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Preliminary Geologic Map of the
Socorro County, New Mexico
David J. McCraw, David W. Love, and Sean D. Connell
Open-file Digital Geologic Map OF-GM 121
This work was supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program (STATEMAP) under USGS Cooperative Agreement 02HQAG0051 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.
Includes 1:24,000 scale digital geologic map.
The Abeytas Quadrangle covers the junction between the Rio Puerco and Rio Grande at the south end of the Albuquerque Basin. Rift-related Plio-Pleistocene basin fill of the Santa Fe Group exposed in the Abeytas Quadrangle came from four sources: the Rio Grande from the north, Abo Pass and Los Pinos uplift to the southeast, Rio Puerco and Rio San Jose from the northwest, and Rio Salado from the southwest. Quaternary post-basin-fill episodic entrenchment of the Rio Grande is shown by three major fluvial terraces preserved on the eastern side of the Rio Grande valley and by a correlative high terrace along the Rio Puerco. Local tributaries formed several inset levels of alluvial terraces in response to climate change and episodic downcutting and lateral planation by the Rio Grande. Thick valley fills of the Rio Grande and Rio Puerco are the hallmark of both major streams in this area. Several north-trending Quaternary faults displace deposits from less than one to more than 10 m. The Sabinal fault dies out southward in a monoclinal structure. The West Ceja fault dies out southward as the hanging wall becomes a ramp and joins the footwall at the same elevation. The newly-discovered Contreras Cemetery fault offsets the lowest Rio Grande terrace by 2 to more than 4 m and was active at the time of terrace deposition.
The Abeytas Quadrangle lies in central New Mexico about 50 miles south of Albuquerque along the Rio Grande and Interstate 25. The area covered by the quadrangle, which extends from 34°22’30”N to 34°30’N and 106°45’W to 106°52’ 30”W, is approximately 158 km2 (61 mi2). Elevations range from approximately 1,436 m (4,710 ft) where the Rio Grande flows out of the quadrangle to the south to the highest point on the southern Llano de Albuquerque, 1,564 m (5,132 ft). Villages located in the quadrangle are Abeytas, Bernardo, Contreras, Las Nutrias, Sabinal, and San Francisco.
The southeastern and southwestern map areas fall within the largely roadless Sevilleta National Wildlife Refuge and Long-Term Ecological Research area (Sevilleta LTER), which is closed to the public and requires a research permit for access. Access to the southeastern quadrant of the map is gained through a locked gate east of the town cemetery of Contreras on N.M. Route 304 (formerly N.M. 47 as depicted on the quadrangle’s base), but there are no maintained roads beyond the gate. Access to the southwestern part of the area is gained by a gated, unpaved road that heads southwest from a county road southwest of Bernardo, approximately 1 km southwest of the old decommissioned U.S.Highway 85 bridge over the Rio Puerco. Accessibility for the remainder of the quadrangle is quite good via paved roads (Interstate 25, U.S. Highway 60, and N.M. Route 304) and numerous unpaved county roads.
Notable geomorphic features include the Rio Grande and Rio Puerco valleys which meet in southern part of the quadrangle at the confluence of the Rio Puerco and Rio Grande, the southern tip of the Llano de Albuquerque at Picho Hill, the southwestern edge of the extension of the Llano de Manzano, and the northeast edge of the Llano de Salado/Ladrones piedmont (Cliff Surface). The Rio Grande valley is about 4 to 6.4 km (2.5 to 4 mi) wide and 105 to 125 m (roughly 350 to 400 feet) deep with broad, gently sloping valley borders. The Rio Puerco valley is about 3.2 km (2 mi) wide and 90 to 105 m (roughly 300 to 340 ft) deep with broad valley borders.
