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SPECIAL SECTION: LOS ALAMOS NATIONAL LABORATORY

Geologic Framework of a Groundwater System on the Margin of a Rift Basin, Pajarito Plateau, North-Central New Mexico

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.

	ABSTRACT

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
The Pajarito Plateau is an important source of abundant potable groundwater for Los Alamos National Laboratory (LANL) and the communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau-wide hydrogeological investigation to develop conceptual models of the groundwater system as a framework for numerical simulations of groundwater flow. The Pajarito Plateau is located in the western part of the Española basin where rocks of the Jemez and Cerros del Rio volcanic fields overlie and interfinger with Neogene basin-fill sedimentary rocks. The vadose zone is about 200 m (600 ft) thick beneath mesas on the east side of the plateau and more than 375 m (1245 ft) thick on the west side. Groundwater occurs as shallow groundwater in canyon-floor alluvium, moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone. The most productive rocks of the regional aquifer occur in a westward-thickening wedge of coarse-grained, Miocene and Pliocene volcaniclastic rocks derived from the Jemez volcanic field. Eastern aquifer rocks consist of fine-grained, somewhat less productive Miocene sedimentary deposits derived from highland sources to the east and north. Intermediate and mafic lavas interbedded with the Miocene and Pliocene sedimentary deposits are components of the regional aquifer locally. The hydrogeology of the Pajarito Plateau is probably typical of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their recharge from adjacent mountainous areas.

Abbreviations: LANL, Los Alamos National Laboratory

	INTRODUCTION

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
THE PAJARITO PLATEAU lies at the volcanically and seismically active boundary between the Colorado Plateau and the Rio Grande rift in north-central New Mexico (Fig. 1) . The Rio Grande rift is a major geologic feature that consists of north-trending, fault-bounded basins extending from central Colorado to northern Mexico (Riecker, 1979; Keller and Cather, 1994; Chapin and Cather, 1994). The local area of subsidence, the Española basin, is between two larger basins, the Albuquerque basin to the south and San Luis basin to the north (Kelley, 1978). The Española basin is about 70 km (44 mi) long and 60 km (37 mi) wide.

View larger version (42K): [in this window] [in a new window]   Fig. 1. Map of the major tectonic features in northern New Mexico. Major fault systems are shown schematically with ball on downthrown side. VC is the Valles caldera complex, NFZ is the Nacimiento fault zone, CCFZ is the Cañada del Cochiti fault zone, PPFZ is the Picuris-Pecos fault zone, EFZ is the Embudo fault zone, and PFZ is the Pajarito fault zone. Figure is modified from Gardner and Goff (1984).

View larger version (212K): [in this window] [in a new window]   Fig. 2. Map of the Pajarito Plateau and surrounding region showing significant geographic features.

View larger version (78K): [in this window] [in a new window]   Fig. 3. Location map of the central Pajarito Plateau. Yellow shaded area is the Los Alamos National Laboratory. Also shown are the municipalities of Los Alamos and White Rock. East- and southeast-trending canyons are incised into the plateau. Water supply wells are shown as blue stars and the water supply well fields are indicated in blue shading; additional wells of Guaje well field extend north of this map. The Buckman well field provides water to Santa Fe. Water supply wells LA-1 through LA-6 are no longer used for municipal water production. New regional aquifer wells installed since 1998 are shown as red dots. Older test wells are shown as black dots. Line A–A' shows location of cross section in Fig. 5. Main elements of the Pajarito fault zone are shown in blue. PFZ is the main trace of the Pajarito fault zone, RCF is the Rendija Canyon fault, GMF is the Guaje Mountain fault, and DDG is the Diamond Drive graben. Faults modified from Gardner et al. (2001) and Lewis et al. (2002).

View larger version (43K): [in this window] [in a new window]   Fig. 5. Interpretive east–west cross section showing stratigraphic relations for geologic units of the Pajarito Plateau. Line of section is shown in Fig. 3.

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.

	REGIONAL GEOGRAPHIC SETTING

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
The Jemez Mountains are a nearly circular volcanic field 72 km (45 mi) in diameter that overlaps and covers the boundary between the Colorado Plateau and the Española basin (Smith et al., 1970). The central part of these mountains is occupied by the bowl-like depression of the Valles caldera that is 24 km (15 mi) in diameter (Fig. 2). The elevation of the caldera floor is about 2600 m (8500 ft). The Sierra de los Valles is the arc of mountains rimming the east side of the caldera. These mountains, which lie between the Valles caldera and the Pajarito Plateau, include a number of peaks more than 3050 m (10000 ft) in elevation. Precipitation in the Sierra de los Valles is a major source of recharge for the regional groundwater system beneath the Pajarito Plateau (Griggs, 1964; Purtymun, 1995).

