowland deposits associated with one or more andesitic stratovolcanoes. Local,
but conspicuous fresh-water limestone beds within the formation, including travertine
mounds, are interpreted to be spring deposits that fed local fresh-water ponds and
cienegas (Seager and Mack, 2003).
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Within the map area, the Palm Park Formation is mostly buried beneath thin piedmontslope gravels. Beneath these gravels, however, its subcrop, like that of the Love Ranch
Formation, is extensive, both because of low dips and the substantial thickness of the
formation. It certainly underlies much of the surficial deposits east and north of Prisor Hill
and Upham Hills, as well as those north of Point of Rocks Hills. Outcrops are restricted to
the southwestern side of Upham Hills and the northeastern corner of Point of Rocks Hills.
Light purple to bluish gray, tuffaceous breccia and conglomerate are poorly
exposed in both areas. Clasts range up to boulder size, are matrix supported and comprise a
suite of intermediate –composition porphyries containing hornblende and biotite; the
matrix consist of a poorly sorted mix of broken crystals, ash, and smaller clasts. All of the
outcrops appear to be lahar deposits.
Bell Top Formation. The Bell Top Formation (Kottlowski, 1953; Mack et al., 1994a;
Seager and Mack, 2003) conformably overlies the Palm Park Formation. Based on
Radiometric ages of ash-flow tuffs inter-bedded within the unit, the Bell Top Formation
is Oligocene in age, ranging from 35.7 to 28.6Ma. Regionally, the formation fills the
Goodsight-Cedar Hills half graben, a broad, shallow basin that extends 100km northward
from west of Las Cruces to the Caballo Mountains and Prisor Hill. Alluvial fan and
fluvial sediments, syneruption, tuffaceous sandstones, and ash-flow tuff outflow sheets
comprise the bulk of the basin fill, totaling approximately 450m thick near the basin
center in the Sierra de las Uvas (Mack et al., 1994a). The Bell Top Formation is exposed
in Prisor Hill, Upham Hills and in Point of Rocks Hills, but at each locality the quality of
outcrops is generally poor and no complete section is present. However, in adjacent
Alivio quadrangle, at the northwestern corner of Point of Rocks Hills, a complete section,
nearly 240m thick, is well exposed Seager and Mack, 2003), and is representative of
rocks in the study area. From bottom to top the section contains: basal ash-flow tuff 5,
sedimentary sequence with medial ash-flow tuff 6, and ash-flow tuff 7 at or near the top
of the formation.
The base of the Bell Top Formation is marked by a prominent ridge or cuesta-forming
ash-flow tuff, informally called ash-flow tuff 5 (Clemons and Seager, 1973). McIntosh et al.
(1991) report a Ar40/Ar39 age 34.8Ma for the tuff. Pumice and crystal rich, the light gray to
grayish brown ash-flow tuff contains broken fragments of sanidine, plagioclase, and
bipyramidal quartz, as well as biotite. It is rather densely welded and is a simple cooling
unit only 10 to 12 m thick in the map area. Locally a few meters of white, fallout tuff or
tuffaceous sandstone underlie tuff 5, separating it from the underlying Palm Park
Formation.
Above the basal tuff 5, the bulk of the Bell Top Formation consists of inter-bedded white to
tan, tuffaceous sandstone and inter-bedded conglomerate. Sandstones are thin to medium
bedded and contain a mixture of glass shards, pumice, quartz, sanidine, and biotite, together
with numerous lumps of pumice. Conglomerate beds consist of poorly sorted to moderately
sorted, rounded boulders and cobbles of Kneeling Nun ash-flow
tuff, intermediate-composition porphyries and some clasts of Paleozoic limestone.
Boulders approach 1m in diameter, especially in Prisor Hill outcrops. Both grainsupported and matrix-supported types of conglomerate and conglomeratic sandstone are
present.
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Ash-flow tuff 6 occurs near the middle of the sedimentary rock sequence just described. An
Ar40/Ar39 age of 33.6 Ma. was determined by McIntosh et al (1991). The tuff is a pale
pinkish-orange to grayish-red crystal ash-flow tuff, somewhat less welded compared to tuff
5, and contains fewer and smaller crystals. Broken sanidine, quartz, biotite, and plagioclase
crystals are set in a matrix of devitrified glass shards. Like tuff 5, ash-flow tuff 6 is a simple
cooling unit that weathers to a ridge or cuesta above the surrounding softer Bell Top rocks.
Ash-flow tuff 7 marks the top of the Bell Top Formation in many places, although locally
one or two flows of Uvas Basaltic Andesite are inter-bedded within Bell Top strata just
below tuff 7. McIntosh et al. (1991) report an 40Ar/39Ar age of 28.6Ma for the tuff. Within
the map area, tuff 7 is only a meter or less thick, has no notable outcrop, its presence noted
only by the occasional, but conspicuous, float fragments. The tuff weathers to grayish
brown and consists almost entirely of modestly welded glass shards and small pumice
fragments; the few crystals present are small and inconspicuous.
A similar Ar40/Ar39 age for tuff 5 and the Kneeling Nun Tuff of the Black Range area has
led McIntosh et al. (1991) to suggest that tuff 5 is the distal part of the Kneeling Nun Tuff
outflow sheet, erupted from the Emory cauldron in the Black Range. Similarly, these
authors correlate ash-flow tuff 7 with distal parts of the Vicks Peak Tuff, erupted from
the Nogal Canyon cauldron in the San Mateo Mountains. Apparently, these major outflow
sheets spread from their source cauldrons into the Goodsight-Cedar Hills Basin. The lightcolored, tuffaceous sandstones within the Bell top Formation are interpreted to be
“syneruption,” fallout tephra reworked by sedimentary processes on the distal parts of
alluvial aprons that surrounded such volcanoes as the Nogal Canyon or Emory cauldrons.
Parts of these aprons clearly extended into the subsiding or topographically low GoodsightCedar Hills Basin. Conglomerate and conglomeratic sandstone beds were deposited by
sheetflood or shallow stream-flow processes on these same volcanic aprons and/or on
alluvial fans adjacent to block faulted margins of the Goodsight-Cedar Hills half graben
(Mack et al, 1994a). Matrix-supported conglomerate probably represent debris flow
deposits on volcanic piedmont slopes or alluvial fans. Clasts of porphyritic igneous rocks
are similar to clasts in the McRae and basal Love Ranch Formation. These, and well
rounded and case hardened Paleozoic limestone clasts may be recycled from those older
formations, suggesting uplift of the older units at least locally around the margin of the
Goodsight-Cedar Hills Basin (Mack et al., 1994a).
Uvas Basaltic Andesite. Named by Kottlowski (1953), the Uvas Basaltic Andesite
conformably overlies the Bell Top Formation. Radiometric ages of 25,9 to 28 Ma establish
the formation as Oligocene in age (Clemons and Seager, 1973; Clemons, 1979; Seager and
Mack, 2003). Regionally, the formation, together with correlative units, forms a vast sheet
of flood basalts that once covered large parts of southwestern New Mexico and northern
Chihuahua (Cameron et al., 1989). Averaging 100 to 150m thick, the formation thickens
and thins modestly, probably in response to inter-fingering with both underlying and
overlying formations, or to widespread erosion of upper flows involved in faulting during
early stages of block faulting within the Rio Grande rift; locally, as in the central Caballo
Mountains, flows thin and pinch out within tuffaceous, syneruption sandstone beds of the
Bell Top and Thurman Formations (Seager and Mack, 2003).