The Abeytas Quadrangle is in the southern part of the Albuquerque basin, a large structural basin of the Rio Grande rift. The basin narrows to the south into the Socorro constriction of the Rio Grande rift (Kelley, 1977). The quadrangle is above the northeastern margin of the Socorro Magma Body (Sanford and Long, 1965; Balch and others, 1997) and has undergone significant uplift historically (Reilinger and others, 1980; Larsen and others, 1986; Fialko and Simmons, 2001). The area has had a recent earthquake swarm (1989-1990) with the largest quakes of M 4.7 (Sanford and others, 2002). Several faults with Quaternary movement cut deposits in the quadrangle or in the surrounding quadrangles (Kelley, 1977; Machette and McGimsey, 1982; Machette and others, 1998). The COCORP group developed a seismic image of the basin fill, crust, and upper mantle across the southern Albuquerque basin and the Abeytas Quadrangle (Brown and others, 1979; DeVoogd and others, 1988). The deepest oil well in the quadrangle, drilled on the west side of the Llano de Albuquerque, reached a depth of 907.7 m (2,978 ft) and while well-log interpretations suggested Cretaceous rocks at the base of the hole, we consider this very doubtful.
The Rio Grande and the Rio Puerco in this quadrangle have been subjects of USGS Water Resources Division research for more than 40 years (e.g. Nordin, 1963; Friedman and others, in prep.). Davis and others (1994) and Davis and Cross (1994) surveyed reports concerning the Rio Puerco and found more than 1100 citations. It is one of the most sediment-laden streams in the world with a record 680,000 parts per million suspended sediment concentration (Bondurant in Nordin, 1963), and while it contributes only about four percent of the water, it contributes almost 72 percent of the sediment to the Rio Grande downstream from the confluence (Gorbach and others, 1996). In recent years, the amount of suspended sediment has declined (Gellis, 1991).
Groundwater exhibits significant changes in chemistry across the quadrangle (Spiegel, 1955; Roybal, 1991) with poor quality, high sulfate water on both margins of the Rio Grande valley.
Comments to Map Users
This quadrangle map has been Open-filed in order to make it available to the public. The map has not been reviewed according to New Mexico Bureau of Geology and Mineral Resources standards, and due to the ongoing nature of work in the area, revision of this map is likely. As such, dates of revision are listed in the upper right corner of the map and on the accompanying report. The contents of the report and map should not be considered final and complete until published by the New Mexico Bureau of Geology and Mineral Resources.
A geologic map graphically displays information on the distribution, nature, orientation, and age relationships of rock and surficial units and the occurrence of structural features such as faults and folds. Geologic contacts are irregular surfaces that form boundaries between different types or ages of units. Data depicted on this geologic map are based on field geologic mapping, compilation of published and unpublished work, and photogeologic interpretation. Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given contact onto a topographic base map; therefore, the accuracy of contact locations depends on the scale of mapping and the interpretation of the geologist. Significant portions of the study area may have been mapped at scales smaller than the final map; therefore, the user should be aware of potentially significant variations in map detail. Site-specific conditions should be verified by detailed surface mapping or subsurface exploration. Topographic and cultural changes associated with recent development may not be shown everywhere.
The cross-sections are constructed based on exposed geology, and where available, subsurface and geophysical data. The cross sections are interpretive and should be used as an aid to understand the geologic framework and not used as the sole source of data in locating or designing wells, buildings, roads, or other structures.
The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of either the State of New Mexico or the U.S. Government.
All depositional units of the Abeytas Quadrangle are late Neogene in age and include those related to basin fill in the Albuquerque basin and units that post-date the basin fill, either overlying or inset against the basin fill. No pre-Late Neogene bedrock crops out in the quadrangle, but both upper Paleogene and Proterozoic rocks crop out a few miles to the southeast (Myers and others, 1986; De Moor and others, 2005). The basin fill (Santa Fe Group) consists of sediments derived from the basin margins and fluvial sediments from beyond the basin margin transported by larger streams such as the Rio Grande, Rio Puerco, Rio Salado, and Abo and Salas Arroyos. We recognize the following formations of the Santa Fe Group in the Abeytas Quadrangle: Sierra Ladrones axial facies (QTsa), Sierra Ladrones piedmont facies (QTsp), the western fluvial fan facies of the Ceja Formation (Tc) and a western-southwestern mixed fluvial facies of the Ceja Formation (Tcs). The timing of arrival of the Rio Grande in the southern Albuquerque basin is not well constrained. To the north, the Rio Grande was established by 6 million years ago. In the Socorro area and farther south, the Rio Grande became a through-flowing stream in the rift by about 4.5 million years ago. The Albuquerque basin continued to aggrade along the Rio Grande until about 800,000 years ago (Connell and others, 2005). Tributaries are either graded to the top of the basin fill, or are graded to several inset levels between the top and the Rio Grande valley.
