Geologic Maps of the Upham Hills and Prisor Hill Quadrangles, Sierra County, New Mexico By William R. Seager June, 2005 New Mexico Bureau of Geology and Mineral Resources Open-file Digital Geologic Map OF-GMs 113 and 114 Scale 1:24,000 This work was supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program (STATEMAP) under USGS Cooperative Agreement and the New Mexico Bureau of Geology and Mineral Resources. New Mexico Bureau of Geology and Mineral Resources 801 Leroy Place, Socorro, New Mexico, 87801-4796 The views and conclusions contained in this document are those of the author and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government or the State of New Mexico. Geology of the Upham Hills and Prisor Hills Quadrangles, Sierra County, New Mexico INTRODUCTION The Prisor Hill and Upham Hills 7 ½ minute quadrangles are located in the south-central part of the Jornada del Muerto, approximately 72 km north-northwest of Las Cruces and 45km southeast of Truth or Consequences, New Mexico (Fig. 1). Access to the area is provided by a paved road that joins Interstate 25 at the Upham interchange, then traverses the Jornada del Muerto northward past the Spaceport site to New Mexico highway 51 at Engle. This road skirts the western boundaries of both the Upham Hills and Prisor Hill quadrangles. Maintained, but unpaved, 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 unpaved roads in the quadrangles can become impassable or nearly so following heavy rains. Figure 1— Location map, Prisor Hill and Upham Hills quadrangles. 2 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 is Jornada Draw (Fig. 2), a south-flowing, axial ephemeral stream, which delivers runoff from the western piedmont slopes of the San Andres Mountains and eastern 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 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 3 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. 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 4 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. (1987) provides more stratigraphic information, but fails to identify the important Jornada Draw fault zone, as well as certain surficial deposits. Open-File geologic maps (1:24,000) of the adjacent Alivio, Upham, and Cutter quadrangles are available on line (Seager, 1995a; Seager, 1995b; Seager, 1995c). 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 quadrangles well as from 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; middle to late Miocene paleocanyon fill; Plio-Pleistocene Camp Rice Formation; and late Pleistocene and Holocene surficial deposits. Except for 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 5 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-flank to basin-center deposits. Love 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.