The Uvas Basaltic Andesite is well exposed in Prisor Hill, Upham Hills, and especially
across all of the Point of Rocks Hills, where it is at least 160m thick, the thickest section of
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Uvas Basaltic Andesite flows known. These are also the easternmost outcrops of the
formation. Dark gray to black lavas, many of which are vesicular and/or amygdaloidal, are
conspicuous members of the formation. Other flows or flow interiors are massive and
dense; some exhibit platy jointing approximately parallel to flow tops. Locally, brown
sandstone and conglomeratic sandstone containing mostly basaltic grains and clasts is interbedded, but the numbers and thickness of such beds is in doubt owing to the effective cover
of colluvium across large parts of the formation. Opposing flanks of a cinder cone, at least
one kilometer in diameter, are preserved near the base of the formation in the central part of
Point of Rocks Hills (Fig. 4). Dikes and plugs of basaltic andesite near the cinder cone and
in the northwestern part of Point of Rocks Hills cut the Bell Top Formation. The dikes are
part of a northwest-trending system that extends into the Elephant Butte area and across the
Caballo Mountains. Some dikes can be traced upward into basal Uvas Basaltic Andesite
flows; one such dike yielded an Ar40/Ar39 age of 26.8Ma (Esser, 2003).
Figure 4— Partial eastern flank of Uvas Basaltic Andesite cinder cone, exposed in central Point of Rocks
Hills. Cinder beds strike southeast, dip approximately 25 degrees northeast. View looks southeast.
The thickness of basaltic andesite flows in Point of Rocks suggest that the basaltic plateau
extended much farther north and east than the limit of present outcrops might suggest.
Clearly some flows were the product of eruptions from northwest-trending fissures,
suggesting northeast-southwest directed extensional stresses prevailed in the region 26-28
Ma. Scattered cinder cones were also constructed. The cinder cone in the Point of Rocks
Hills formed early in the history of eruption, was largely, if not entirely buried by
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subsequent flows, then, during later uplift by block faulting, was breached by erosion and
exhumed, leaving a circular valley one kilometer in diameter in its place, surrounded by
high ridges or hills of basaltic andesite flows.
Early Miocene paleocanyon fill (Hayner Ranch Formation)
Unconformably overlying the Uvas Basaltic Andesite and Bell Top Formation are boulder
conglomerate beds assigned to the Hayner Ranch Formation (Seager and Hawley, 1971;
Mack et al., 1994b). The formation is considered to be latest Oligocene and early Miocene
in age because in the Caballo Mountains and Rio Grande valley area the formation
conformably overlies strata dated 27 Ma (Thurman Formation) and is beneath the 9.6 Ma
Selden Basalt (Seager and Mack, 2003). In the same region, the Hayner Ranch Formation
consists mostly of footwall alluvial fan deposits, as much as 1,300m thick, that document
early rise of fault blocks in the Rio Grande rift. Less commonly, paleocanyon fill on
hanging wall dip slopes, has also been assigned to the Hayner Ranch Formation (Mack et
al., 1994b; Seager and Mack, 2003). In the map area,rocks assigned to the Hayner Ranch
Formation unconformably overlie Uvas Basaltic Andesite in the Upham Hills and Point of
Rocks Hills, but are unconformable above both Uvas Basaltic Andesite and Bell Top
Formation at Prisor Hill. The unconformity appears to be deep and irregular and is
interpreted to represent paleovalleys cut into the Bell Top and Uvas rocks. The formation is
composed entirely of boulder/cobble conglomerate consisting of angular to sub-rounded
clasts of Uvas Basaltic Andesite and Bell Top ash- flow tuffs. Clasts range up to 3/4m in
length. Unfortunately, clasts are everywhere disaggregated from matrix, at least at the
surface, resulting in “outcrops” that are surficial lag deposits. That the formation is not
merely a modern surficial deposit is proven by the facts that it ranges up to 100m thick (top
not exposed), forms part of the summit of the Prisor Hill fault block, and contains clasts that
could not have been delivered by the Plio-Pleistocene and younger drainage systems.
The Hayner Ranch Formation is interpreted to be colluvial and alluvial fill of paleovalleys
that drained the eastern dip slope of incipient Caballo Mountain fault blocks during the late
Oligocene or early Miocene. At this time, valley sidewalls and/or headwater regions
exposed Uvas Basaltic Andesite, as well as Bell Top rock units. The absence of substantial
basin-fill deposits in the central Jornada del Muerto indicates that paleovalley drainage
probably turned southward, transporting sediment out of the southern Jornada del Muerto
area, perhaps to the deeply subsiding “early” rift basin in the San Diego Mountains area,
where Hayner Ranch and younger basin fill accumulated to 1,900m thick. Mack et al.
(1994b) have shown that parts of this basin fill was derived from the eastern Caballo
Mountains dip slope.
Camp Rice Formation
The Camp Rice Formation may overlie any older formation, usually on a conspicuous
angular unconformity. Named by Strain (1966), the formation has been the subject of
numerous subsequent studies (e.g. Hawley et al., 1969; Hawley and Kottlowski, 1969;
Mack and James, 1993; Mack et al., 1994c; Mack et al., 1997; Mack et al., 1998).
Radiometric ages, reversal magnetostratigraphy and vertebrate fauna indicate the
formation and the correlative Palomas Formation range in age from approximately 5Ma to
0.7Ma, Pliocene to middle Pleistocene (e.g. Lucas and Oakes, 1986; Repenning and May,
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1986; Bachman and Mehnert, 1978; Seager et al., 1984; Mack et al., 1996; Mack et al,
1998; see Seager and Mack, 2003 for a review). Axial/fluvial deposits of the ancestral Rio
Grande comprise much of the Camp Rice deposits along the Rio Grande valley and
in adjoining basins, but piedmont-slope alluvium, which grades to the fluvial deposits, is
also an important component of the formation. The constructional top of the axial/fluvial
facies, a gently sloping surface known as the La Mesa surface, is widely preserved and
marked by stage IV or V petrocalcic paleosol. In outcrops, the formation does not exceed
approximately 100m in thickness, but thicker sections may be present in the subsurface in
some basins. In the study area both piedmont-slope alluvium and axial fluvial facies of
the formation are present.
Piedmont-slope alluvium consists exclusively of alluvial-fan deposits that form a thin
<10m-thick) veneer above a shallowly buried pediment surface, both adjacent to bedrock
hills in the area as well as across the broad stretches of desert plains. The deposits are
entirely locally derived, consisting of boulders and cobbles of Uvas Basaltic Andesite and
Bell Top ash-flow tuffs in small fans adjacent to bedrock hills, but including a wider
variety of predominantly limestone pebbles or cobbles on the distal parts of huge fans
draining the Caballo and San Andres Mountains. Generally unlithified, the uppermost
meter or two of the deposits is tightly cemented by stage IV soil carbonate. Red clayey
horizons are also locally present but in most places they have been removed by wind or
sheetflood erosion. Gypcrete soils have developed on Camp Rice and younger fans
surrounding the Upham Hills and on the piedmont slopes draining the southeastern Point of
Rocks Hills. However, the gypsum appears to be of eolian origin, blown onto the fans
surfaces well after the fans were abandoned as new generations of younger fans developed.
Camp Rice and younger generations of alluvial fans have similar provenance and ranges
in size, making it somewhat difficult to distinguish between them. Three characteristics of
Camp Rice fans are helpful. Camp Rice fans are the highest fan surfaces, especially in
medial and proximal parts of the fans where younger fans are usually inset below them. In
this setting, Camp Rice fan segments may be isolated by erosion, standing above their
surroundings as mesas or cuestas capped by fan gravels and calcrete paleosols. Parallel,
incised drainage patterns are also typical of Camp Rice fan surfaces. Otherwise, the fan
surfaces are stable and “high and Dry” during heavy rain events. The stability of the
surfaces has resulted in the development of state IV or greater petroalcic horizons,
perhaps the most distinguishing feature of Camp Rice fans.