Basin fill of the Santa Fe Group.
The lowest exposures on the east side of the Rio Grande consist of partially to well-cemented red to yellowish red (2.5YR4/6 to 5YR5/6) sandstone, siltstone, and claystone, and paler, grayish pink (7.5YR7/2) well-cemented conglomeratic cross-bedded sandstones and less cemented finer-grained planar-low-angle cross-bedded sandstones. These cemented units exhibit dips in various directions on the order of 1 to 4 degrees. The finer-grained cross-bedded sandstone exposures near the present Rio Grande floodplain margin are mapped as QTsa, but their affiliation and provenance are not determined. Some channel-form conglomeratic units contain subangular intermediate volcanic clasts with imbrications to the northwest. The channels are interbedded with red claystone and red eolian (?) sandstones. We lumped these in with QTsp, but some of this unit may be much older because these exposures are reminiscent of Popatosa Formation (Miocene, closed rift basin deposits) northeast of Socorro.
Ancestral Rio Grande deposits included in the basin fill (QTsa) and known as Sierra Ladrones Formation (Machette, 1978a, 1978b; Connell and others, 2005) crop out only on the eastern side of the quadrangle. As summarized above, exposures low on the landscape are extremely rare and may or may not be part of the Rio Grande system or may be part of the pre-Quaternary basin fill. The contact between typical Rio Grande crossbedded pebbly sands and underlying partially indurated red sand, silt and claystones and pinkish gray conglomerates of QTsp is traceable throughout most of the southeastern corner of the map. This is also the base of deposits known to have pebbles of Rabbit Mountain obsidian from the Jemez Mountains (40Ar/39Ar age of 1.428 ± 0.003 Ma; Love and others, 2004) at an elevation of 1,504 m (4,935 ft). The upper surface of QTsa is confined to the highland south of Pino Draw and extends eastward into the Black Butte and Becker SW quadrangles. The aggradational top is preserved at an elevation of about 1,530 m (5,020 ft) beneath piedmont deposits that form the constructional top of the Santa Fe Group (diagonal map pattern of QTsp) in the southeastern part of the quadrangle. The ancestral Rio Grande deposits interfinger with coarse alluvial channel deposits from the east toward the eastern edge of the quadrangle and up section (see cross section B-B') and are mapped as a transitional facies, QTst.
On the west side of the Abeytas quadrangle are deposits we correlate to the Ceja Formation (Tc) farther north in the Albuquerque basin. The Ceja Formation of Connell (in prep., c.f. Kelley, 1977) ranges to 700 m thick near Albuquerque and consists of coarse and fine alluvial deposits derived from the Rio Puerco and Rio San Jose on the west side of the Albuquerque basin. Clasts in the gravel portion of gravelly sand channels in the unit tend to be bimodal, with larger clasts (ranging to small boulders in size) commonly basalt, intermediate volcanic rocks, granite, Cretaceous sandstone, and Miocene chalcedony ("Pedernal Chert"). The smaller pebbles are well-rounded, reworked siliceous clasts derived from Paleogene and Mesozoic conglomerates, early Cenozoic and Cretaceous petrified wood, and Cretaceous pelcypod fossils. The Ceja Formation also includes several intervals of fine-grained sediments cut by steep-sided arroyo-channel-like sediments (Davis and others, 1993). The upper 53 m of the Ceja Formation in the southern Albuquerque basin include obsidian from East Grants Ridge, which traveled down the Rio San Jose drainage (Morgan and others, 2001). This obsidian has a K-Ar age of 3.2±0.3 Ma (Lipman and Mehnert, 1980) so the upper 50 m of the formation is less than about 3 million years old. This age is corroborated by the presence of Blancan fossils in the section, including a Blancan-age horse leg bone (G. Morgan, personal commun., 2006) found near the West Ceja fault on the Abeytas quadrangle during the course of mapping. The Llano de Albuquerque at the top of the Ceja Formation represents the quasi-stable top of aggradation in this part of the Albuquerque basin. Pedogenic calcium carbonate accumulation in the soil of the Llano de Albuquerque is Stage III+ with a thickness on the order of 1.5-2 m.