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).

	REGIONAL GEOLOGIC FRAMEWORK

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
The hydrogeology of the Pajarito Plateau reflects Miocene through Quaternary tectonism, volcanism, and sedimentation along the western margin of the Española basin. The plateau overlies the deepest part of the basin and is bounded on the west by the Pajarito fault zone, one of the basin's major bounding faults. The 1.61 to 1.22 Ma Bandelier Tuff, erupted from the adjacent Jemez volcanic field, caps the mesas of the plateau and gives the area its distinctive tableland appearance. The Bandelier Tuff covers a diverse assemblage of Miocene to Pliocene basin-filling sedimentary and volcanic rocks.

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).

View larger version (47K): [in this window] [in a new window]   Fig. 4. Pajarito Plateau stratigraphy and hydrogeologic units as used in this paper. The bedrock geologic framework shows the stratigraphy of the plateau and the adjacent Sierra de los Valles. Units with italicized names are not exposed or penetrated by boreholes in the immediate vicinity of the plateau, but they are coeval units of the Jemez volcanic field that may be important source rocks for plateau deposits. The hydrogeologic framework shows units that are defined for site-wide numerical modeling.

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).

View larger version (86K): [in this window] [in a new window]   Fig. 6. The Tesuque Formation of the Santa Fe Group is an important component of the regional aquifer beneath the Pajarito Plateau. The Tesuque Formation is typically made up of fluvial and lacustrine deposits of pink to buff siltstone and silty sand. Lenses of pebble conglomerate and clay are also present. Top photograph shows typical bedding characteristics of stratified Tesuque deposits. Middle photograph is close up view of silts and silty sands that are the predominant rock types. The bottom photograph shows channels of pebble conglomerate interbedded with silty sands. Photographs taken in lower Los Alamos Canyon.

View larger version (33K): [in this window] [in a new window]   Fig. 7. Radial histograms showing bedding orientations for the Tesuque Formation in Well R-16 and pumice-rich volcaniclastic rocks in Well R-20. Bedding in the Tesuque Formation dips 11° toward the west-northwest. The pumice-rich volcaniclastic rocks in R-20 dip 5° to the south-southwest. Bedding in both units was rotated during subsidence of the Española basin. Dip azimuth and angles were determined using oriented Formation Microimager logs.

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).

View larger version (57K): [in this window] [in a new window]   Fig. 8. Formation Microimager log showing clast size and bedding characteristics of older fanglomerate deposits in Well R-6. Two representative depth intervals are shown (351.7–357.5 m, 1154–1173 ft and 388.6–395.9 m, 1275–1299 ft). The older fanglomerate is a coarse-grained deposit with thick cobble and gravel beds separated by well-stratified sandstones. During logging, four sensors pads in contact with the borehole wall produce oriented, high-resolution resistivity maps. Resistive dacite cobbles and gravels are indicated by lighter colors, and conductive clay-rich beds and water-bearing zones are indicated by darker colors. Thin black horizontal lines indicate bedding planes used to determine bedding orientations.

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.

View larger version (100K): [in this window] [in a new window]   Fig. 9. Axial ancestral Rio Grande deposits of the Totavi Lentil. Top photograph shows typical coarse-grained deposits made up of subrounded silicic to intermediate volcanic rocks and rounded Precambrian quartzite, granite, and pegmatite. Middle photograph shows unconsolidated, cross-bedded, arkosic sands. The bottom photograph is a close-up view of well-rounded, clast- and matrix-supported cobbles and gravels. Photographs taken at type locality near Totavi.

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.

View larger version (76K): [in this window] [in a new window]   Fig. 10. Formation Microimager log showing typical grain size and bedding characteristics for the pumice-rich volcaniclastic rocks in Well R-19. Two representative depth intervals are shown (516.9–520.3 m, 1696–1707 ft and 534.8–538.0 m 1754.5–1765 ft). These strata are fine grained and thinly bedded compared with the overlying Puye Formation and underlying older fanglomerate unit. Subordinate gravel and cobble beds (e.g., 535.5- and 537.1-m, 1757- and 1762-ft depths) are interbedded with the sandstones of this unit. Green horizontal lines indicate bedding planes used to determine bedding orientations.

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).

View larger version (89K): [in this window] [in a new window]   Fig. 11. Fanglomerate facies of the Puye Formation. Top photograph shows beds of light-colored, ash-rich fluvial deposits and gray gravel and cobble deposits. Deposits are made up primarily of crystalline dacite detritus. Middle photograph shows gray, poorly sorted gravel and cobble beds overlying light-colored, ash-rich sands and gravels. Bottom photograph is mudflow deposit with crystalline dacite clasts supported in a silty to sandy matrix. Top photograph taken near Totavi. Middle and bottom photographs taken in Rendija Canyon.