Axial fluvial facies of the Camp Rice Formation underlies the La Mesa surface in the
southwestern part of the Upham Hills quadrangle and overlies an irregular erosion
surface cut primarily on Uvas Basaltic Andesite. Scattered bedrock hills project through
the fluvial deposits and rise above the La Mesa surface. Because of relief on bedrock, the
thickness of the axial fluvial deposits must vary significantly, but probably does not
exceed 100m. Only the upper 15m of the formation is poorly exposed in the Jornada Draw
fault escarpment. The upper few meters consist of fine-grained, gray sand capped by stage
IV or V soil carbonate, which underlies the La Mesa surface. Below the gray sand,
approximately 10 or 12m of gray, well-sorted sand and sandstone is locally exposed; it
contains well-rounded pebbles of granite and chert from distant upstream sources.
Although much of the sand is unlithified, some is cemented by selenite. Calcic paleosols
occur throughout the fluvial facies. The fine-grained sand at the top of the section may
11
represent overbank or eolian deposits, but the sand and sandstone carrying granite and
chert pebbles are clearly fluvial deposits of the ancestral Rio Grande. Apparently fluvial
channels or floodplains were occasionally abandoned for sufficient lengths of time to
develop petrocalcic horizons. Deposition of gypsum from groundwater locally lithified the
sand.
Surficial deposits
Surficial deposits include: older piedmont-slope alluvium; younger piedmont-slope
alluvium; basin-floor deposits; eolian sand; and colluvium. Older piedmont-slope alluvium
is late Pleistocene in age, based on stages of soil development, geomorphic position in the
landscape, dated basalt flows associated with the alluvium, and scattered mammal remains
(Gile et al., 1981). The eolian sand, basin-floor deposits, and younger piedmont-slope
alluvium have been deposited in active depositional systems and exhibit little or no soil
development; they are considered to be mostly of Holocene age, perhaps ranging back to
the latest Pleistocene (15,000 years). Radiocarbon dates from charcoal in these “younger”
deposits elsewhere in the region (Gile et al., 1981) confirm the Holocene age. Colluvial
deposits range in age from middle Pleistocene components of the Camp Rice Formation to
Holocene. Like Camp Rice piedmont-slope deposits, the surficial deposits are part of a thin
veneer of sediment that has buried the pediment that truncates Hayner Ranch and older
rock units. Total thickness of surficial deposits probably does not exceed 15 or 20m, and is
generally much less.
Older piedmont-slope alluvium. Older piedmont-slope alluvium comprises deposits of
gravel, sand, and silt that accumulated on valley side slopes, as pediment veneers, and
especially as large alluvial fans. Like Camp Rice fans, “older” fan alluvium is locally
derived and coarse grained on small fans adjacent to bedrock hills in the area, and
somewhat finer grained on the distal parts of huge fans that enter the map area from the
San Andres and Caballo Mountains. Except for stage II, III, or IV calcic or calcrete
paleosols in the upper meter or less, “older” piedmont-slope alluvium is unlithified.
Gypcrete of eolian origin caps older piedmont-slope alluvium adjacent to the Upham hills,
in the same manner that it caps Camp Rice fans there. On valley sideslopes, such as Rincon
arroyo or the Jornada Draw fault escarpment, older piedmont-slope alluvium consist
mostly of stage II or III calcic paleosols developed on underlying bedrock (mostly axial
fluvial Camp Rice sand), rather than discrete deposits of alluvium. The soils are generally
covered by eolian sand. “Older” alluvial fans are distinguished from Camp Rice fans
primarily by the inset relationship of the former with the latter, especially on medial and
proximal parts of the fans, and by the less mature soil profiles (stage II, III or IV).
Commonly, two and sometimes three generations of “older” fans may be distinguished,
based on inset relationships and soil development. Down-slope, however, older
piedmont-slope deposits commonly bury distal parts of Camp Rice fans. Like Camp Rice
fans, highest and oldest of the “older” fans exhibit parallel and incised drainage and are
“prevailingly “high and dry” following rain events. The youngest generations of “older”
fans, however, may be complexly associated with younger piedmont-slope alluvium
(discussed next), and be an integral part of an active fan drainage system.
Younger piedmont-slope alluvium. Younger piedmont-slope alluvium includes sand,
gravel and silt on arroyo floors, large and small alluvial fans, and pediment veneers, all
12
graded to or within a meter or so of the surface of Flat Lake playa. Generally
unconsolidated, the upper few centimeters may be weakly coherent due to clay
accumulation or may even exhibit stage I or II soil carbonate accumulation. Arroyo
alluvium generally occupies narrow to very broad, entrenched channels, inset against older
fan alluvium on upper and medial slopes of piedmonts but commonly overlaps and
spreads laterally as sheets of sediment across older alluvium on distal portions of piedmont
slopes. The composition reflects local source areas and is predominantly basaltic andesite
adjacent to the small groups of hills in the map area, but includes Paleozoic limestone and
sandstone, as well as reworked clasts of Precambrian granite and Cretaceous or lower
Tertiary rocks that crop out on distant piedmont slopes or in adjacent mountain ranges.
Major drainages, such as Rincon Arroyo, Aleman and Yost Draws, carry predominantly
sand or pebbly sand with lesser amounts of cobble gravel. Smaller drainages adjacent to
the groups of hills in the map area carry a range of clast sizes from boulder alluvium on
proximal parts of fans to silt and clay at the distal confluences with Jornada Draw or other
major drainage systems. Eolian sand locally covers large parts of the younger piedmontslope alluvium and probably is locally inter-bedded with it. Some active fans or other
drainages in the area consist entirely of younger piedmont- slope alluvium whereas others
are more complex, consisting of a complex pattern of both younger and older piedmontslope alluvium. The latter areas, mapped as Qpa, Qpad and Qpa(g), include the largest
active depositional and sediment-transport sites in the map area. Following periods of
heavy rainfall, the surfaces of these deposits are subject to sheet floods and to runoff in
countless shallow, branching and anastomosing drainage ways; weeks may be required for
the deposits to dry. Qpa refers to fans or other drainage systems where bodies of both
younger and older alluvium exist side by side in a complex pattern, or where extensive
deposits of older alluvium are covered by a thin veneer of younger alluvium. The symbol
is also used when soil exposures or inset relationships are insufficiently clear to
distinguish between “younger” and “older” alluvium. Qpad is used for the large, barren or
grass-covered bodies of fine-grained alluvium at the toes of large Camp Rice alluvial fans
draining the San Andres Mountains. The deposits appear to be shallowly inset below and
to locally bury Camp Rice fans. Whether the alluvium is predominantly “younger” or
“older” alluvium or a complex of both is in doubt because of the lack of soil exposures,
but active transport and deposition of sediment on and across these deposits is clear.
Qpa(g) is similar to Qpad except that Qpa(g) contains disseminated gypsum nodules
throughout. The deposit is located along the eastern shore of Flat Lake playa. Basin-floor
deposits. Basin-floor deposits consist primarily of dark reddish brown to grayish brown,
fine sand, silt, and clay on the bed of Jornada Draw, on the broad alluvial plain adjacent to
Jornada Draw, on the floor of Flat Lake playa, and as fan deltas that encroach onto the
floor of the playa. Aleman and Yost Draws also deliver large volumes of sand and pebbly
gravel that form fluvial fans at the confluence of these drainages with Jornada Draw. None
of the deposits appears to be gypsiferous at the surface, but trenching may show that the
sediments are gypsiferous at depth. In fact, exposed lake beds along the southern and
eastern shores of Flat Lake playa contain abundant selenite and these may extend beneath
the surface of Flat lake playa.
Eolian deposits. Broad expanses of the desert floor are mantled with pale red eolian
sand, especially the La Mesa surface, the Jornada Draw fault escarpment, the piedmont
slopes west of Jornada Draw, the valley side slopes of Rincon Arroyo, and the southern
and eastern shores of Flat lake playa. Much of the sand is in the form of coppice dunes,
13
but fields of weakly parabolic dunes, tending toward transverse ridges, form substantial
dune fields on the distal parts of alluvial fans draining westward from the San Andres
Mountains. The dunes do not exceed 5-7m in height, except where they have piled up
against bedrock hills. Most of the dunes are stabilized by vegetation, but the higher
dunes, as well as many low sheets or sand mounds, are active.