On the southwest edge of the Abeytas quadrangle and in adjacent quadrangles to the west and south, fine-grained (fine sand, silt, and clay) alluvium and pebbly to bouldery sand channels are stacked in a set of units at least 150 m thick. Large angular and subangular cobbles and boulders of Precambrian igneous and metamorphic rocks, Paleozoic limestone, Paleogene volcanic rocks derived from the Rio Salado and Sierra Ladrones are added to obsidian from Grants and the regular suite of pebbles of the Ceja Formation. We map these exposures as Ceja Formation with a Salado component (Tcs), but recognize that this additional facies with sediment contributions from the west may warrant a separate formation name. The top surface of these deposits was termed the Cliff Surface by Machette (1978b). We suspect that there is more than one surface west of our quadrangle, so we suggest the addition of the Llano de Salado and Ladron piedmont surfaces.
Major River Terraces.
In the Abeytas quadrangle, three terraces of Rio Grande gravels form discontinuous exposures along the eastern side of the map area and on adjacent quadrangles. As is the case farther north (see description of the terraces in the Albuquerque area below), the top of the basin fill is not considered to be a terrace. All three terraces rest on older basin fill (see cross sections), although the lowest terrace may also rest on earlier terrace deposits at least locally.
The first (highest) Rio Grande terrace-gravel deposit (Qrgt1) ranges from 3 to 13 m thick with a base about 50 m above the Rio Grande floodplain (Figure 1). The ridge
Figure 1. Well cemented trough-crossbedded sandstone of QTsp overlain by gravel of terrace Qrgt1 exposed in Maes Arroyo. Not shown are 15 + m of Maes terrace alluvium Qam2 overlying Qrgt1.
illustrated in cross section B-B' displays a well-exposed buttress unconformity at the eastern limit of this terrace. The second terrace gravel deposit (Qrgt2) crops out in isolated places, is only 3 m thick, and has a base 38 m above the Rio Grande floodplain. The third level terrace deposit (Qrgt3) is the most extensive, both north-south and laterally from the floodplain east several hundred meters (see cross sections). This terrace's base is just above floodplain level to 29 m above the floodplain (24-27 m thick). Lateral incision of older basin fill during aggradation of this terrace formed a paleo-bluff line (buttress unconformity) that was later overtopped with a laterally more extensive gravel, sand, and floodplain deposit (Figure 2). Where the lower gravel and bluff line are stripped by tributary erosion, they were initally misinterpreted as a fourth terrace level near the margin of the valley. To the east and above the eroded gravel-lag surface, finer sand, silt, and clay of the ancestral river interfinger with alluvium from the eastern tributaries (Qam2). The tributary alluvial deposits are discussed below.
In the southwestern part of the Abeytas Quadrangle, we found a broad terrace (Qrpt1) about 45-50 m above the Rio Puerco valley floor. Terrace deposits appear to be 3-4 m thick. While these terrace sands and gravels exhibit a Rio Puerco / Rio San Jose provenance, we tentatively correlate this terrace with Qrgt1 on the opposite side of the Rio Grande valley.
North of the Abeytas Quadrangle, Rio Grande terraces in the greater Albuquerque area have been described by numerous workers, most lately by Connell and Love (2001). Each terrace has been given a separate formation name in order to distinguish similar-looking deposits at different geomorphic positions along the valley borders. The highest
Figure 2. Basal exposure of Rio Grande terrace Qrgt3 overlying red sandstone (note small uncovered area to the leftmost center). Well-sorted, well rounded river gravel is overlain and interfingers with tributary arroyo alluvium of Qam2.
