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Hydrology, Geochemistry, and Geology Group, P.O. Box 1663, Los Alamos National Laboratory, Los Alamos, NM, 87544
* Corresponding author (broxton{at}lanl.gov)
Received 28 April 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL GEOGRAPHIC SETTINGREGIONAL GEOLOGIC FRAMEWORKRELATION BETWEEN GEOLOGIC AND...GEOLOGIC SETTING OF GROUNDWATER...CONCLUSIONSREFERENCES |
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Abbreviations: LANL, Los Alamos National Laboratory
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOGRAPHIC SETTINGREGIONAL GEOLOGIC FRAMEWORKRELATION BETWEEN GEOLOGIC AND...GEOLOGIC SETTING OF GROUNDWATER...CONCLUSIONSREFERENCES |
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Water quality is typically good, but the effects of LANL operations can be detected in parts of the groundwater system. Locally, shallow and intermediate perched groundwater systems of the vadose zone contain elevated concentrations of mobile contaminants such as tritium, nitrate, high explosives, and perchlorate (LANL, 2001). Although the regional groundwater system shows little effect from LANL activities, low concentrations of tritium, nitrate, and perchlorate are found in several test and water supply wells. In 1998, a hydrogeologic work plan (LANL, 1998) was prepared to address New Mexico Environment Department concerns about LANL's impact on local and regional groundwater systems. A major component of the hydrogeologic work plan included installation of up to 32 deep characterization wells in the regional aquifer. These wells are designed to provide a better understanding of the hydrogeologic framework at LANL, to define areas of groundwater contamination, and to provide information needed to design and implement a detection-monitoring program. Since 1998, 27 new regional-aquifer wells have been installed (Fig. 3).
We describe the geologic framework of groundwater systems beneath the Pajarito Plateau based on these new wells and provide a hydrogeologic framework for discussions by other papers in this special volume. The physical and geologic setting of the plateau is described, and the influence of geologic factors on hydrology is discussed. Interpretations based on the new data are used to refine hydrogeologic conceptual models for the Pajarito Plateau. These conceptual models provide a framework for site-wide geologic, geochemical, and numerical flow and transport models for both the vadose zone and the regional saturated zone.
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REGIONAL GEOGRAPHIC SETTING |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOGRAPHIC SETTINGREGIONAL GEOLOGIC FRAMEWORKRELATION BETWEEN GEOLOGIC AND...GEOLOGIC SETTING OF GROUNDWATER...CONCLUSIONSREFERENCES |
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The Pajarito Plateau is an east-sloping plateau bounded on the west by the Sierra de los Valles and on the east by the valley of the Rio Grande, including White Rock Canyon (Fig. 2). The plateau is deeply dissected, and it consists of numerous fingerlike mesas separated by deep canyons containing east- to southeast-draining streams that are mostly ephemeral and intermittent. The canyons tend to be deep and narrow in the western part of the plateau where streams are incised in the welded tuff units. The canyons become wider and shallower eastward where nonwelded tuffs overlie resistant basalt and coarse volcaniclastic deposits. Mesa top elevations range from approximately 2350 m (7700 ft) on the flanks of the Jemez Mountains to about 1900 m (6200 ft) at their eastern termination above the Española valley. The eastern margin of the plateau stands 100 to 300 m (300–1000 ft) above White Rock Canyon, a 25-km (14-mi)–long gorge cut by the Rio Grande that separates the Pajarito Plateau from the Caja del Rio Plateau (Fig. 2). Elevations along the Rio Grande range from 1676 m (5500 ft) at Otowi crossing to 1634 m (5360 ft) at the mouth of Frijoles Canyon 18.5 km (11.5 mi) downstream (Fig. 2 and 3).
The crest of the Sierra de los Valles forms a surface-water divide for the eastern Jemez Mountains (Griggs, 1964; Purtymun, 1995). Streams on the east slopes of the Sierra de los Valles and on the Pajarito Plateau flow eastward to the Rio Grande, the master drainage for the region. Streams west of the divide flow west across the Valles caldera via the Jemez River that joins the Rio Grande south of the Jemez Mountains (Griggs, 1964; Purtymun, 1995). The creek in Frijoles Canyon (Rito de los Frijoles), south of LANL (Fig. 3), is the only perennial stream on the Pajarito Plateau (Griggs, 1964). Other canyons on the plateau support short reaches of perennial flow fed by springs or by releases of treated municipal and industrial wastewater. Within these reaches surface water travels down canyon for short distances before infiltrating alluvial deposits beneath the channel. Ephemeral and intermittent flow in canyons occurs because of snowmelt and runoff from rain. Runoff from large snowmelt or thunderstorm events occasionally reaches the Rio Grande (Griggs, 1964).
Los Alamos has a semiarid, temperate mountain climate. Summer temperatures range between 10 and 16°C (50 and 60°F) at night and are usually below 32°C (90°F) during the day (Bowen, 1990). Winter temperatures typically range from about –9 to –4°C (15–25°F) during the night and from –1 to 10°C (30–50°F) during the day. From 1910 through 1989, the annual precipitation ranged from about 36 cm (14 in) on the east side of the plateau to about 46 cm (18 in) on the west side. Forty percent of the annual precipitation normally occurs from thundershowers during July and August. Winter precipitation falls primarily as snow, with accumulations of about 130 cm (51 in) annually. Annual potential evapotranspiration is much greater than the amount of annual precipitation. Annual mean evaporation of shallow freestanding water (Class A pan evaporation) ranges from approximately 165 cm (65 in) in the Jemez Mountains to 190 cm (75 in) at White Rock (Bowen, 1990).