Colluvium. Colluvial deposits are in the form of aprons of boulders and cobbles that
mantle middle to lower slopes of bedrock hills in the map area. Composed of Uvas
Basaltic Andesite boulders and, to a lesser extent, Bell Top ash-flow tuff clasts, the
colluvium moves slowly down-slope, mostly by gravity. Most colluvial deposits are
cemented by stage IV soil carbonate and grade down-slope to the surface of Camp Rice
fans or pediment veneers; these deposits are clearly part of the Camp Rice Formation.
Less commonly, colluvium with weaker petrocalcic cement grades to either younger or
older piedmont-slope deposits. In all cases the colluvium provides a hillside armor which
seemingly slows erosion and effectively obscures underlying bedrock over wide areas.
STRUCTURE
The Jornada del Muerto syncline and Jornada Draw fault zone are the central structures in
the Prisor Hill and Upham Hills quadrangles. Largely covered by Camp Rice piedmontslope and other surficial deposits, the structures must be inferred from limited outcrops.
Jornada del Muerto syncline
The northerly trending Jornada del Muerto syncline is a product of the eastward tilting of
the Caballo uplift and westward tilting of the San Andres uplift. Westerly dipping rocks in
easternmost parts of both Prisor Hill and Upham Hills apparently are part of the eastern
synclinal limb, whereas easterly dipping rocks in the northeastern corner of Point of
Rocks Hills (Alivio quadrangle) are part of the western limb. Southerly dips of bedding or
lava flows between these outcrops --in the Point of Rocks Hills, the Yost- Aleman Draw
area, and northern end of Prisor Hill-- indicate that the synclinal hinge is broad and dips
southerly a few degrees, a slope that may have enabled Miocene and Pliocene hangingwall sediment from both the San Andres and Caballo uplifts to be transported to the south,
out of the syncline. Point of Rocks Hills seemingly lie in the broad, south-dipping trough
of the Jornada del Muerto syncline. The Tertiary section exposed within the Point of
Rocks Hills, mostly Uvas Basaltic Andesite, is broken into grabens or half grabens by
easterly to northwesterly trending normal faults. One such fault forms the northern
boundary of the hills, creating a north-facing fault-line escarpment (Fig. 5). However, the
escarpment is a good example of topographic inversion; downthrown, resistant Uvas
Basaltic Andesite flows in the hanging wall of the normal fault form the escarpment,
whereas uplifted, soft Palm Park strata in the footwall underlie adjacent lowlands to the
north. A second, important fault trends northwesterly, crossing the central part of the Point
of Rocks Hills diagonally. Downthrown to the north, the fault exhibits approximately
300m of stratigraphic separation, sufficient to uplift and expose uppermost Bell Top rocks
and to exhume an Uvas Basaltic Andesite cinder cone that formed near the base of the
Uvas Basaltic Andesite. Southerly dips in this fault block probably carry Uvas Basaltic
Andesite and older rocks to great depth to the south, creating the deep basin near San
Diego Mountain in which 1,900m of Miocene rift-basin deposits accumulated (Seager et
al., 1971; Mack et al., 1994b). The northerly trending Jornada Draw fault zone truncates
14
the Point of Rocks Hills on the east, breaking the hinge area of the Jornada del Muerto
syncline for many kilometers, both to the north and to the south.
Figure 5— Westward-looking view of the northern escarpment of Point of Rocks Hills with Caballo
Mountains on skyline. Uvas Basaltic Andesite forms all hills in the escarpment. Down-to-the south (left)
boundary fault at the base of the escarpment can be seen in right, middle distance as a line of vegetation
along the edge of a basaltic andesite hill.
Jornada Draw fault zone
The Jornada Draw fault, a normal fault, downthrown toward the east, was identified and
named by Seager and Mack (1995) from geologic mapping in the Cutter and Engle
quadrangles. Although the trace of the fault is clear in the Cutter and Engle quadrangles,
where it is a single fracture, its course across the Prisor Hill and Upham Hills quadrangles
is mostly inferred because of limited exposures. Outcrops in Prisor Hill, Upham Hills and
in the Jornada Draw fault escarpment suggest the fault zone in this area has divided into a
series of right-stepping, en echelon faults (Fig.6). Prisor Hill is on strike with exposures
of the Jornada Draw fault in the Cutter quadrangle to the northwest, and it is reasonable to
infer that the Bell Top and younger Tertiary rocks exposed at Prisor Hill are on the
downthrown side of the fault, juxtaposed against Love Ranch strata that crop out across
Jornada Draw only a short distance away. Stratigraphic separation here is estimated to be
1,000m. At this point, the Jornada Draw fault trends northwestward, is inferred to parallel
the southwestern base of Prisor Hill, and continues an unknown distance to the southeast,
buried by Camp Rice and younger piedmont alluvium. The fault separates Prisor Hill from
15
Upham Hills, a seemingly necessary structure to account for the repeated middle Tertiary
section in the two areas. Similarly to Prisor Hill, Bell Top and younger rocks exposed at
Upham Hills are inferred to be on the downthrown, hanging-wall side of a fault zone that
juxtaposes them with mostly covered Palm Park strata to the west. The fault, which
probably follows the western base of the hills, is considered to be part of the Jornada Draw
fault zone. A fault splay in this zone is poorly exposed along the western slope of the
Upham Hills.
Figure 6— Map of Jornada Draw fault zone, showing en echelon arrangement of fault segments.
Trending north-northwest, parallel to the hills, the fault splay is downthrown to the east
16
and juxtaposes Bell Top and Uvas Basaltic Andesite, except at the south end where Uvas
flows and Palm Park strata are in fault contact. At this point, stratigraphic separation is
estimated to be 600m. East of the fault splay, Uvas and Bell Top rocks in Upham Hills are
bent into a narrow, north-northwest-trending syncline whose east-dipping western limb
probably results from drag along the Jornada Draw fault zone. Location of the
Jornada Draw fault north of Upham Hills is uncertain. Although its northerly trend carries
it at an angle to the Jornada Draw fault segment at Prisor Hill, the two fault segments are
considered to be en echelon members of the same fault zone. South of Upham Hills, the
fault zone apparently turns southeastward, separating the Upham Hills from uplifted Bell
Top And Uvas rocks exposed in the small hill one kilometer south of Upham Hills.
The Jornada Draw fault escarpment apparently is a third segment of the Jornada Draw
fault zone. Located to the south and west of the Upham Hills segment, the east-facing
escarpment extends from the eastern edge of the Point of Rocks Hills southeastward for
20km or more. It was created by down-to-the east displacement of the La Mesa surface
and underlying fluvial facies of the Camp Rice Formation, the latter of which crop out in
or underlie the much-degraded scarp. Displacement apparently decreases northward
along the eastern margin of Point of Rocks Hills as suggested by hills of basaltic andesite,
located on either side of the inferred fault trace, that require little or no faulting between
them. It is therefore doubtful that the Jornada Draw fault escarpment segment connects
with the Upham Hills segment. Consequently, available data suggest that the southern
25km of the Jornada Draw fault zone is composed of three, right-stepping, en echelon fault
segments which separate Prisor Hill, Upham Hills, and Point of Rocks Hills (Fig.6). The
total length of the fault zone is nearly 65km in length, and may approach 75km in length if
the faulting that breaks the La Mesa surface north of the Dona Ana Mountains is included
in the fault zone (Fig. 6).