surface with Rio Grande deposits within the Albuquerque basin, the Sunport, or Mesa del Sol surface is the top of the basin fill and is not considered a terrace. The highest terrace, called the Lomatas Negras Formation, is typically 70-75 m above the Rio Grande valley floor, is commonly less than 5 m thick, and includes Lava Creek B ash (0.639±0.002 Ma; Lanphere and others, 2002). The Edith Formation has a base typically 12-24 m and a top about 30-40 m above the valley floor. It contains Rancholabrean fossils but is older than the Los Duranes Formation. The Los Duranes Formation is a predominantly non-gravelly fluvial terrace with a base just below the modern valley floor and a top 40-52 m higher. A basalt flow from the Albuquerque volcanoes is a tongue within the Los Duranes Formation and has an age of 156±20 ka (Peate and others, 1996). The Cat Hills basalt overlies the formation west of Isleta Pueblo and has an age of 98-110 ka (Maldonado and others, 1999). The Menaul Formation is a 3-m thick gravel unit which interfingers with piedmont deposits above the Edith Formation, but below the top of the Los Duranes Formation. The Menaul Formation may be a coarser facies of the Los Duranes Formation. The lowest terrace, the Arenal Formation, is inset against the Los Duranes Formation, is 3-6 m thick and has a top 15-21 m above the valley floor. The valley in the Albuquerque area is underlain by 15-29 m of fill, primarily of sandy Rio Grande fluvial deposits called the Los Padillas Formation (Connell and Love, 2001).
Terraces and age-determined deposits along the Rio Puerco upstream of Abeytas include lower terraces at 3 m, 6-8 m, 24-30 m, 35-40 m, and 55-61 m, as well as the Llano de Albuquerque at the top of the Ceja Formation 145 m above the valley floor (Love and Connell, 2005). The top of the Ceja Formation contains 2.6 Ma pumice. Lava Creek B ash is found 67 m above the valley floor, and a lava flow with K-Ar ages from 0.28 to 0.32 Ma is 27 m above the valley floor. Valley fill beneath the current valley floor is 34-41 m thick, with archaeological sites ranging back to about 4,000 years ago in the upper 1-2 m. Several cycles of Holocene arroyo cut-and-fill have been described by Love and others (1982). The age of the lower valley fill of the Rio Puerco has yet to be determined. Upstream, the Rio San Jose tributary to the Rio Puerco achieved maximum incision and subsequent valley fill before late Pleistocene time (before 120 ka). On the other hand, the Rio Grande apparently achieved maximum incision in the past 20,000 years and accumulated 20 m of valley fill in the past 10-12,000 years (Love and Connell, 2005).
Machette (1978b; 1985) described the Cliff Surface north of the Rio Salado in the San Acacia quadrangle to the southwest of the Abeytas quadrangle as being post-Sierra Ladrones Formation. Machette also described two terraces north of the Rio Salado below that. The higher terrace projects about 63 m above the Rio Salado and is offset by about 6 m along the Cliff fault. The lower terrace has a constructional top 32-34 m above the Rio Salado.
Tracing the Rio Grande terraces from north to south along the river bears on the history of surface uplift and deformation related to the Socorro Magma Body. Ouchi (1983; 1986) reported that terraces ascended across the surface uplift to the south. He did not report actual terrace locations and how he correlated them. We found no evidence of north or eastward tilting of any of the terraces on the Abeytas Quadrangle. In fact, we found only a fault offsetting the lowest terrace (see below).
Valley floor deposits.
The alluvium of the Rio Grande, occupying the valley floor running north to south through the Abeytas Quadrangle, averages 4.4 km (2.7 mi) wide and is thought to be approximately 25 m deep. While technically a braided stream system, the channels of the Rio Grande collectively have aggraded and meandered back and forth across (and thus widening) its floodplain throughout the Holocene, revealing cross-cutting relationships amongst these fluvial deposits. Pearce and Kelson (2004) developed a surficial geologic map of these deposits along a reach from Isleta to San Acacia, crossing some ten 1:24,000 quadrangles including Abeytas, based upon 1935-vintage aerial photography. Their study focused on both historic geomorphic and vegetative changes along this reach by comparing these 1935 photo data to 2001 aerial photos. Their 1935 stereo-photographic analysis was extremely useful for delineating surficial geologic units because this dataset predated the 1940s-1950s infestation of tamarisk, or salt cedar (Tamarix pentandra) that covered floodplains of the southwest, especially the middle Rio Grande and its tributaries, and floodplain agriculture was less extensive and disruptive at this time as well.
For the purposes of this map, we deemed that there was very little modification of the work of Pearce and Kelson (2004) necessary. We reorganized their 1935 data into a more useful morpho-stratigraphic framework by combining several of their units (described below), reinterpreted the western floodplain margin adjacent to the confluence of the Rio Puerco with the Rio Grande, with field mapping and additional photo-interpretation using 1993-vintage, large scale (1:7,920) aerial photography, and redefined the modern channel of the Rio Grande (Qrgc2) using 1996 orthophotograph quarter quads at a scale of 1:12,000. We did not reclassify areas that have undergone extensive modification (bulldozing, etc.) for tamarisk removal and farming.