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REGIONAL GEOLOGIC FRAMEWORK |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOGRAPHIC SETTINGREGIONAL GEOLOGIC FRAMEWORKRELATION BETWEEN GEOLOGIC AND...GEOLOGIC SETTING OF GROUNDWATER...CONCLUSIONSREFERENCES |
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Structural Setting
The Española basin is superimposed on an eroded terrane of Cretaceous-Eocene Laramide uplifts (Baltz, 1978; Cather, 1992, 2004; Smith, 2004). The greater San Luis uplift was a broad, north-trending Laramide highland in north-central New Mexico and south-central Colorado (Cather, 2004). Recent work suggests the southern end of the San Luis uplift, called the Pajarito uplift by Cather (1992)(2004), was tectonically inverted during formation of the Española basin by Neogene extension. The Pajarito uplift is bounded on the east by the Picuris-Pecos fault zone in the Sangre de Cristo Range and on the west by the Pajarito fault. During Laramide compression, the Pajarito fault was a westward-verging reverse fault (Cather, 1992), but it was reactivated as a down-to-the-east normal fault when the Española basin formed (Cather, 1992, 2004; Smith, 2004). The late-Laramide El Rito and Galisteo basins flank the Pajarito uplift on the west and south, respectively. These basins contain synorogenic red-bed sequences of sandstone, conglomerate, and mudstone (Cather, 2004). In the vicinity of the Pajarito Plateau, the El Rito basin lies between the Pajarito fault and the east flank of the Laramide Nacimiento uplift (Cather, 2004), where it is covered by Miocene to Pleistocene rocks of the Jemez volcanic field.
Neogene extension in northern New Mexico was likely part of a continuum of deformation from the Laramide orogeny and exploited structures produced by faults of Laramide and older ancestry (Cather, 2004; Smith, 2004). The structural development of the Española and adjacent basins began about 30 Ma or earlier (Chapin, 1979; Baldridge et al., 1980, 1984; Seager et al., 1984; Smith, 2004).
The Española basin is a west-tilted, half graben (Kelley, 1952, 1978; Biehler et al., 1991; Ferguson et al., 1995; Smith, 2004) filled by Santa Fe Group sediments derived from Precambrian-cored highlands located primarily to the east and north (Galusha and Blick, 1971; Cavazza, 1989; Ingersoll et al., 1990) and by volcaniclastic sediments derived from the Jemez volcanic field (Smith et al., 1970; Ingersoll et al., 1990; Smith, 2004). The western front of the Sangre de Cristo Range marks the eastern-hinged margin of the Española basin (Kelley, 1978; Biehler et al., 1991; Ferguson et al., 1995). The western structural margin of the Española basin is partly covered by rocks of the Jemez volcanic field, but probably includes a broad zone of north-trending faults such as the Cañada de Cochiti fault zone (Fig. 1) that cut older volcanic units in the south-central part of the volcanic field (Gardner and Goff, 1984).
Gravity data (Biehler et al., 1991; Ferguson et al., 1995) indicate the deepest part of the Española basin coincides with three deep, intrabasinal grabens arrayed along the Pajarito and Embudo fault systems. From north to south, these subbasins include the Velarde graben (Manley, 1979, 1984), a north-northeast–trending basin beneath Santa Clara pueblo, and a north-trending basin near Los Alamos. The Pajarito fault zone forms the western boundary the Los Alamos subbasin (Biehler et al., 1991; Ferguson et al., 1995; Smith, 2004). Gravity data suggest that the eastern boundary is bounded by buried faults that lie east of the southern projections of the Rendija Canyon and Guaje Mountain (Ferguson et al., 1995).
The present active boundary on the western side of the Española basin is the Pajarito fault zone, a narrow band of north- and northeast-trending normal faults that delineate the western margin of the Pajarito Plateau (Fig. 3) (Griggs, 1964; Smith et al., 1970; Golombek, 1983; Gardner and Goff, 1984). Neogene displacement along the Pajarito fault zone is dominantly down to the east with episodic faulting indicated by progressively larger offsets in older rock units (Griggs, 1964). The fault, which forms a 120-m (400-ft)–high escarpment on the western margin of the plateau, has the surface expression of a large, north-trending, faulted monocline (Gardner et al., 1999, 2001). Along strike the fault varies from a simple normal fault to broad zones of small faults, faulted monoclines, and unfaulted monoclines. These varied styles of deformation are all considered expressions of deep-seated normal faulting (Gardner et al., 1999). The amount of fault displacement for older rock units is not known because thick deposits of Bandelier Tuff cover critical relations. Stratigraphic separation on the Tshirege Member of the Bandelier Tuff (1.22 Ma) ranges between 80 and 120 m (260–400 ft) along the segment of the fault west of LANL (Gardner et al., 1999). Holocene movements and historic seismicity indicate this fault system is still active (Gardner and House, 1987; Gardner et al., 1990).
