Geologic Map of the Prisor Hill Quadrangle, Sierra County, New Mexico

Transcription

1 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, 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.

2 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, 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

3 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. 2

4 The two quadrangles occupy the central, topographically lowest part of the Jornada del Muerto, an area near 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 3

5 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) 4

6 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 5

7 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 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). 6

8 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. 7

9 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 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 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 8

10 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 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 9

11 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, 10

12 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

13 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

14 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

15 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

16 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

17 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

18 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 Ma stocks, which may or may not represent magma chambers beneath volcanoes. 17

19 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

20 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

21 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 ( 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

22 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 ( 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

23 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

24 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