Thus, we have subdivided the alluvium of the Rio Grande into two channel deposits (Qrgc2 and Qrgc1), two meander-bend deposits (Qrgm2 and Qrgm1), which mark former channel localities, and undifferentiated valley bottom alluvium (Qrg). For the modern channel, we combined Pearce and Kelson’s (2004) units “W35” (the 1935 active channel), “Rib35” (1935 channel in-stream bars), and “Rsb35” (Recent [i.e., early Historic time--16th-19th centuries to 1935] channel scroll bar deposits). Older channel (predominantly sand) deposits (Historic to Holocene) were combined from their units “Hch” (channel deposits), “Hib,” (in-stream bars), and “Hsb” (scroll bar deposits). Likewise, youngest meander-bend deposits (Qrgm2 -- Historic) were combined from “Rch35” (Recent channel deposits), “Rcs35” (Recent crevasse splay deposits), and “Rcb35” (Recent meander bend deposits), while older meander-bend deposits (Qrgm1 – Historic to Holocene) were combined from units “Hcb” (channel meander-bend deposits) and “Hcs” (channel crevasse splay deposits).
The alluvium of the Rio Puerco valley is divided into several general units based also on geomorphic position and cross-cutting relationships. The deposits on broad alluvial slopes of the valley margins that are a combination of reworked Ceja Formation and eolian sand are mapped as Qaeo if they contain significant pedogenic horizons that indicate surface stability. Younger footslope alluvial and eolian deposits that lack near-surface pedogenic horizons are mapped Qae. The broad valley floor (Qarp) is alluvium deposited primarily by overbank flow of the Rio Puerco and tributaries that used to flow down-valley parallel to the main stream. Alluvium accumulated below the valley floor includes both paleochannels and overbank deposits and probably is at least 20 m thick. Upstream, the valley fill is up to 40 m thick. Inset below the level of the valley floor along the arroyo margins are intermediate alluvial terraces and an older floodplain and oxbow channels (Qarpm). Below that is the active Rio Puerco channel (Qarpc2) and active floodplain (Qarpc1; cross section B-B'). Bryan (1925) and Bryan and Post (1927) attempted to establish when the Rio Puerco incised its valley floor in the 19th century. Historic photographs of the Rio Puerco channel in the vicinity of the railroad and U.S. Highway 85 bridges show remarkable channel changes during the 20th century from a broad, shallow, braided channel to one trapezoidal silt-sided meandering channel between aggraded tamarisk-stabilized banks now (Qarpc1 and Qarpc2; J.Wall and C. Gorbach, unpublished compilation). Elliot (1979), Meyer (1989), and Gellis (1991) addressed the possible evolution of the modern Rio Puerco channel through time, while Love and others (1982) and Love and Young (1983) described several buried Holocene arroyo channels within the Rio Puerco valley fill upstream from the Abeytas Quadrangle.
We mapped a separate deposit inset below the Rio Puerco valley floor on the northern margin of the valley (Qarps). This deposit partially fills an eastward-trending swale eroded into the valley floor and probably fed by runoff from the valley border to the north. This swale appears to have developed in response to gradient increases due to trimming Rio Puerco valley floor deposits by the Rio Grande east of Bernardo.
Tributary stream-valley alluvium and other alluvium.
Alluvium associated with the development of tributaries to the Rio Grande and Rio Puerco is found at several levels along the drainage courses. In the southeastern part of the quadrangle, the alluvium is a bedded mixture of sand and coarse cobble and boulder gravel. The coarse clasts are subangular to subrounded pebbles and cobbles of limestone, sandstone, granitic, metamorphic, and sparse volcanic rock types indicating derivation from drainages from uplands to the east. The oldest alluvium (Qao) is inset below the top of basin fill and either parallels incised drainages or spreads out as piedmont deposits not readily associated with modern drainages. Its surface and soil horizons are mostly stripped. Inset below the oldest tributary alluvium are flights of at least three tributary terraces along major drainages (but not smaller ones) and designated as Qam1, Qam2, and Qam3. Locally, a fourth terrace, Qam4, is adjacent to the modern floodplain and modern tributary channels (Qay and Qafy). Soil development increases from negligible on modern floodplains and Qam4, to pedogenic carbonate Stage I on Qam3, to Stage II on Qam2, to Stage III on Qam1.