Other major faults on the Pajarito Plateau include the Rendija Canyon and Guaje Mountain faults (Fig. 3). The Rendija Canyon fault, located in the northern part of the plateau, is a north-trending normal fault with down-to-the-west displacement. The Diamond Drive graben (DDG in Fig. 3), which displaces Bandelier Tuff, is bounded by the main trace of the Pajarito fault on the west and the Rendija Canyon fault on the east (Gardner et al., 1999). The Rendija Canyon fault dies out as a simple normal fault on the north side of LANL. Southward across LANL it is replaced by a broad arc of small-displacement faults that trend in a southwesterly direction toward the main trace of the Pajarito fault (Gardner et al., 1999, 2001). The Guaje Mountain fault lies east of and is generally parallel to the Rendija Canyon fault (GMF in Fig. 3). It is also a north-trending normal fault with down-to-the-west displacement. Surface traces of the Guaje Mountain fault die out north of LANL.
Where well exposed on the east side of the Española basin, Santa Fe Group rocks are cut by numerous, north-trending normal faults (Kelley, 1978; Koning and Maldonado, 2001; Koning, 2002; Koning et al., 2002). Similar fault densities and orientations are probably present in the older rocks beneath the Pajarito Plateau beneath the cover of Bandelier Tuff.
Volcanic Setting
Jemez Volcanic Field
The Jemez volcanic field lies at the intersection of the northeast-trending Jemez lineament, a major crustal structure of Precambrian ancestry, and north-trending faults of the Rio Grande rift (Mayo, 1958; Goff and Gardner, 2004; Karlstrom et al., 2005). Volcanism during the last 14 Ma built up the constructional highlands of the Jemez Mountains, while contemporaneous tectonic rifting resulted in subsidence of the area extending from the Valles caldera to the western margin of the Sangre de Cristo Mountains. During this time of coeval volcanism and rifting, the Jemez volcanic highlands were a source of Miocene and Pliocene volcaniclastic sediments that were deposited as alluvial fans in the rifted lowlands. To the east, these alluvial fan deposits interfinger with basin-fill sediments derived from Precambrian sources to the east and north.
The Jemez volcanic field began to develop between about 14 and 10 Ma with the eruption of predominantly basaltic and rhyolitic rocks of the Keres Group (Gardner et al., 1986; Goff and Gardner, 2004). Early basalts were erupted from vents located in the southern and northeastern parts of the volcanic field. Ages of southern Keres Group basalts (13.2 ± 1.2 to 8.96 ± 0.76 Ma) overlap with those for the northeastern Lobato basalts (14.05 ± 0.33 to 7.6 ± 0.4 Ma) (Gardner et al., 1986; Chamberlin et al., 1999; Goff and Gardner, 2004). The Canovas Canyon Rhyolite of the Keres Group is made up of rhyolite domes and their associated pyroclastic deposits that were erupted from 12.4 ± 2.0 to 8.8 ± 0.7 Ma from vents aligned along faults of the Cañada de Cochiti fault zone (Gardner et al., 1986; Goff et al., 1990; Goff and Gardner, 2004). From 10.6 ± 1.4 to 7.1 ± 0.2 Ma, 1000 km3 (240 mi3) of andesite and subordinate basalt and rhyodacite were erupted as part of the Paliza Canyon Formation (Gardner and Goff, 1984; Gardner et al., 1986; Chamberlin et al., 1999). High-silica rhyolite plugs, domes, and tuffs of the Bearhead Rhyolite, including thick tuffaceous deposits of the Peralta Canyon Member, were erupted from 6.01 ± 0.05 to 7.1 ± 0.2 Ma (Justet, 1996; Smith, 2001) along faults of the Cañada de Cochiti fault zone. The period from 6 to 7 Ma also coincided with a transition to predominantly dacitic volcanism throughout the volcanic field (Gardner et al., 1986). Porphyritc dacitic lavas of the Tschicoma Formation of the Polvadera Group were erupted primarily between 5 and 3 Ma (Goff and Gardner, 2004; G. WoldeGabriel, 2003, personal communication) from large, overlapping dome complexes typified by the extensive exposures of this formation in the highlands of the Sierra de los Valles west of the Pajarito fault zone.
Volcanism in the Jemez volcanic field reached a climax with eruption of the Bandelier Tuff from the Valles and Toledo calderas and (Griggs, 1964; Smith and Bailey, 1966; Bailey et al., 1969; Smith et al., 1970; Self et al., 1986). The Bandelier Tuff has two members, each consisting of a basal pumice fall overlain by a petrologically related succession of ash-flow tuffs (Bailey et al., 1969). Eruption of the two members was accompanied in each case by caldera collapse. The Otowi Member (1.61 Ma; Izett and Obradovich, 1994; Spell et al., 1996) was erupted from the Toledo caldera that was nearly coincident with and was largely destroyed by the younger Valles caldera (Smith and Bailey, 1966; Bailey et al., 1969; Smith et al., 1970; Self et al., 1986). The Valles caldera formed during the eruption of the Tshirege Member (1.22 Ma; Izett and Obradovich, 1994; Spell et al., 1990, 1996). About 300 km3 (72 mi3) of high-silica rhyolite magma was erupted for each of the two Bandelier Tuff members. Deposits of Bandelier Tuff form tuff plateaus (Pajarito on the east and Jemez on the west, Fig. 2) that dip away from the central volcanic highlands. The Pajarito Plateau is made up of Bandelier Tuff that flowed more than 21 km (13 mi) across the western Española basin.
A period of about 400 Ka separated the eruptions of the two Bandelier members (Izett and Obradovich, 1994; Spell et al., 1996). During this time, Cerro Toledo Rhyolite erupted from domes located northeast and southeast of the earlier Bandelier caldera (Smith et al., 1970; Self et al., 1986; Heiken et al., 1986; Stix et al., 1988; Spell et al., 1996). Tephras from these domes were deposited as ash and pumice falls over the Sierra de los Valles and Pajarito Plateau.
