Preliminary Geologic Map of the
Prisor HillQuadrangle,
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 06HQPA0003
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 PRISOR HILL AND UPHAM HILLS QUADRANGLES,
SOUTHERN JORNADA DEL MUERTO< NEW MEXICO
By
William R. Seager
INTRODUCTION
The Prisor Hill and Upham Hills 7 ½ minute quadrangles are located in the south-central
part of the Jornada del Muerto, approximately 72km 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.
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.
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 armorlike 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 relationship results from the superior durability and resistance to weathering
of hanging-wall 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.
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) by Seager et al. (!987) 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 xposures 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 Ranch strata in the map area become finer grained upward. Stratigraphically lowest
exposed beds consist of interbedded 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 (1m) 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,
grain-supported 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 finer-grained sediment. Sandstones are mostly 1 to3m- thick beds of coarse to
medium-grained, pale red or tan sand, much of which is crossbedded 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
clast-supported 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).
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 interbedded 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 40Ar/39Ar age of 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 interbedded white
to tan, tuffaceous sandstone and interbedded 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.
Ash-flow tuff 6 occurs near the middle of the sedimentary rock sequence just described.
A 40Ar/39Ar age of 33.6Ma 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 interbedded 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 40Ar/39Ar 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 light-colored, 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
Goodsight-Cedar 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 interfingering 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 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 a 40Ar/39Ar age of
26.8Ma (Esser, 2003).
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 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 subrounded 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 (eg. 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 (eg. Lucas and Oakes, 1986; Repenning and
May, 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 stage IV or greater petrocalcic 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
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 sideslopes, 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. Downslope, 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
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 piedmont-slope alluvium and probably is locally interbedded with it.
Some active fans or other drainages in the area consist entirely of younger piedmontslope alluvium whereas others are more complex, consisting of a complex pattern of both
younger and older piedmont-slope 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
sheetfloods and to runoff in countless shallow, branching and anastomosing
drainageways; 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 sideslopes of Rincon Arroyo,and the southern
and eastern shores of Flat lake playa. Much of the sand is in the form of coppice dunes,
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 downslope, mostly by gravity. Most colluvial deposits are
cemented by stage IV soil carbonate and grade downslope 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 YostAleman 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
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.
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 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 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, hangingwall 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.
Ttrending north-northwest, parallel to the hills, the fault splay is downthrown to the east
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 hangingwall 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.
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 (Fig7). 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.
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
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).
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. Expolsive
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 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).
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
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). 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 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).
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).
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 hangingwall 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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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.
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. 257263.
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.
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Mountain area Dona Ana County, New Mexico: New Mexico Bureau of Mines
and Mineral Resources, Bulletin 97, 38 pp.
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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
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from basalts and the evolution of the southern Rio Grande rift: Geological Society
of America Bulletin, v. 95, pp. 87-99.
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County, Texas: Texas Memorial Museum, Austin, Bulletin 10, 55pp.
Figure captions
Figure 1. Location map, Prisor Hill and Upham Hills quadrangles.
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.
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.
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.
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.
Figure 6. Map of Jornada Draw fault zone, showing en echelon arrangement of fault
segments.
Figure 7. Paleogeographic map of south-central New Mexico in middle Eocene time
during deposition of the Love Ranch Formation.
Figure 8. Paleogeographic map of south-central New Mexico in late Eocene time during
deposition of the Palm Park Formation.
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.
Figure 10. Paleogeographic map of south-central New Mexico in Oligocene time (28.025.9 Ma) during emplacementof the Uvas Basaltic Andesite.
Figure 11. Paleogeographic map of south-central New Mexico in latest Oligocene or
early Miocene time during deposition of the Hayner Ranch Formation
Figure 12. Paleogeographic map of south-central New Mexico Pliocene to middle
Pleistocene during deposition of the Camp Rice Formation Formation.
Description of Units
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, Qlg
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
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 alluvial-fan 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 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, wellsorted 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, wellbedded 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 50m is exposed.
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.
Tb7
Bell Top Formation, ash-flow tuff 7—Light grayish-brown, vitric ashflow 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 pumice-rich 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.
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 intermediatecomposition 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 channelform 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 (1m 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.
Symbols
Qs/Qpo/Qcf; Qc/Tu; etc
Shows stratigraphy of surficial units above bedrock
in selected areas. Units shown in color on the
geologic map are those chosen to be emphasized.
Geologic contact, dashed where approximately
located
Normal fault, dashed where approximately
located or inferred, dotted where buried; ball on
downthrown side; arrow shows direction of dip
Synclinal hinge, dotted where buried
Strike and dip of bedding
Horizontal beds
Estimated strike and dip of lava flows or bedding
In sedimentary rocks
Figure 2
Figure 3
Figure 4
Figure 5