Alluvium at the mouth of Abo Arroyo (lumped as Qam) in the northeastern corner of the Abeytas quadrangle is very extensive and quite thick (up to 21 m). The bulk of the accumulation overrides the top of Rio Grande terrace (Qrgt3) and probably buries Qrgt2 east of the edge of the quadrangle as well. The lack of soil development on this accumulation is surprising, but perhaps the stable top has been stripped, much like Qao to the south. North of Abeytas Quadrangle the valley of Abo Arroyo is inset 45 m below this surface and is graded to the modern Rio Grande.
Alluvium, colluvium, and eolian deposits deposited downhill from fault scarps and raised footwalls on the Llano de Albuquerque are mapped as Qase (slope deposits) and as Qag (graben fill). The footwall block making the east side of the graben on the adjacent Veguita quadrangle is more than 20 m higher than the graben floor and the slope is several hundred m wide so Qase is extensive. The thickness of graben fill is unknown on the Abeytas quadrangle. On the Veguita quadrangle, exposures of Qag show multiple soils developed on fill more than 10 m thick (see Figure 7-- a photograph of similar soils in hanging- wall deposits of West Ceja fault below).
Eolian and mixed deposits.
Eolian deposits are most extensive on the Llano de Albuquerque and overlie the soil developed there. Thin sand cover is mapped as Qe/Tc and thicker sand is mapped as Qe. More extensive vegetationally stabilized eolian sand sheets and low dunes that appear to be older are mapped as Qedi. Larger active dunes are mapped as Qed. Interdune areas are mapped as Qe/Tc.
Slope deposits along valley borders are mapped as Qae and Qaeo, showing that they are deposits from colluvial, alluvial, and eolian processes. The old deposits (Qaeo) have significant pedogenic calcium carbonate accumulation in soils on slopes and are found on slopes below the top of basin fill and the highest river terraces. Younger slope deposits along the Rio Grande can be divided into Qae1 on higher alluvial sloping surfaces and Qae2 inset below and adjacent to the Rio Grande floodplain.
Discussion of stratigraphic relationships.
The correlation chart shows relative times when various deposits are thought to have accumulated. Notice that Tc, Tcs, QTsa, QTst, and QTsp are all thought to have accumulated at the same time during the Pliocene and perhaps early Pleistocene. However, nowhere in the quadrangle do we see the relationship between the ancestral Rio Grande (QTsa) and its western tributaries, the Rio Puerco (Tc) and Rio Salado (and Puerco, Tcs). Therefore, on the cross sections, we have placed the contact between Tc and QTsa and between Tcs and QTsp under the present Rio Grande Valley.
Cross-cutting stratigraphic relationships in southeastern Abeytas Quadrangle are complicated. Tributary stream alluvium Qam2 cross-cuts Rio Grande terrace Qrgt2 (cross section B-B'), interfingers with the uppermost deposits of Rio Grande terrace Qrgt3, and progrades across Qrgt3 to its westernmost exposures (Figure 3). Therefore it must post-date Qrgt2 and is contemporaneous to and largely post-dates Qrgt3. Tributary stream alluvium Qam3 and Qam4 are inset below Qrgt3 and are younger than that terrace.
The extent of Rio Puerco Valley floor deposits Qarp southwest of the Rio Puerco Valley margin, and perhaps east to the middle of the Rio Grande valley suggests that sediment delivery to the Rio Grande valley dominated deposition by either river, but the Rio Grande had larger erosive power to remove Rio Puerco deposits farther east. These deposits need to be investigated in detail.
Figure 3. Mouth of Maes Arroyo looking southwest. Red unit of sand, silt and clay (QTsp) on opposite bank underlies terrace Qrgt3 and forms a buried bluff line near the center of the photo. Terrace Qrgt3 continues east (to left and in foreground). Road in middle ground is on Maes Arroyo terrace Qam3 and Maes Arroyo channel is between Qam3 and QTsp.
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