Post-Valles caldera volcanic vents are restricted to the floor of the Valles caldera. Early rhyolitic domes, lava flows, and tuffs of the Valles Rhyolite were erupted during resurgence of the caldera floor and during emplacement of the circular arrangement of domes in the caldera moat believed to represent eruptions along ring-fracture faults (Smith and Bailey, 1968; Smith et al., 1970). Resurgent dome growth and associated volcanic activity was probably completed within 50 Ka of caldera collapse (Goff et al., 2003). The northern moat rhyolite domes were erupted between 1.13 ± 0.01 and 0.52 ± 0.01 Ma (Spell and Harrison, 1993). After a hiatus of about 470 Ka, the youngest rocks of the Jemez volcanic field, including rhyolitic fall deposits, lava flows, and ash-flow tuffs, were erupted from vents in the southern moat area between 60 and 35 Ka (Ogoh et al., 1993; Toyoda et al., 1995; Reneau et al., 1996a; Phillips et al., 1997). Valles Rhyolite ash and pumice falls were deposited across large parts of the southeastern Pajarito Plateau where they were reworked and incorporated into Pleistocene fluvial deposits. Late Pleistocene and Holocene erosion largely removed these deposits, but remnants are preserved on some mesas.
Cerros del Rio Volcanic Field
The Cerros del Rio volcanic field is mainly exposed as the Caja del Rio basalt plateau on the east side of the Rio Grande (Fig. 2). The surface of the plateau ranges in elevation from 1829 to 2254 m (6000–7396 ft). The exposed part of the volcanic field extends about 40 km (26 mi) in a north-south direction and is up to 20 km (12 mi) wide. The volcanic field extends an additional 11 km (7 mi) beneath the Pajarito Plateau, where Bandelier Tuff covers it. Black Mesa, located 6 km (3.5 mi) to the north, is an erosional outlier of this volcanic field (WoldeGabriel et al., 2001). The exposed portion of the volcanic field is made up of about a dozen volcanoes and more than 70 vents of cinder cones, plugs, and tuff rings (Kelley, 1978). Basalts and related intermediate-composition lavas are the predominant rock types, and most were erupted between 2.3 and 2.8 Ma (WoldeGabriel et al., 1996, 2001; Sawyer et al., 2002). The Rio Grande cuts a south-southwesterly course through the northwestern part of the basalt plateau, forming the 300-m (1000-ft)–deep, 1- to 2-km (0.7- to 1.25-mi)–wide White Rock Canyon.
Stratigraphy of the Pajarito Plateau
Rock units of the Pajarito Plateau (Fig. 4)
are described below from oldest to youngest. An east–west interpretive geologic cross section across the plateau is shown in Fig. 5
. The geological map published by Smith et al. (1970) shows a generalized distribution of these bedrock units across the Pajarito Plateau and the Jemez Mountains. Other geological maps covering the area are those by Griggs (1964), Kelley (1978), Goff et al. (1990), Kempter and Kelley (2002), and Dethier (1997). More detailed geological maps, some with cross sections, covering portions of the LANL include those by Baltz et al. (1963), Rogers (1995), Vaniman and Wohletz (1990), Reneau et al. (1995), Goff (1995), Goff et al. (2002), Lewis et al. (2002), and Lavine et al. (2003).
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Kelley (1978) found that the Chamita Formation could not be mapped as a distinctive lithologic unit in the central and western parts of the Española basin, and he included it in the Tesuque Formation and the Ojo Caliente Sandstone (which he raises to formational rank) in the those areas. Smith (2004) noted that similar problems occur when trying to map some members of the Tesuque Formation away from the small areas where they are defined. Recent mapping investigations use sediment provenance as a basis for defining lithostratigraphic map units (e.g., the lithosomes of Cavazza, 1989; Koning and Maldonado, 2001; Koning, 2002; Koning et al., 2002). Based on the new mapping projects currently underway in the basin, it is likely that the Chamita Formation and members of the Tesuque Formation will be revised or replaced in the near future (Smith, 2004).
The stratigraphy, lithology, and geochronology of the Santa Fe Group beneath the Pajarito Plateau are known primarily through well data because Bandelier Tuff covers these older rock units. Based on exposures near the Rio Grande and new drill hole data, the Santa Fe Group beneath the Pajarito Plateau is believed to include, in ascending order, the Tesuque Formation, older fanglomerate deposits of the Jemez volcanic field, Totavi Lentil and older river deposits, pumice-rich volcaniclastic rocks, and the Puye Formation. Based on recent mapping of basin sediments north and east of Los Alamos, the Tesuque Formation, as used in this paper, may include rocks of the Chamita Formation (Daniel Koning, 2005, personal communication). The older fanglomerate and pumice-rich volcaniclastic rocks are new units that are given provisional informal names. These units are generally similar to the Puye and Cochiti Formations, but are older than rocks normally assigned to them. Redefining the Puye and Cochiti Formations is beyond the scope of this study, and the older fanglomerates and pumice-rich volcaniclastic rocks are treated as informal units until they can be incorporated into the new stratigraphic framework being developed for the Española basin (e.g., Smith, 2004).