Seager and Mack (1995) discussed the age of the Jornada Draw fault zone. Because of a
lack of Miocene basin fill on the hanging-wall side of the fault, they suggested that the
fault was not initiated until latest Miocene to early Pleistocene, at which time faulting
helped accommodate the growing structural relief between the Jornada del Muerto
syncline and uplifted ranges to the west and east. Middle to late Pleistocene movement
along the northernmost segment of the fault zone near Engle, as well as east and south of
Point of Rocks Hills, created fault escarpments in Camp Rice or correlative units that
persist to today, albeit in degraded form.
SUMMARY OF CENOZOIC GEOLOGIC HISTORY
Figures7-12 are paleogeographic/paleotectonic reconstructions of what south-central
New Mexico may have looked like during the time intervals represented by each of the
seven Cenozoic formations in the Prisor Hill and Upham Hills quadrangles. The
reconstructions are based not only on the outcrops in these two quadrangles, but also on
exposures of these formations throughout the region. Because the outcrops on which
these maps are based are scattered across a large region, parts of the maps are
diagrammatic, designed to give an overall interpretation of the character of the landscape.
For example, the location of some stratovolcanoes in the Palm Park Formation map is
based on outcrops of 46-37 Ma stocks, which may or may not represent magma chambers
beneath volcanoes.
17
Late Cretaceous-early Tertiary (Laramide) crustal shortening in southwestern New
Mexico resulted in a series of northwest-trending block uplifts yoked to intermontane
basins (Seager, 2004). The Love Ranch basin seemingly was one of the largest of these
basins and was filled with alluvial fan and fluvial deposits derived from the adjacent Rio
Grande uplift (Fig. 7). By middle to late Eocene time the uplift was drained by lowgradient fluvial systems that deposited mostly fine-grained sediment across much of the
basin floor. The fine-grained, alluvial flat and fluvial deposits exposed in the Prisor Hill
Quadrangle occur near the top of the Love Ranch section and are interpreted to record
waning stages of deposition on the distal slopes of the Love Ranch basin.
Figure 7— Paleogeographic map of south-central New Mexico in middle Eocene time during
deposition of the Love Ranch Formation.
By late Eocene time, Laramide uplifts were onlapped and nearly buried by Love Ranch
clastics, and a prolonged period dominated by volcanic activity was initiated. Continental
arc volcanism commenced in the late Eocene when andesitic stratovolcanoes formed
across southwestern New Mexico (McMillan, 2004). Lava flows, as well as lahar and
pyroclastic debris, mantled volcanic slopes; lahars, especially, formed aprons of alluvium
18
far down volcano flanks and onto intravolcano lowlands. In the Prisor Hill and Upham
Hills quadrangles such aprons of lahar deposits are represented by the Palm Park
Formation (Fig. 8).
Figure 8—Paleogeographic map of south-central New Mexico in late Eocene time during
deposition of the Palm Park Formation.
Andesitic arc volcanism changed to weakly bimodal basalt-rhyolite volcanism beginning
approximately 36Ma. This change has been interpreted as documenting the transition to an
extensional stress field in a crust long affected by contractional ones (eg. McMillan,
1998; McMillan et al., 2000). In south-central New Mexico, basaltic volcanism was clearly
subordinate to explosive silicic volcanism, the latter long referred as the “ignimbrite flareup” Large volume ash flows were erupted from the Organ, Emory, Nogal Canyon, and Mt
Withington calderas between 35.8 and 27.4Ma, the outflow sheets spreading far across
surrounding lowlands (Fig. 9). Ash-flow tuffs 5 (Kneeling Nun Tuff?) and 7 of the Bell
Top Formation in the Prisor Hill and Upham Hills quadrangles represent distal parts of
outflow sheets whose source probably was the Emory and Nogal Canyon calderas,
respectively. The Emory, Nogal Canyon, and Mt Withington calderas must have been huge
volcanic edifices, rivaling the Jemez volcano in size. Explosive plinian eruptions mantled
the volcanic slopes with thick deposits of pumice, which were reworked by gully and
sheet-flow runoff, then deposited in surrounding lowlands as tuffaceous sandstones of the
19
Bell Top Formation. Coarser-grained alluvial fan and fluvial
deposits also accumulated in lowlands as the pumiceous deposits were stripped away, and
these, too, are an important component of the Bell Top Formation in the south-central
Jornada del Muerto region (Fig.9).
Figure 9—Paleogeographic map of south-central New Mexico in latest Eocene and
Oligocene time (34.8-28.6 Ma). during deposition of the Bell Top Formation.
With the possible exception of the Goodsight-Cedar Hills Basin, Bell Top and other
major ash-flows in southwestern New Mexico “saw” little or no fault-block topography.
Apparently regional extension was sufficiently weak to preclude extensive faulting of the
crust. However, according to Mack et al. (1994a), the Goodsight Cedar-Hills half graben
was one of the earliest extensional structures in the region; Bell Top strata and ash-flow
tuffs from distant volcanoes, as well as locally derived alluvial- fan and fluvial sediment,
accumulated to unusual thickness in the basin. In contrast, Chamberlain (personal
communication, 2001) has suggested that the basin may have been a topographic lowland
20
between primary volcanic features that may have simply filled with Bell Top tuffs and
sediment.
Accelerating crustal extension in late Oligocene time is suggested by the outpouring of
huge volumes of basaltic andesite in southwestern New Mexico and northern Mexico,
creating a basalt plateau (Cameron et al., 1989). The Uvas Basaltic Andesite is part of
this plateau and is associated with a swarm of west-northwest-trending basaltic dikes,
some of which seemingly fed lava flows (Fig 10).
Figure 10— Paleogeographic map of south-central New Mexico in Oligocene time (28.025.9 Ma) during emplacementof the Uvas Basaltic Andesite.
The dikes suggest that north-northeast extensional stresses were operative in south-central
New Mexico during the late Oligocene. Locally, cinder cones, such as the one exposed in
Point of Rocks Hills, were constructed on the basalt plateau, and one basaltic diatreme is
known (Clemons and Seager, 1973). By latest Oligocene or earliest Miocene time, the
crust was sufficiently extended so that block faulting in the southern Rio Grande rift
began, documented by the alluvial fan and basin-floor deposits of the Hayner Ranch
Formation (Mack et al., 1994b). The Caballo and probably San Andres ranges began to
21
form, uplifting and tilting middle Tertiary volcanic rocks. Opposing dips of these ranges
created a shallow, incipient Jornada del Muerto syncline, whose gentle southerly plunge
probably facilitated movement of hanging wall sediment southward to a rapidly subsiding
basin near San Diego Mountain (Fig.11). Except for the thin paleovalley deposits of
Hayner Ranch Formation exposed in the Prisor Hill and Upham Hills quadrangles, basin
fill never accumulated in the Jornada del Muerto syncline throughout its Neogene history,
suggesting that sediment consistently bypassed the syncline on its way to the deep basin
near San Diego Mountain (Fig.11).
Figure 11— Paleogeographic map of south-central New Mexico in latest Oligocene or early Miocene
time during deposition of the Hayner Ranch Formation.
22
Fault-block ranges continued to evolve throughout the Miocene. Early ranges grew higher
and were deeply eroded while new fault blocks were initiated from time to time (Mack et
al., 1994b; Seager and Mack, 2003). Faulting appears to have culminated in the latest
Miocene as new basaltic volcanism increased (Seager et al., 1984; Mack et al.,
1994b). At this time the Jornada Draw fault zone was probably initiated to help
accommodate growing structural relief between the floor of the Jornada del Muerto
syncline and the Caballo and San Andres uplifts.
Until the early Pliocene, rift basins were closed structures, internal drainage prevailed,
and playa lakes were common. By approximately 5Ma, however, the ancestral Rio
Grande entered the basins from the north, spread periodically into six contiguous basins
of southern New Mexico (Fig. 12), filled them with fluvial and piedmont-slope deposits
of the Camp Rice and correlative formations, and finally emptied into Lake Cabeza de
Vaca south of El Paso and in northern Chihuahua (Strain, 1966).