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) .

View larger version (98K): [in this window] [in a new window]   Fig. 12. Cerros del Rio basalt is largely buried by Bandelier Tuff on the Pajarito Plateau. Top photograph shows a thick stack of basalt flows overlain by Bandelier Tuff near the mouth of Frijoles Canyon. Individual flows are separated by interflow breccia, scoria, sediment, and ash. Lower photograph shows well-developed columnar joints in dense flow interiors near White Rock. A thin interflow breccia separates two flows near the bottom of the photograph.

View larger version (40K): [in this window] [in a new window]   Fig. 13. Structure contour for the top of Cerros del Rio basalt and western dacite on the Pajarito Plateau. Green dashed line indicates the northern and western extent of the Cerros del Rio volcanic field. Blue line indicates western extent of dacitic lavas that were contemporaneous with the basalts. Top of Cerros del Rio basalts formed broad north-trending highland on east side of plateau. This highland is now covered by Bandelier Tuff.

View larger version (42K): [in this window] [in a new window]   Fig. 14. Structure contour for the base of Cerros del Rio basalt with isopachs showing the cumulative thickness of flows. Green dashed line indicates the northern and western boundary of the Cerros del Rio volcanic field. The maximum thickness of basalt corresponds with structural-contour lows suggesting that the basalts accumulated in topographic basins.

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.

View larger version (87K): [in this window] [in a new window]   Fig. 15. Thick Bandelier Tuff deposits form the mesas of the Pajarito Plateau. Upper photograph, looking across Pueblo Canyon toward the northwest, shows, in ascending order, the Otowi Member (Qbo), the Cerro Toledo interval (Qct), and the Tshirege Member (Qbt 1 g to Qbt 3, see text). Subunits of the Tshirege Member are defined by changes in welding and crystallization properties within this compound cooling unit. Middle photograph, in upper Pueblo Canyon, shows well-stratified, tuffaceous sandstones of the Cerro Toledo interval (Qct) between light-colored, slope-forming, nonwelded, vitric ash-flow tuffs of the Otowi Member (Qbo, below) and the cliff of nonwelded, vitric ash-flow tuffs of subunit Qbt 1 g of the Tshirege Member (above). Bottom photograph, east of the confluence of Pueblo and Los Alamos Canyons, shows stratified pumice-fall deposits of the Guaje Pumice Bed (Qbog) overlain by Otowi Member ash-flow tuffs (Qbo, massive light-colored deposits in slope). The Guaje Pumice Bed overlies Cerros del Rio basalt (Tb4).

View larger version (61K): [in this window] [in a new window]   Fig. 16. Structure contour and isopach map for the Otowi Member of the Bandelier Tuff. Structure contours are for base of Guaje Pumice Bed and show the paleotopography before eruption of the Otowi Member. Otowi ash-flow tuffs filled a broad north-trending paleovalley bound by the Sierra de los Valles highlands on the west and the Cerros del Rio basaltic highland on the east. The variable thickness of the Otowi Member on the western side of the plateau represents deep erosion of these poorly consolidated nonwelded tuffs before eruption of the Tshirege Member.

View larger version (38K): [in this window] [in a new window]   Fig. 17. Structure contour and isopach map for the Cerro Toledo interval. Structure contours for base of unit indicate that Cerro Toledo filled a broad southeast-trending paleovalley incised into the Otowi Member (see isopach map for Otowi Member in Fig. 16). The thickest Cerro Toledo deposits coincide with the axis of the paleovalley.

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 coarse dacitic deposits commonly define the axial portions of channels incised into the underlying Otowi Member.

The Cerro Toledo interval contains a large component of crystal-rich tuffaceous detritus in the western part of the plateau. These tuffaceous sediments represent reworked Otowi tuff that accumulated in drainages incised into the Otowi Member before emplacement of the Tshirege Member. These reworked Otowi deposits are interbedded with other volcaniclastic deposits derived from Cerro Toledo and Tschicoma sources.

Tshirege Member, Bandelier Tuff
The Tshirege Member is the upper member of the Bandelier Tuff and is the most widely exposed bedrock unit of the Pajarito Plateau (Griggs, 1964; Smith and Bailey, 1966; Bailey et al., 1969; Smith et al., 1970). It is a multiple-flow, ash and pumice sheet that forms prominent cliffs throughout the plateau (Fig. 15). It also underlies canyon floors in all but the middle and lower reaches of Los Alamos Canyon and in canyons to the north. The Tshirege Member is generally >61 m (200 ft) thick in the north-central part of LANL and is >183 m (600 ft) thick near the southern edge of LANL (Broxton and Reneau, 1996).