The total thickness of the Santa Fe Group in the eastern and northern part of the Española basin is as much as 1450 m (4800 ft) (Galusha and Blick, 1971). The Yates La Mesa #2 exploration well penetrated 1200 m (3966 ft) of Tesuque Formation in the south-central part of the basin (Myer and Smith, 2004). However, the thickest Santa Fe Group deposits are believed to occur in the western Española basin beneath the Pajarito Plateau (Kelley, 1978; Biehler et al., 1991; Ferguson et al., 1995; Smith, 2004). The thickness of these deposits is poorly known because the deepest wells on the plateau (e.g., PM-5 depth 950 m; 3110 ft) do not fully penetrate the basin-fill sediments. Biehler et al. (1991) estimated that the Santa Fe Group in the central basin might be as much as 2000 m (6650 ft) thick based on gravity data. Cross sections by Kelley (1978) and Koning and Maldonado (2001) showed up to 2750 to 3300 m (9000–10000 ft) of Santa Fe Group deposits in the central and western parts of the basin. Drill hole data and outcrops indicate that Santa Fe Group deposits are considerably thinner (<500 m, <1640 ft) west of the Pajarito fault (Goff and Gardner, 2004).
Prerift and Early Rift Rocks
Precambrian rocks exposed in the cores of the nearby Nacimiento, Sangre de Cristo, and Brazos uplifts include granite, granitic gneiss, schist, and greenstone. Precambrian quartzite forms extensive outcrops in the Brazos and Sangre de Cristo uplifts. Prebasin strata exposed around the margins of, and possibly underlying, the Española basin include upper Paleozoic (Mississippian to Permian) and Mesozoic marine and terrestrial sedimentary rocks and upper Tertiary Laramide synorogenic deposits (Goff and Grigsby, 1982; Biehler et al., 1991). Smith (2004) and Cather (2004) believe the upper Paleozoic and Mesozoic rocks were stripped from the Española basin area during uplift of the Laramide Pajarito uplift. Oligocene volcaniclastic sediments from the Ortiz Mountains (Espinaso Formation) are exposed in the southern part of the basin. Early rift deposits that may extend beneath the Pajarito Plateau from the north may include the Miocene Abiquiu Formation and Chama-El Rito Member of the Tesuque Formation, but these deposits are absent in the Yates La Mesa #2 well in the southern basin (Myer and Smith, 2004).
Tesuque Formation
The Miocene Tesuque Formation is partially penetrated by wells in the eastern part of the Pajarito Plateau where it makes up a significant portion of the aquifer for local communities and LANL (Purtymun, 1995). It is primarily made up of thick fluvial deposits consisting of partly lithified, arkosic sediments derived from Precambrian granite, pegmatite, and sparse sedimentary rocks of the Sangre de Cristo Range and volcanic sediments derived from Tertiary intermediate to felsic volcanic rocks of northern New Mexico and possibly southern Colorado (Cavazza, 1989). Individual beds are generally <3 m (10 ft) thick and consist of massive to planar- and cross-bedded light pink to buff siltstone and sandstone, with minor lenses of pebbly conglomerate (Fig. 6)
. Exposures of the Tesuque Formation just east of the Rio Grande are comprised primarily of the Pojoaque Member (Koning and Maldonado, 2001). In Well PM-5, a 110-m (360-ft)–thick series of basalt flows within the Tesuque Formation yielded a 40Ar/39Ar date of 11.39 ± 0.40 Ma (WoldeGabriel et al., 2001).
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Older Fanglomerate
In this paper, we use the informal "older fanglomerate" to describe a thick sequence of late Miocene fan deposits that were shed from the Jemez volcanic field into the western Española basin. These deposits, which are found only in deep boreholes, are important for the development of high-yield, low-draw-down municipal and industrial water supply wells on the Pajarito Plateau (Purtymun, 1995).
Purtymun (1995) called these deposits the "Chaquehui Formation" and assigned them a post-Chamita and pre-Puye age. As originally defined, the Chaquehui Formation consists of up to 457 m (1500 ft) of gravels, cobbles, and boulders derived from the Jemez volcanic field and volcanic, metamorphic, and sedimentary rocks derived from highlands to the north and east (Purtymun, 1995). However, recent stratigraphic studies indicate the type section in Chaquehui Canyon consists of late Pliocene phreatomagmatic deposits that are younger than Miocene deposits identified as Chaquehui Formation in deep wells (Heiken et al., 1996; WoldeGabriel et al., 2001).
Based on the geology encountered in supply wells in Guaje Canyon, Griggs (1964) included the older fanglomerates with undifferentiated Santa Group deposits primarily made up of the Tesuque Formation. He recognized that these older volcaniclastic rocks formed westward-thickening deposits derived from the Jemez volcanic field and that they interfinger with arkosic sediments derived from eastern source areas. Griggs differentiated these fanglomerates from those of the Puye Formation because they were overlain by axial Rio Grande gravels of the Totavi Lentil, which is generally found at the base of the Puye Formation in exposures on the eastern side of the plateau. Basalt flows within the older fanglomerates in Wells R-9, R-22, and Otowi 4 yield 40Ar/39Ar ages between 8.45 ± 0.21 and 8.97 ± 0.08 Ma (WoldeGabriel et al., 2001; Broxton et al., 2001). These ages overlap those reported for an 8.48 ± 0.14 Ma white lapilli bed in the Tesuque Formation on the northeast side of the Caja del Rio Plateau (Koning and Maldonado, 2001) and for a 9.3 ± 0.2 Ma basalt in Tesuque Formation rocks at the mouth of Ancho Canyon (WoldeGabriel et al., 1996).