Figure 12— Paleogeographic map of south-central New Mexico Pliocene to middle
Pleistocene during deposition of the Camp Rice Formation.
23
Approximately 3Ma to 0.8Ma, the ancestral Rio Grande made an excursion from the Rio
Grande Valley near Rincon into the Jornada del Muerto (Mack et al., 1998), where the
river deposited fluvial sediment along the southern margin of Point of Rocks Hills before
turning south and flowing along the axis of the Jornada del Muerto toward the Dona Ana
Mountains (Fig.12). The constructional top of these deposits, the La Mesa surface, is still
preserved over a broad area south of Point of rocks. Gile (2003) suggests that periodic
movement along southern segments of the Jornada Draw fault created gypsiferous playas
of Lake Jornada on the hanging-wall side of the fault zone between approximately one
and.0.3Ma (Fig.12). Such faulting during Camp Rice time and in the late Pleistocene
resulted in exposures of the Camp Rice fluvial facies in the Jornada Draw fault
escarpment. As fluvial sediments accumulated along the ancestral Rio Grande, thin
piedmont-slope alluvium of the Camp Rice Formation buried the widespread pediments
adjacent to the Caballo and San Andres uplifts, as well as those surrounding Point of
Rocks, Upham Hills and Prisor Hill.
Deposition of the Camp Rice Formation ended approximately 0.78 Ma (Mack et al,
1998), when the ancestral Rio Grande and its tributaries began to alternately incise and
backfill the basins, creating a stepped sequence of terraces on valley sideslopes (Gile et al.,
1981). Similarly, in the Jornada del Muerto, several generations of late Pleistocene and
Holocene alluvial fans were inset below Camp Rice fans or partially buried them. Eolian
gypsum, perhaps derived from the gypsiferous bed of Lake Jornada, accumulated on the
oldest of these late Pleistocene fans, as well as on Camp Rice fans adjacent to Upham Hills
and Point of Rocks Hills; the gypsum is now in the form of a gypcrete cap on the alluvial
fan deposits. Although faulting in late Pleistocene time created or renewed relief on the
Jornada Draw fault escarpment, younger alluvial fans, deposition along the
axial drainage of Jornada Draw, and extensive eolian sand have concealed the trace of the
fault across most of the area. Eolian sand has also buried piedmont slopes and the La Mesa
surface over wide areas.
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Lucas, S. G. and Oakes, W., 1986, Pliocene (Blancan) vertebrates from the Palomas
Formation, south-central New Mexico; in Clemons, R. E., King, W. E., Mack, G. H., and
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25
G. H. and Seager, W. R., in press, Geology of the Engle quadrangle, Sierra County, New
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sedimentation as influenced by intrabasinal faulting, southern Rio Grande rift: Geological
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Mack, G. H., Love, D. W., and Seager, W. R., 1997, Spillover models for axial rivers in
regions of continental extension: the Rio Mimbres and Rio Grande in the southern Rio
Grande rift, USA: Sedimentology, v. 44, pp. 637-652.
Mack, G. H., McIntosh, W. C., Leeder, M. R., and Monger, H. C., 1996, Plio-Pleistocene
pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA: Sedimentary
Geology, v. 103, pp. 1-8.
Mack, G. H., Nightengale, A. L., Seager, W. R., and Clemons, R. E., 1994a, The
Oligocene Goodsight-Cedar Hills half graben near Las Cruces and its implications to the
evolution of the Mogollon-Datil volcanic field and to the southern Rio Grande rift; in
Chamberlin, R. M., Kues, B. S., Cather, S. M., Barker, J. M. and McIntosh, W. C., (eds.),
Guidebook of the Mogollon slope, west-central New Mexico and east-central Arizona: New
Mexico Geological Society, Guidebook 45, pp. 135-142.
Mack, G. H., Salyards, S. L., McIntosh, W. C., and Leeder, M. R., 1998, Reversal
magnetostratigraphy and radioisotopic geochronology of the Plio-Pleistocene Camp Rice
and Palomas Formations, southern Rio Grande rift; in Mack, G. H., Austin, G. S. and
Barker, J. M. (eds.), Guidebook of the Las Cruces country II: New Mexico Geological
Society, Guidebook 49, pp. 229-336.
Mack, G. H., Seager, W. R., and Kieling, J., 1994b, Late Oligocene and Miocene faulting
and sedimentation, and evolution of the southern Rio Grande rift, New Mexico, USA:
Sedimentary Geology, v. 92, pp. 79-96
40
39
McIntosh, W. C., Kedzie, L. L., and Sutter, J. F., 1991, Paleomagnetism and Ar/ Ar
ages of ignimbrites, Mogollon-Datil volcanic field, southwestern New Mexico: New
Mexico Bureau of Mines and Mineral Resources, Bulletin 135, 79 pp.
McMillan, N. J., 1998, Temporal and spatial magmatic evolution of the Rio Grande rift; in
Mack, G. H., Austin, G. S. and Barker, J. M. (eds.), Guidebook of the Las Cruces country
II: New Mexico Geological Society, Guidebook 49, pp. 107-115.
McMillan, N.J., 2004, Magmatic record of Laramide subduction and the transition to
Tertiary extension: Upper Cretaceous through Eocene igneous rocks in New Mexico, in
Mack, G.H. and Giles, K.A., The geology of New Mexico: a geologic history: New Mexico
Geological Society Special Publication 11, pp.249-270.
McMillan, N.J., Dickin, A.P. and Haag, D., 2000, Evolution of magma source regions in
the Rio Grande rift, southern New Mexico: Geological Society of America Bulletin, v.
112, pp. 1582-1593.
26
Repenning, C. A. and May, S. R., 1986, New evidence for the age of the lower part of the
Palomas Formation, Truth or Consequences, New Mexico; in Clemons, R. E., King, W. E.,
Mack, G. H., and Zidek, J., (eds.), Guidebook of the Truth or Consequences region: New
Mexico Geological Society, Guidebook 37, pp. 257-263.
Seager, W.R., 2004, Laramide tectonics of southwestern New Mexico, in Mack, G.H. and
Giles, K.A., eds., The Geology of New Mexico: a geologic history: New Mexico
Geological Society, Special Publication 11, pp.183-202.
Seager, W. R., in press, Geology of Alivio quadrangle, New Mexico: New Mexico
Bureau of Mines and Mineral Resources, Bulletin.
Seager, W.R. and Hawley, J.W., 1971, Geology of San Diego Mountain area, Dona Ana
County, New Mexico; New Mexico Bureau of Mines (Geology) and Mineral Resources,
Bulletin 97, 38pp.
Seager, W. R. and Hawley, J. W., 1973, Geology of Rincon quadrangle, New Mexico: New
Mexico Bureau of Mines and Mineral Resources, Bulletin 101, 42 pp.
Seager, W. R. and Mack, G. H., in press, Geologic map of Cutter and Upham quadrangles:
New Mexico Bureau of Mines and Mineral Resources, Geologic Map, scale
1:24,000.
Seager, W. R. and Mack, G. H., 1995, Jornada Draw fault: a major Pliocene-Pleistocene
normal fault in the southern Jornada Del Muerto: New Mexico Geology, v. 17, pp. 37-43.
Seager, W.R. and Mack, G.H., 2003, Geology of the Caballo Mountains, New Mexico:
New Mexico Bureau of Geology and Mineral Resources, Memoir 49, 136pp
Seager, W. R., Hawley, J. W., and Clemons, R. E., 1971, Geology of San Diego Mountain
area Dona Ana County, New Mexico: New Mexico Bureau of Mines and Mineral
Resources, Bulletin 97, 38 pp.