The Tshirege Member differs from the Otowi Member most notably in its generally greater degree of welding compaction (Fig. 15). Time breaks between the successive emplacements of ash-flow units caused the tuff to cool as several distinct cooling units. For this reason the Tshirege Member is a compound cooling unit, consisting of at least four cooling subunits that display variable physical properties vertically and horizontally (Smith and Bailey, 1966; Crowe et al., 1978, Broxton and Reneau, 1995). These variations in physical properties reflect zonal patterns of varying degrees of compaction, welding, and glass crystallization (Smith, 1960a, 1960b). The welding and crystallization zonation in the Tshirege Member produce vertical variations in properties such as density, porosity, hardness, composition, color, and surface weathering patterns. The degree of welding in each of the cooling units generally decreases from west to east, reflecting higher emplacement temperatures and thicker deposits closer to the Valles caldera.

The Tsankawi Pumice Bed forms the base of the Tshirege Member (Bailey et al., 1969). Where exposed, it is commonly 50 to 75 cm (20 to 30 in) thick. This pumice-fall deposit contains sorted pumice lapilli (diameters reaching about 6.4 cm, 2.5 in) in a crystal-rich matrix. Several thin ash beds are interbedded with the pumice-fall deposits.

Because the thick Tshirege deposits make up a significant portion of the upper vadose zone, brief descriptions are provided below for the major subunits of the member. These subunits form widespread mappable units across the Pajarito Plateau and represent tuffs with distinctive lithologic characteristics. A photograph of Tshirege Member subunits in the central part of the plateau is shown in Fig. 15.

Qbt 1 g is the lowermost subunit of the thick ash-flow tuffs that overlie the Tsankawi Pumice Bed. It consists of porous, nonwelded, and poorly sorted ash-flow tuffs. The "g" in this designation stands for "glass" because none of the glass in ash shards and pumices shows crystallization by devitrification or vapor phase alteration. Qbt 1 g is poorly indurated but nonetheless forms steep cliffs because a resistant bench near the top of the unit forms a hard, protective cap over the softer underlying tuffs (Fig. 15, top and middle photographs). A thin (10–25 cm, 4–10 in), pumice-poor, pyroclastic surge deposit is sometimes present at the base of Qbt 1 g.

Qbt 1v forms alternating cliff-like and sloping outcrops composed of porous, nonwelded, but crystalline tuffs. The "v" stands for vapor-phase crystallization that together with crystallization of glass in shards and pumices (devitrification) transformed the rock matrix into microcrystalline aggregates of silica polymorphs and sanidine. The base of this unit is a thin, horizontal zone of preferential weathering that marks the abrupt transition from glassy tuffs below to crystallized tuffs above. This feature forms a widespread mappable marker horizon (locally called the vapor-phase notch) throughout the Pajarito Plateau. In some locations the vapor-phase notch grades laterally into a prominent bench developed on top of the glassy tuff (Fig. 15, top photograph). The lower part of Qbt 1v, which is designated Qbt 1vc, is a colonnade tuff that is orange brown, resistant to weathering, and has distinctive columnar cooling joints. The upper part of the unit, designated Qbt 1vu, consists of white, variably compacted, alternating cliff- and slope-forming tuffs. The tuffs of Qbt 1v are generally nonwelded and have an open, porous structure.

Qbt 2 forms a distinctive, medium brown, vertical cliff that stands out in marked contrast to the slope-forming, lighter colored tuffs above and below (Fig. 15, top photograph). A series of surge beds commonly mark its base in the eastern part of the Laboratory. In the central and western part of the Laboratory, the boundary between Qbt 2 and Qbt 1v is gradational, and the distinction between the two units is somewhat arbitrary. Qbt 2 is typically the most strongly welded tuff in the Tshirege Member. Welding tends to increase up section through the subunit. On the western side of the plateau, a partial cooling break occurs in the middle of the densely welded portion of the unit. Qbt 2 is characterized by lower porosity and higher density relative to the other units of the Tshirege Member, and because of its brittle nature, it contains more abundant and continuous joints. Vapor-phase crystallization of flattened shards and pumices is extensive in this subunit.