Data from recently installed wells and reinterpretation of Purtymun's Chaquehui Formation indicate the older fanglomerates are widespread beneath the Pajarito Plateau. These deposits are mostly made up of volcanic detritus derived from Keres Group rocks and possibly from early Tschicoma Formation centers. They are characterized by dark, lithic sandstones and gravel and cobble deposits dominated by fresh to silicified, subangular to rounded andesite, latite, and porphyritic dacite (Fig. 8) . Subordinate clasts (<10%) include subangular to rounded rhyolite and basalt, and rounded quartzite. Rounded granite and angular chert clasts are generally rare (<1%). The nearest exposures of potential intermediate-composition source rocks include exposures of Paliza Canyon hornblende dacite in Frijoles Canyon west of the Pajarito fault zone (Goff et al., 2002), 8.72 ± 0.05 Ma clinopyroxene trachyandesite near Los Alamos Canyon reservoir (Broxton and WoldeGabriel, 2003, unpublished data), and Keres Group intermediate lava flows near Pine Spring (Kempter and Kelley, 2002).
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The maximum thickness of older fanglomerate is 500 m (1650 ft) in Well Otowi-4. However, thicknesses could be greater to the west, where drill holes did not fully penetrate the unit. The older fanglomerate deposits correspond to the zone of thick, highly productive aquifer rocks that extend northeastward across the central plateau described by Purtymun (1995). The western boundary of this thick sequence of sediments is poorly defined, but these rocks probably extend to the Pajarito fault zone. The older volcaniclastic deposits abruptly thin eastward between east–west pairs of wells such as R-23/R-22 and Otowi-1/Otowi-4. The transition zone generally corresponds to the eastern boundary of the gravity low beneath the Pajarito Plateau described by Ferguson et al. (1995). The interpretive east–west cross section (Fig. 5) shows the older fanglomerate deposits interfingering eastward with the upper Tesuque Formation, an interpretation that is consistent with stratigraphic relations described above for Guaje Canyon wells. Alternatively, the older fanglomerate may have accumulated the structural trough associated with the subsidence of Los Alamos subbasin, as defined by the gravity data. Because younger rocks cover critical geologic relations, these alternative interpretations cannot be tested until additional information is collected about the nature of the transition zone.
Totavi Lentil and Older River Deposits
The Totavi Lentil is made up of river-channel sands and gravels that crop out along the Puye escarpment, in lower Los Alamos and Guaje Canyons, and along White Rock Canyon. These rocks are also penetrated by a number of wells on the Pajarito Plateau. These axial-channel deposits were named the Totavi Lentil of the Puye Formation for a type section in Los Alamos Canyon (Griggs, 1964). Griggs recognized their importance as ancestral Rio Grande deposits, and he used them to delineate the base of the Puye Formation with which they are conformable at the type section.
The Totavi Lentil is a poorly consolidated conglomerate containing well-rounded cobbles and gravels of Precambrian quartzite, granite, and pegmatite with subrounded to subangular cobbles and boulders of silicic to intermediate and rarer basaltic volcanic rocks (Fig. 9) . Precambrian clasts typically make up >80% of the clasts in the deposits. Though commonly subordinate in abundance, clasts of volcanic rocks from the Jemez volcanic field make up to 50% of the deposit in some interbedded horizons. Loose, well-sorted, fine to coarse, quartz and microcline sands occur as lenses within the conglomerate. Totavi deposits are generally about 15 m (about 50 ft) thick near the Rio Grande but thicken in a northwest direction (Griggs, 1964). Totavi conglomerate is intercalated with lower Puye fanglomerate in outcrops along White Rock Canyon (Dethier, 1997) and in drill holes on the eastern side of the Pajarito Plateau (e.g., Well R-16). At the mouth of Frijoles Canyon, two layers of axial pebble conglomerate are interbedded with late Pliocene maar deposits of the Cerros del Rio volcanic field (Goff and Gardner, 2004). An unusually thick sequence of quartzite-rich conglomerate (>98 m, >323 ft) was partially penetrated in Well R-31 located in Ancho Canyon (Vaniman et al., 2002). A number of wells on the plateau (e.g., R-5, R-9, R-12, R-32) did not encounter the Totavi Lentil, indicating that these river deposits may form lenticular deposits of limited lateral extent.
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Based on new well data, it is evident that ancient river deposits in the Pajarito Plateau area are coeval with variety of stratigraphic units that span a longer time interval than previously recognized. River gravels occur beneath the pumice-rich volcaniclastic rocks in Wells R-13, R-15, R-20, R-33, R-34, PM-1, PM-2, and PM-5. These river gravel deposits are generally 10 to 30 m (30–100 ft) thick and include abundant well-rounded gravels of quartzite, angular to subangular basalt, andesite, and dacite, and minor metavolcanics. Granitic clasts are rare to absent. Clast compositions suggest these gravels were derived from the Tusas Mountains and were transported southward by the ancestral Rio Chama, with tributaries draining the Jemez volcanic field. Preliminary radiometric ages, described below, indicate the overlying pumice-rich volcaniclastic rocks are late Miocene in age. A late Miocene age for the early riverine deposits is consistent with geologic interpretations that through-going rivers were established in the Española basin before about 6.96 Ma (Smith et al., 2001; Smith, 2004).