Seager, W. R., Mack, G. H., and Lawton, T. F., 1997, Structural kinematics and
depositional history of a Laramide uplift-basin pair in southern New Mexico: Implications
for development of intraforeland basins: Geological Society of America, Bulletin, v. 109,
pp. 1389-1401
Seager, W. R., Shafiqullah, M., Hawley, J. W., and Marvin, R. F., 1984, New K-Ar dates
from basalts and the evolution of the southern Rio Grande rift: Geological Society of
America Bulletin, v. 95, pp. 87-99.
Strain W.S., 1966, Blancan mammalian fauna and Pleistocene formations, Hudspeth
County, Texas: Texas Memorial Museum, Austin, Bulletin 10, 55pp.
27
DESCRIPTION OF UPHAM HILLS AND PRISOR HILLS UNITS
Q: Quaternary sediments— Cross-sections only.
Qs: Eolian sand, coppice dunes—Pale red to pale orange sand, mostly in the form of
coppice dunes, but also including thin sand sheets, as well as mounds and aprons, the
thickest of which may be nearly barren of vegetation; best developed against the bedrock
hills above the La Mesa surface; along the southeastern margins of Flat Lake playa; on the
valley sideslopes of Rincon arroyo; along the western flanks of both the Upham Hills
and Prisor Hill; and on the Jornada Draw fault escarpment west of Flat Lake; widespread,
but discontinuous on the La Mesa surface and on the distal piedmont slopes (especially
Qcp) of the San Andres Mountains; as much as 3m thick.
Qsp: Eolian sand, parabolic dunes—Pale red to orange sand in the form of narrow,
arcuate, weakly parabolic dunes, which tend to form discontinuous transverse ridges;
generally 1 to 2 m in height, although locally they may exceed 4m; except for the highest,
the dunes are largely stabilized by vegetation; forms distinctive fields of dunes on distal
parts of alluvial fans derived from San Andres Mountains; dunes overlap both older Qcp
and younger Qpa, Qpo and Qpy deposits, and probably interfinger downward with the
latter; interdune areas are fine-grained or pebbly deposits of Qpy, Qpo or Qcp; generally
1 to 2 m thick.
Ql, Ql(g): Playa deposits—Pale reddish-brown to tan silt, clay and fine sand on the floor
of Flat Lake playa; surficial deposits appear to be non gypsiferous (Ql), but older, buried
beds may be gypsiferous as indicated by selenite-rich lake sediment (Qlg) exposed along
the southeastern margin of the lake; little or no soil development and little or no vegetation
across broad areas of Ql; at least 1m thick and probably much more.
Qa: Axial channel deposits—Brown, pale red, to dark reddish-gray sand, silt and minor
gravel on the bed of Jornada Draw, an axial drainage of the Jornada del Muerto basin; 2m
thick or more.
Qap: Alluvial-plain deposits—Pale reddish-brown to tan silt, fine sand and clay
adjacent to the lower reaches of Jornada Draw; gradients of the alluvial plains are
generally less than 10 ft/mi; non gypsiferous, at least in the exposed uppermost parts;
little or no soil development and locally no vegetation across broad areas; at least 1m
thick and perhaps much more.
Qpy: Younger piedmont-slope alluvium—Gravel, sand and silt on arroyo or canyon
floors of upland areas, filling shallow drainageways on pediments or alluvial fans, and
forming small alluvial fans at the mouths of such drainageways; includes broad but thin
veneers of sediment on middle or distal parts of large alluvial fans. Deposits are graded to or
within a meter or two of the floor of Flat Lake playa and are actively moving downslope by
sheetflood and channelized runoff. Clast composition reflects local source areas, ranging from
predominantly Uvas Basaltic Andesite adjacent to Point of Rocks, Upham Hills, and Prisor
Hill, to Paleozoic limestone and sandstone derived from the Caballo and San Andres
Mountains; unconsolidated, although the uppermost few centimeters may be weakly coherent
because of incipient (stage I) soil development; as much as 2-3m thick.
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Qpo: Older piedmont-slope alluvium—Gravel, sand, and silt of canyon floors, arroyos, alluvial
fans and pediment veneers; generally inset against older Camp Rice deposits on
upper parts of piedmont slopes but overlap and bury Camp Rice deposits downslope. At least
two generations of Qpo deposits exist, an older deposit distinguished on upper piedmont slopes
by a geomorphic position just below the surface of camp Rice fans, as well as by stage III-IV
soil carbonate, and and a younger deposit, inset against the older, displaying stage II soil
carbonate. Like Qpy deposits, clast composition reflects local source areas. The surface of Qpo
alluvial-fan deposits adjacent to Upham Hills exhibit gypcrete soil, as much as 2m thick, that
apparently was developed on eolian gypsum that mantled the fans in late Pleistocene time.
Along the sideslopes of Rincon Arroyo and on the Jornada Draw fault escarpment, deposits
mapped as Qpo are merely stage III-IV soil carbonate developed on underlying Camp Rice
strata- the erosion surfaces on which the soils are present being correlative with the surface of
Qpo deposits elsewhere; Except for these soils, Qpo deposits are at least 2-3m thick.
Qpa, Qpad, Qpa(g): Undifferentiated Qpy and Qpo—Qpa includes medium to large
alluvial fans or other piedmont-slope deposits on which patterns of Qpy and Qpo are
complex, or where Qpo is locally buried by thin but extensive veneers of Qpy. Qpad
refers to fine-grained, distal piedmont-slope deposits derived from the San Andres
Mountains and located along the eastern margins of Jornada Draw; these consist of light
gray to white, fine sand and silt, are of uncertain age but probably correlative with Qpo,
Qpy or Qpa elsewhere. Qpa(g) consists of fine-grained, tan to dark gray, distal alluvialfan deposits containing disseminated gypsum. Qpa, Qpad, and Qpa(g) all are located on
active depositional surfaces subject to sheetfloods and to anastomozing, closely spaced,
channelized runoff.
Qc: Colluvium—Bouldery hillside deposits that are slowly moving downslope, mostly
by gravity; most deposits are cemented by stage IV carbonate and grade downslope to
piedmont-slope alluvium of the Camp Rice Formation and therefore represent the most
proximal part of the formation. Less commonly, colluvial deposits grade downslope into
Qpo or Qpy alluvium. In any case, the deposits provide a hillside armor which
seemingly slows erosion and effectively obscures underlying bedrock relationships over
wide areas; mapped boundaries between colluvium and other alluvial deposits are
entirely gradational and are generally portrayed on the geologic map somewhat
diagrammatically Furthermore, small outcrops of unmapped bedrock (Uvas Basaltic
Andesite, especially), may locally project through the colluvium. One to 2m thick.
Qcp: Camp Rice Formation, piedmont-slope deposits—Boulder to pebble
conglomerate, gravel, conglomeratic sandstone, pebbly sand, sand and silt forming
pediment veneers and alluvial fans adjacent to local hills and mountains. Forming the
highest constructional surfaces near mountain fronts, the deposits generally are buried
downslope by younger piedmont-slope alluvium (Qpo, Qpy, Qpa), upslope on hillsides,
the deposits grade into bouldery colluvium (Qc), unconsolidated to well cemented, the
cementation a product of stage IV soil carbonate development in the upper 1 to 2 m of the
deposit. Clast composition and grain size reflect local sources. Basaltic boulder
conglomerate is distinctive of proximal deposits adjacent to Point of Rocks, Upham Hills,
and Prisor Hill, whereas limestone/sandstone pebble or cobble gravel and gravelly sand is
characteristic of distal parts of pediments or alluvial fans draining the San Andres and
Caballo Mountains. Gypcrete soil, as much as 2m thick, caps Camp Rice piedmont–slope
29
deposits adjacent to Upham Hills and along the southeastern flank of Point of Rocks;
these outcrops are shown on the map as Qcp(g). Apparently of eolian origin, the gypcrete
also overlies younger (Qpo) deposits and so is younger than both Qcp and Qpo; as much
as 4m thick
Qcl: Camp Rice Formation, La Mesa surface—Constructional top of the fluvial facies
of the Camp Rice Formation, marked by stage IV calcrete; covered over broad areas by
coppice dunes; as much as 1.5m thick.