Qbt 3 is a nonwelded to partly welded, vapor-phase altered tuff that forms the cap rock of mesas in the central part of the Pajarito Plateau (Fig. 15, top photograph). Its base consists of a purple-gray, unconsolidated, porous, crystal-rich, nonwelded tuff that underlies a broad, gently sloping bench developed on top of strongly welded Qbt 2. This basal, nonwelded portion forms relatively soft outcrops that weather into low, rounded outcrops with a white color, which contrast with the partly welded, cliff-forming tuffs in the middle and upper portions of the subunit. Qbt 3 becomes moderately to densely welded in the western part of the plateau. In the extreme western part of the Laboratory, an additional subunit, Qbt 3t, is present above Qbt 3 (Gardner et al., 2001). Qbt 3t is a moderately to densely welded ash-flow tuff that has petrographic and geochemical characteristics that are transitional between Qbt 3 and Qbt 4.

Qbt 4 is a complex unit consisting of nonwelded to densely welded ash-flow tuffs. The lower part of the subunit is made up of nonwelded to partly welded ash-flow tuffs characterized by small, sparse pumices and intercalated surge deposits. The upper part of the subunit includes pumiceous, densely welded ash-flow tuffs that form mesa cap rocks. Devitrification and vapor-phase alteration are typical in this unit, but thin zones of rhyolitic, vitric ash-flow tuff occur locally. The occurrence of Qbt 4 is limited to the western part of the Pajarito Plateau. Lewis et al. (2002) divided Qbt 4 into local subunits and provided detailed descriptions of this heterogeneous unit.

Alluvium
Holocene and late Pleistocene canyon-floor alluvium consists of stratified, lenticular deposits of unconsolidated fluvial sands, gravels, and cobbles (Reneau et al., 1996b). Smaller canyons whose headwaters are located on the plateau contain detritus exclusively of Bandelier Tuff. Larger canyons that head in the Sierra de los Valles contain Bandelier detritus mixed with dacite detritus derived from the Tschicoma Formation. Active and inactive channels and floodplains form complex, cross-cutting deposits. These fluvial sediments interfinger laterally with colluvium that contains large blocks (up to 3 m, 10 ft in diameter) derived from canyon walls. In Pueblo Canyon alluvium is about 3.4 m (11 ft) thick on the west side of the plateau and about 5.5 m (18 ft) thick near the confluence with Los Alamos Canyon. Mortandad Canyon has 0.3 to 0.6 m (1–2 ft) of alluvium near its headwaters and more than 30 m (100 ft) of alluvium near the eastern LANL boundary.

Alluvium of probable early Pleistocene age overlies Bandelier Tuff on mesas throughout the plateau (Reneau and McDonald, 1996). The alluvial deposits form fairly continuous deposits on the western side of the plateau, but only remnants of these deposits are preserved further east. These alluvial deposits are primarily made up of coarse dacitic detritus from the Sierra de los Valles, but some locations also contain Valles Rhyolite (Cerro del Medio and El Cajete) fall deposits or their reworked equivalents. These deposits record the locations post-Tshirege alluvial fans and streams that predate incision of canyons on the plateau (Reneau and McDonald, 1996).

	RELATION BETWEEN GEOLOGIC AND HYDROGEOLOGIC FRAMEWORKS

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
The geologic units of the Pajarito Plateau are organized into more generalized hydrogeological units that form the framework for flow and transport numerical models. Hydrogeological units are subdivided based on lithological characteristics that are believed to reflect differences in hydrological properties. A comparison of geologic and hydrologic frameworks for the plateau region is provided in Fig. 4. The hydrologic framework supports both vadose zone and regional aquifer modeling efforts. Definition of hydrogeological units is an iterative process, and further refinement of hydrogeological units is expected as additional hydrological measurements for rock units become available. Keating et al. (2005) discussed the development and application of numerical models for estimating fluxes through the groundwater system beneath the Pajarito Plateau.

Basement Confining Layer
Pre-Tesuque rocks are not penetrated by any of the wells on the Pajarito Plateau, and the prebasin stratigraphy is inferred from exposures in nearby uplifts and from basin deposits exposed in surrounding areas. All rock units below the Tesuque Formation are grouped together and treated as the basement confining layer in the three-dimensional geologic model. These may include Precambrian igneous and metamorphic rocks, Paleozoic and Mesozoic sedimentary rocks, and mid to upper Tertiary terrestrial sediments.

Santa Fe Group Rocks
Hydrogeologic unit Ts is generally equivalent to the Tesuque Formation. Ts is defined as a hydrogeologic unit based on its sandy and silty lithology that is markedly finer grained than overlying volcaniclastic deposits. Though a significant aquifer unit, wells completed entirely in Ts, such as those in lower Los Alamos Canyon, typically have lower yields (2270 L m^–1, 600 gpm) than wells penetrating coarser deposits to the west (3800 L m^–1, 1000 gpm) (Purtymun, 1995). Ts contains basalts and intercalated sedimentary deposits collectively designated Tb1 (10.9–13.1 Ma basalts). Results for a dynamic spinner log run in Supply Well PM-4 suggest that the Miocene basalts are less productive than the sedimentary rocks making up the regional aquifer (Koch et al., 1999). These spinner log results are in general agreement with lithologic descriptions that indicate these basalts are altered, with much of the permeability associated with fractures and with interflow breccias plugged by secondary minerals.