Pumice-Rich Volcaniclastic Rocks
The pumice-rich volcaniclastic rocks are characterized by well-bedded horizons of light-colored, reworked, tephra-rich sedimentary deposits and subordinate primary ash- and pumice-fall deposits. These rocks consist mainly of tuffaceous sandstones and contain a few beds of lava-rich gravels (Fig. 10)
. The underlying older fanglomerate and overlying Puye Formation are more heterogeneous and contain higher percentages of gravel and cobble beds. In a number of wells, pumice-rich volcaniclastic rocks are separated from the older fanglomerate by ancestral Rio Chama deposits. The pumice-rich volcaniclastic rocks thin northeastward across the central part the plateau and are absent north of Pueblo Canyon.
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Most lapilli are aphyric or contain sparse phenocrysts of quartz, sanidine, and plagioclase, but the presence of some biotite-, hornblende-, pyroxene-phyric varieties indicates that multiple volcanic sources supplied tephra to these deposits. Two 40Ar/39Ar feldspar ages of 7.00 ± 0.63 Ma and 7.50 ± 0.30 (G. WoldeGabriel, 2003, personal communication) were obtained from crystal-poor pumice falls in Well R-19 at depths of 1595 ft and 1900 ft, respectively. The younger age overlaps the 6.01 ± 0.05 to 7.1 ± 0.2 Ma range of ages reported for the Bearhead Rhyolite (Justet, 1996; Smith, 2001), and the older age is slightly older. Additional work is taking place to investigate the relation between the pumice-rich volcaniclastic rocks and Keres Group silicic volcanism.
Bedding for the pumice-rich volcaniclastic rocks dips predominantly toward the south-southwest in Well R-20 (Fig. 7). The mean dip is 5°. Strata in Wells R-2, R-7, and R-19 also dip predominately to the southwest. Because the source area for these deposits is located to the west, the observed dips probably represent both westerly and southerly tilting of these rocks during subsidence of the Española basin. The pumice-rich volcaniclastic rocks in R-20 and the Tesuque Formation in R-16 have markedly different bedding orientations (Fig. 7). Part of the difference can be attributed to original bedding orientations for the two units, but the lack of a southerly component of dip in the R-16 deposits suggests that the two wells may be separated by a buried fault.
Tschicoma Formation
The Tschicoma Formation (Griggs, 1964) of the Polvadera Group (Smith et al., 1970) makes up the rugged Sierra de los Valles highlands west of Los Alamos. The Tschicoma Formation was encountered in Wells TW-4, H-19, CDV-16-3(i), and CDV-R-37-2 in the western part of the Pajarito Plateau but is absent in boreholes to the east.
The Tschicoma Formation consists of thick dacite to low-silica rhyolite lava flows erupted from large overlapping dome complexes. Major peaks in the Sierra de los Valles, including Cerro Grande, Pajarito Mountain, Caballo Mountain, and Tschicoma Mountain, are compositionally distinct lava domes that represent separate volcanic source areas. Low-silica rhyolite erupted from a deeply eroded dome complex in the upper Rendija Canyon and Guaje Mountain area yielded three 40Ar/39Ar feldspar ages between 4.95 ± 0.06 and 5.32 ± 0.02 Ma (G. WoldeGabriel, 2003, personal communication). Dacites of the Cerro Grande, Pajarito Mountain, and Caballo Mountain centers have 40Ar/39Ar ages between 2.91 ± 0.06 Ma and 3.34 ± 0.16 Ma (G. WoldeGabriel, 2003, personal communication).
Outcrops of the Tschicoma Formation in the Sierra de los Valles are primarily gray to purplish-gray lavas characterized by pronounced jointing and flow foliation. Flow interiors consist of dense, massive rock that is commonly devitrified to form a microcrystalline groundmass, giving the rocks a stony appearance. Chilled volcanic glass is sometimes preserved in brecciated interflow zones. Fragmental deposits of ash and lava debris occur in the distal parts of the formation. The Tschicoma Formation has a variable thickness due to the lenticular shapes of its lava flows, and it is at least 762 m (2500 ft) thick in the Sierra de los Valles. The Tschicoma Formation thins eastward under the western Pajarito Plateau, where it interfingers with the Puye Formation.
Puye Formation
The Puye Formation is a large apron of overlapping alluvial and pyroclastic fans that were shed eastward from the Jemez volcanic field into the western Española basin (Griggs, 1964; Bailey et al., 1969). The Puye Formation unconformably overlies rocks of the Tesuque Formation, and the Otowi Member of the Bandelier Tuff unconformably overlies it. The Puye Formation is intersected by most deep wells on the Pajarito Plateau, and it crops out along White Rock Canyon, in lower Los Alamos Canyon, and in canyons to the north. Turbeville et al. (1989) estimated its areal distribution at 518 km2 (200 mi2) and its volume at about 15 km3 (about 3.6 mi3). Because its primary source area was volcanic domes in the Sierra de los Valles, the Puye Formation overlaps and post-dates the Tschicoma Formation in age.
The formation reaches a maximum thickness of 330 m (>1100 ft) in Well R-25 on the western side of the plateau (Broxton et al., 2002). In the central and eastern portions of LANL, it is about 200 m (about 600 ft) thick, and the upper part contains interbedded basaltic lavas of the Cerros del Rio volcanic field. The Puye Formation is subdivided into a fanglomerate and lacustrine facies.