QTcf: Camp Rice Formation, fluvial facies—Light gray, fine-grained, well-sorted sand
and loamy sand, as much as 7m thick, that underlies the La Mesa surface and probably
represents overbank or eolian deposits associated with the ancestral Rio Grande. These
are underlain by acestral Rio Grande channel deposits consisting of gray, well-sorted,
coarse to medium-grained sand and sandstone containing scattered, well-rounded pebbles
of Precambrian granite and chert derived from distant sources; largely uncemented but
locally well-cemented by gypsum; locally exposed along the Jornada Draw fault
escarpment; but, in general, outcrops are concealed by Qs and/or Qpo soils or by thin Qpo
alluvium; total exposed thickness is at least 15 m, base not exposed.
Thr: Hayner Ranch Formation—Boulder/cobble conglomerate consisting of angular to
sub-rounded boulders of Uvas Basaltic Andesite and Bell Top ash-flow tuffs 5 and 6;
clasts range up to 3/4m in length and are entirely disaggregated from matrix, resulting in
“outcrops” consisting of boulder and cobble lag deposits; unconformably overlies Uvas
Basaltic Andesite and Bell Top Formation on a deep, irregular erosion surface; deposits
are probably alluvial and colluvial fill of paleovalleys; at least 100 m thick, top not
exposed.
Tui: Uvas Basaltic Andesite, dikes and plugs(?)—Northwest-trending basaltic andesite
dikes exposed in the northwestern part of the Upham Hills quadrangle; transect Bell Top
strata and ash-flow tuffs and may merge upward into and “feed” Uvas Basaltic Andesite
flows; also includes possible plugs of basalt that intrude Tbs in the central part of Point of
Rocks Hills; as much as 15 m thick.
Tuc: Uvas Basaltic Andesite, cinder cone—Reddish-brown to tan, well-bedded basaltic
andesite cinder and lapilli tuff breccia containing bombs and impact sag structures; well
cemented with calcium carbonate; initial dips of 25 degrees; interbedded with Uvas
Basaltic Andesite flows near the base of the formation; represents part of northeastern,
northern and northwestern flanks of Uvas Basaltic Andesite cinder cone, which was largely
buried by subsequent flows, then exhumed and almost entirely eroded; a thickness of
approximately 50 m is exposed.
30
Tu: Uvas Basaltic Andesite—Black, gray, reddish-brown and tan basaltic andesite
flows; dense, massive, to vesicular or amygdaloidal (chalcedony) to platy; locally
contains interbedded, very poorly exposed, brown, coarse-grained volcaniclastic beds;
locally a basal flow is interbedded with uppermost beds of the Bell Top Formation;
individual flows range from 4-20m thick; at least 160m thick, top eroded.
Tbt: Bell Top Formation— Cross-sections only.
Tb7: Bell Top Formation, ash-flow tuff 7—Light grayish-brown, vitric ash-flow tuff at
the base of the Uvas Basaltic Andesite, although locally an Uvas Basaltic Andesite flow
underlies the ash-flow tuff; probably represents distal parts of Vicks Peak Tuff, erupted
from the Nogal Peak cauldron in the San Mateo Mountains (McIntosh et al., 1991);
generally less than one meter thick.
Tb6: Bell Top Formation, ash-flow tuff 6—Pale pinkish to orange-gray, crystal-rich
ash-flow tuff; contains broken crystals of quartz, sanidine, biotite and plagioclase in a
matrix of devitrified ash; simple cooling unit; occurs near the middle of Bell Top
sedimentary sequence (Tbs); 7-10m thick.
Tbs: Bell Top Formation, sedimentary member—White to light tan, tuffaceous
sandstone and interbedded cobble to boulder conglomerate; divided into upper and lower
units by medial ash-flow tuff 6 (Tb6); sandstones are medium to thin bedded and consist
of a mixture of glass shards, pumice, quartz, sanidine, and biotite; sand to granule-sized,
white pumice grains are especially abundant and conspicuous. Conglomerate beds are
poorly exposed, generally represented only by disaggregated clasts; these include a variety
of dark gray to reddish-gray porphyries of intermediate composition, similar in appearance
and composition to those of the McRae and basal Love Ranch Formations; generally well
rounded, the clasts may be recycled from McRae and Love Ranch conglomerates;
interpreted to be syneruption, alluvial fan and fluvial deposits on the distal flanks of large
volcanoes, as well as the fill of the Goodsight-Cedar–Hills half graben; approximately
235m thick.
Tb5: Bell Top Formation, ash-flow-tuff 5—Light gray to grayish tan, crystal and pumicerich ash-flow tuff; coarse-grained fragments of sanidine, plagioclase, and bipyramidal
quartz crystals, as well as biotite, are conspicuous in hand specimens; abundant pumice
fragments range from 1 to 3cm in length and weather light brown; unit is rather densely
welded and is a simple cooling unit; white tuffaceous sandstone and air- fall tuff,
approximately 5m thick, underlies tuff 5, separating it from the underlying Palm Park
Formation; approximately 10m thick along northern edge of Point of Rocks Hills, including
basal white, tuffaceous strata.
31
Tpp: Palm Park Formation—Pale grayish-purple to gray conglomerate, breccia, and
tuffaceous, volcaniclastic sandstone that probably represents distal piedmont-slope deposits
of one or more andesitic stratovolcanoes; conglomerate and breccia clasts range up to
boulder size, are matrix supported, and comprise a suite of intermediate- composition
porphyries containing phenocrysts of hornblende and plagioclase; matrix consists of a
poorly sorted mixture of ash, small clasts, and crystals; all lithologies are probably lahar
deposits; prevailingly soft, the unit is poorly exposed only along the northeastern edge of
Point of Rocks Hills and on southwestern slopes of Upham Hills; elsewhere, it’s normal
outcrop area is buried by a thin veneer of alluvial-fan sediments; thickness uncertain but
may be as much as 600m.
Tlrc, Tlrs, Tlrm,Tlrl: Love Ranch Formation—Gray to reddish-gray conglomerate, tan
to reddish-brown conglomeratic sandstone and sandstone, and red to purple mudstone; unit
becomes finer grained upward in the section and toward the east. Outcrops containing
conglomerate were mapped as Tlrc; those consisting of interbedded sandstone and
mudstone are designated Tlrs; and outcrops of mudstone are shown as Tlrm. Conglomerate
clasts include well rounded and grain-supported types interpreted to be fluvial in origin, as
well as minor poorly sorted, angular, matrix-supported types indicative of deposition on
alluvial fans; conglomerate bodies are channe lform in geometry and exhibit trough crossbedding. Although angular boulders are locally present in the fanglomerate, clasts are
generally cobble size, decreasing to pebble size upward in the section. Clasts consist mainly
of Paleozoic limestone and sandstone and Precambrian granite, with lesser amounts of
intermediate-composition porphyries. Sandstones are coarse to medium grained, crossbedded, channelform bodies as much as 7m thick, exposed for tens to hundreds of meters
along strike; enclosed in red mudstone units, the sandstone/mudstone sequences represent
deposition in fluvial channels and on floodplains, respectively. Red mudstone in the
stratigraphically highest and easternmost outcrops of the formation is a basin-floor facies,
probably deposits of alluvial plains; a minor pisolitic limestone bed (1 m thick) (Tlrl) within
mudstone probably indicates the presence of a local fresh- water pond. The formation is the
fill of the Love Ranch basin, a major Laramide intermontane basin. Thickness in map area
is uncertain but may be as much as 900m.
Tlr: Love Ranch Formation, undifferentiated—Cross-sections only.
K: Cretaceous, undifferentiated—Cross-sections only.
32