Hydrogeologic unit Tf is generally equivalent to the older fanglomerate unit. Tf is made up of thick sequences of gravel and cobble beds and interbedded sandstones. These diverse lithologies occur as stacked sequences of stratified deposits, resulting in a wide range of hydrologic properties. Vertical heterogeneity due to stacking of beds of contrasting lithology is much greater than lateral heterogeneity within beds. Hydrologic properties of Tf are probably strongly anisotropic with higher hydraulic conductivities occurring laterally within beds than across beds. Purtymun (1995) identifies these coarse-grained rocks as the most productive horizon for high-yield (3800 L m^–1, 1000 gpm) water-supply wells on the Pajarito Plateau. The dynamic spinner log for Well PM-4 indicates older fanglomerates are among the most hydraulically productive rocks in the well (Koch et al., 1999). Because the older fanglomerates were incorporated into other geologic units in pre-1997 wells, we reinterpreted the earlier well logs to delineate these units using stratigraphic concepts developed for recently installed wells. The contact between Tf and Ts is placed at the transition from predominantly coarse-grained deposits (gravels and cobbles) above to predominately silts and sands below. Basalts and their intercalated sedimentary deposits within Tf are designated as hydrogeologic unit Tb2 (8.4 to 9.3 Ma basalts). Tf interfingers to the west with volcanic rocks of the Keres Group that are designated as hydrogeologic unit Tk.

Hydrogeologic unit Tpt represents the Totavi Lentil and older river deposits. Tpt tends to be a good producer of groundwater in water-supply wells on the Pajarito Plateau (Purtymun, 1995). A spinner log for Supply Well PM-4 indicates that Tpt produces 18% of the water even though it accounts for only 2.5% of the screen interval (Koch et al., 1999). Drillers commonly report high water production and flowing sands when drilling through this unit. Similar ancestral Rio Grande deposits are also an important source of groundwater in the Albuquerque basin (Hawley et al., 1995). Fluvial sands and gravels form a stacked sequence of braided river-channel deposits in a zone as much as 5 km (3 mi) wide beneath the eastern heights of Albuquerque. The deposits of the Albuquerque basin are often much thicker (up to 300 m, 1000 ft) than equivalent rocks in the Española basin (<100 m, <300 ft), though finer in grain size.

Tpp represents the pumice-rich volcaniclastic rocks. As interpreted here, Tpp is a well-stratified sedimentary unit that is generally finer-grained and more porous than the thick, coarse deposits found in underlying and overlying units. Preliminary results from borehole hydrologic tests suggest that the hydraulic conductivity of Tpp is generally greater than the crystalline-rich detritus of Tpf (Stone and McLin, 2003). Because of its well-bedded nature, the vertical heterogeneities within Tpp are greater than lateral heterogeneities, resulting anisotropic hydrologic properties and higher hydraulic conductivities parallel to bedding.

Hydrogeologic unit Tpf is equivalent to the fanglomerate and lacustrine facies of the Puye Formation. Tpf is an important component of both vadose zone and regional aquifer rocks. Tpf is a complex assemblage of coarse-grained, stratified sediments and thin interbedded tephras. Vertical heterogeneity in hydrologic properties results from the stacking of sedimentary deposits with contrasting lithologic characteristics. Lateral heterogeneity is due to the limited lateral extent and cross-cutting relationships of fluvial deposits in an alluvial fan setting. Heterogeneity occurs at scales ranging from hand specimen to outcrop in extent, and it arises from a number of factors, including grain-size variations, sorting characteristics, variations in bedding thicknesses, and lateral continuity of individual beds. Hydraulic conductivity is expected to be greater parallel to bedding than across bedding. Tpf interfingers westward with hydrogeological unit Tt1, which consists of volcanic rocks of the Tschicoma Formation. Tt1 is primarily located within the Jemez Mountains, but it is locally important near the western margin of the plateau.