The fanglomerate facies, the dominant unit of the Puye Formation, is a heterogeneous assemblage of clast- to matrix-supported conglomerates, and of gravels and lithic-rich sandstones. Clasts in the coarsest deposits consist of subangular to subrounded cobbles and boulders of latite, dacite, rhyolite, and tuff in a poorly sorted matrix of ash, silts, and sands (Fig. 11) . Consolidated mudflow deposits are common throughout the unit, and tend to be cliff-forming units (Bailey et al., 1969). At least 25 ash beds of dacitic to rhyolitic composition are interbedded with the conglomerates and gravels (Turbeville et al., 1989).
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Basaltic Rocks of the Cerros del Rio Volcanic Field
Basaltic rocks of the Cerros del Rio volcanic field crop out primarily in White Rock Canyon and east of the Rio Grande (Griggs, 1964; Smith et al., 1970; Kelley, 1978; Sawyer et al., 2002). In the subsurface, these basalts extend westward to the central part of the Pajarito Plateau (Dransfield and Gardner, 1985; Broxton and Reneau, 1996). Cerros del Rio basalts interfinger with the upper Puye Formation west of the Rio Grande and overlie the Tesuque Formation east of the river. Bandelier Tuff unconformably overlies the basalts near White Rock.
Cerros del Rio basalts typically form thick sequences of stacked lava flows separated by interflow breccia, scoria, sediment, and ash (Fig. 12) . These rocks are mostly basalts and basaltic andesites, but subordinate dacite is present within thick basalt stacks in the east and central plateau (e.g., Ball et al., 2002) or is found as isolated flows on the western side of the volcanic field. Cerros del Rio lavas were erupted from vents located both east and west of the Rio Grande (Smith et al., 1970; Aubele, 1978; Kelley, 1978; Sawyer et al., 2002). On the Pajarito Plateau, the top of these basalts (Fig. 13) formed a broad north–south–trending highland that was buried by Bandelier Tuff (Broxton and Reneau, 1996). Cerros del Rio deposits are generally 61 to 183 m (200–600 ft) thick and reach a maximum thickness of 300 m (983 ft) at R-22. The thickest deposits generally coincide with a south-southwest–draining paleovalley that is defined by structure contours at the base of the unit (Fig. 14) .
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The basaltic rocks of the Cerros del Rio volcanic field include buried remnants of maar volcanoes in White Rock Canyon (Aubele, 1978; Heiken et al., 1996). The aprons of fragmental debris surrounding these buried craters consist of thin layers of basaltic ash and sediments. The maar deposits resulted from steam explosions that occurred where basalt erupted through an aquifer or standing body of water. Thin maar deposits are found at the base of the Cerros del Rio basalt in R-9 and R-12.
Otowi Member, Bandelier Tuff
The Otowi Member crops out in several canyons but is best exposed in Los Alamos Canyon and in canyons to the north. On the plateau it consists of moderately consolidated, porous, nonwelded ash-flow tuffs that form colluvium-covered slopes along the base of canyon walls (Fig. 15)
. The Otowi ash-flow tuffs are vitric and contain light gray to orange pumice supported in a white to tan ashy matrix of glass shards, broken pumice, crystals, and rock fragments (Broxton et al., 1995; Goff, 1995). The Otowi Member is made up of multiple ash flows, but individual ash-flow deposits cannot be traced in the subsurface using core and cuttings from widely spaced boreholes. In some drill holes, a shift in borehole 
measurements in the central part of the unit provides a useful datum for correlations between drill holes. The nonwelded ash-flow tuffs of the Otowi Member collectively form a relatively homogenous rock unit throughout the plateau.
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Tephra and Volcaniclastic Sediments of the Cerro Toledo Interval
The informal Cerro Toledo interval, originally named the Cerro Toledo Rhyolite (Bailey et al., 1969), is a stratified sequence of volcaniclastic sediments and tephra of mixed provenance (Fig. 15, top and middle photographs) (Broxton et al., 1995; Goff, 1995; Broxton and Reneau, 1995). Although it occurs between the ash-flow tuff members, the Cerro Toledo interval is not considered part of the Bandelier Tuff, a usage consistent with the original definition by Bailey et al. (1969).
The Cerro Toledo interval crops out in Los Alamos Canyon and in canyons to the north, and it occurs in many of the wells on the plateau. It unconformably overlies the deeply eroded Otowi Member, as described above, and its thickness is highly variable, ranging from 1 to 120 m (3–390 ft). Structure contours for the base of the Cerro Toledo indicate that this unit fills a broad southeast-draining valley fed by one or more canyons exiting the Sierra de los Valles (Fig. 17). The thickest Cerro Toledo deposits coincide with the axis of this paleovalley.
The predominant rock type in the Cerro Toledo interval is rhyolitic tuffaceous sandstone and tephra (Heiken et al., 1986; Stix et al., 1988; Broxton et al., 1995; Goff 1995). These deposits contain abundant crystal-poor ash and pumice, and clasts of vitric to devitrified rhyolite lava and minor obsidian. They represent the reworked equivalents of Cerro Toledo Rhyolite tephra erupted from the Cerro Toledo and Rabbit Mountain dome complexes located northeast and southeast of the Valles caldera, respectively (Smith et al.,1970; Heiken et al., 1986; Stix et al., 1988). Primary pumice and ash falls are interbedded with the sedimentary deposits in most locations.
Clast-supported gravel, cobble, and boulder deposits of porphyritic Tschicoma dacite derived from the Tschicoma Formation are interbedded with the tuffaceous rocks. In some deposits, the dacitic detritus is volumetrically more important than the tuffaceous detritus. These coars