Basaltic Rocks of the Cerros del Rio Volcanic Field
Hydrogeological unit Tb4 is mainly equivalent to the Cerros del Rio basalt, but it includes subordinate amounts of Puye Formation that is interbedded with flow units in some locations. Tb4 hosts perched water at a number of locations in the central and eastern part of the plateau (Robinson et al., 2005), and it forms the top of the regional aquifer in the southeast part of LANL. It is a heterogeneous unit containing stacked lava flows separated by interflow zones. The interiors of lava flows are characterized by dense, impermeable basalt, and fracture permeability controls the transmission of water. Interflow zones are generally characterized by highly interconnected pore space and represent important lateral pathways for groundwater. However, in some locations, clay fills the voids in clinker beds and scoria, significantly decreasing the permeability of interflow zones.

Bandelier Tuff
Units of the Bandelier Tuff are entirely within the vadose zone. The stratigraphic divisions of Tshirege subunits were retained for the hydrologic model because the subunits are based on widespread stratification of rock properties imparted by variations in welding, fracturing, and post-emplacement alteration. Except for Qbt 4 and Qbt 3t, which occur only in the west, subunits of the Tshirege Member are aerially extensive and can be traced laterally over the entire plateau. Rogers and Gallaher (1995) and Springer (2005) discussed the hydrologic properties of the Bandelier Tuff.

Hydrogeologic units Qbo and Qbog are equivalent to the ash-flow tuffs and fall deposits of the Otowi Member and the Guaje Pumice Bed, respectively. The ash-flow tuffs making up the Qbo are nonwelded and vitric throughout on the Pajarito Plateau. These characteristics result in more uniform lithological characteristics than those of the Tshirege Member. In the western part of the plateau, Qbo shows some variability in vertical density and density–porosity profiles (Fig. 18) . Abrupt vertical changes in density and density porosity probably reflect successive episodes of ash-flow tuff emplacement. Qbo is relatively uniform in the central and eastern parts of the plateau. Qbog is treated as a separate hydrogeological unit because of its strongly stratified nature and because it is better sorted and contains less matrix ash than other Bandelier Tuff units.

View larger version (36K): [in this window] [in a new window]   Fig. 18. Natural , density, density porosity, epithermal-neutron porosity, and resistivity logs for borehole CdV-16-2(i) located in the western part of the plateau. The borehole is filled drilling fluid below 289.6-m (950-ft) depth. Because of higher concentrations of U, Th, and K, the rhyolitic Tshirege and Otowi Members of the Bandelier Tuff have elevated natural signatures relative to the dacitic sedimentary deposits of the Puye Formation. The density porosity was calculated from the density log using a sandstone grain density of 2.65 gm cm–3. While appropriate for the Puye Formation and crystalline tuffs, this value is about 6 to 8% too high for tuff units containing large amounts of volcanic glass (Qbo and Qbt 1 g). The large density and porosity variations in the Tshirege Member reflect vertical compaction and welding variations in this compound cooling unit. The Otowi Member shows less variability, but density and porosity shifts suggest a small increase in compaction below 190.5-m (625-ft) depth. The epithermal neutron porosity log is sensitive to water content in the unsaturated rocks, and it shows a general trend toward higher moisture with depth through the Bandelier Tuff. The relatively high density and low porosity of the Puye Formation reflects the high percentage of dense, crystalline dacite gravels, cobbles, and boulders embedded in a silty matrix.

Within the Tshirege Member alternating sequences of welded and nonwelded tuffs form distinctive cooling units with welded interiors separated by porous, poorly indurated, nonwelded tuffs. Because of their brittle nature, the welded portions of Subunits Qbt 2, Qbt 3, Qbt 3t, and Qbt 4 are typically denser and more highly fractured than the nonwelded tuffs that separate them. Fractures originating in welded zones, which include both cooling joints and tectonic fractures, commonly die out in overlying and underlying nonwelded tuffs. Fracture-filling materials, including illuviated detritus from the surface, clay minerals, and carbonate, are common in near-surface fractures, but decrease with depth.

Hydrogeologic unit Qbt t is equivalent to the stratified pumice fall deposits of the Tsankawi Pumice Bed. Qbt t is overlain by Qbt 1 g, a sequence of massive, vitric ash-flow tuffs at the base of the Tshirege Member. Qbt t and Qbt 1 g are mineralogically unique in that they are the only parts of the Tshirege Member where the original volcanic glass of ash and pumices is extensively preserved. Volcanic glass in units above Qbt 1 g is transformed to silica minerals and alkali feldspars by post-emplacement devitrification and vapor-phase crystallization. The hydrological effects of post-emplacement crystallization is not completely understood, but these processes affect grain-size distributions and may decrease the effective porosity of these tuffs by creating isolated or poorly connected pore spaces.

Qbt 1vc is an indurated, but poorly welded unit characterized by a well-developed system of columnar cooling joints. Overlying Qbt1vu is generally nonwelded to partly welded, but lacks the extensive jointing. In the eastern part the plateau, stratified pyroclastic surge beds up to 0.3 m (1 ft) thick occur between Qbt 2 and Qbt 1v. Westward, however, pyroclastic surge beds are absent, and the contact between Qbt1vu and Qbt 2 is gradational (Fig. 18). Qbt 2 is most strongly welded near its top, and welding generally decreases down section. Density and density–porosity profiles indicate the presence of a partial cooling break in the center of Qbt 2 in the western part of the plateau, but this cooling break is absent toward the east. Qbt 2 is characterized by higher bulk densities and lower density porosities than other subunits of the Tshirege Member (Fig. 18). Qbt 3 is strongly welded in the western part of the plateau, but welding decreases eastward toward the more distal part of the unit. Higher bulk densities and lower density porosities characterize the strongly welded interior of this unit (Fig. 18). Qbt 4 is a complex sequence of nonwelded to strongly welded tuffs of limited extent in the western part of the plateau.

	GEOLOGIC SETTING OF GROUNDWATER OCCURRENCES

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
Groundwater of the Pajarito Plateau has three principal modes of occurrence (Griggs, 1964; Purtymun, 1995): (i) shallow groundwater in canyon-floor alluvium, (ii) moderately deep perched groundwater in bedrock units of the vadose zone, and (iii) deep groundwater associated with the regional saturated zone. Contaminant distributions in groundwater indicate that the alluvial systems are in communication with deep perched and regional groundwater, but that the effect of this communication decreases with depth.

Alluvial Groundwater
Shallow groundwater occurs in canyon-floor alluvium of large watersheds that head in the Sierra de los Valles and in smaller watersheds on the plateau that receive liquid discharges from LANL or municipal sources. The alluvial groundwater most commonly occurs near the base of the alluvial sequence atop weathered bedrock units. Locally, lenses of alluvial water occur above permeability barriers such as silty, fluvial, overbank deposits and soil horizons, particularly where alluvial deposits are particularly thick. The saturated thickness and lateral extent of alluvial groundwater within a canyon is strongly affected by seasonal variations in snow-melt and in storm runoff. Robinson et al. (2005) described the physical characteristics of alluvial groundwater systems. Birdsell et al. (2005) and Kwicklis et al. (2005) discussed conceptual models for vadose zone infiltration and the role of surface water and alluvial groundwater as sources of recharge for deeper perched systems and the regional aquifer.

Deep, Perched Groundwater in Bedrock
Examples of deep, perched groundwater can be found in most types of bedrock units that make up the vadose zone (Robinson et al., 2005). Deep, perched groundwater is most commonly found beneath canyons containing surface and/or alluvial groundwater for significant portions of the year. Though generally associated with canyons with headwaters in the Sierra de los Valles, these perched zones also occur beneath smaller canyons that receive treated effluent from LANL and county sources. Most of these perched zones are believed to be of limited extent, but a few are significant in size and have saturated thicknesses ranging from 30 to 120 m (100–400 ft) (Robinson et al., 2005).

Regional Groundwater
Potentiometric maps indicate that the Sierra de los Valles is an important recharge area for the Pajarito Plateau groundwater system (Purtymun, 1984; Rogers et al., 1996; Keating et al., 2005). Regional groundwater moves generally eastward across the plateau toward the Rio Grande (Purtymun and Johansen, 1974; Purtymun, 1984, 1995). Keating et al. (2005) discussed potentiometric maps, hydraulic gradients, and permeability data for the regional saturated zone.

More than 50 active and inactive water-supply and test wells intersect regional groundwater across the Pajarito Plateau (Griggs, 1955, 1964; Cushman, 1965; Cooper et al., 1965; Purtymun et al., 1990, 1995a, 1995b; Purtymun, 1995; Stoker et al., 1992; McLin et al., 1996, 1998; John Shoemaker, Inc., 1999; Koch and Rogers, 2003). Depth to water is greatest at mesa-top locations in the western part of the plateau (e.g., 375 m, 1245 ft, at Well CdV-R-15-3) and decreases eastward. Well R-16, located near the rim of White Rock Canyon, encountered regional groundwater at a depth of about 200 m (614 ft), about 65 m (210 ft) higher than the Rio Grande, located 1 km (0.6 mi) to the east. In Los Alamos Canyon near the Rio Grande, Supply Wells LA-1, -2, and -3 (Fig. 3) flowed at the surface when installed (Cushman, 1965; Purtymun, 1995). Regional groundwater is believed to discharge through numerous springs and seeps in White Rock Canyon (Purtymun, 1966, 1995).

The distribution of bedrock units at the top of regional saturation is shown in Fig. 19 . Regional groundwater enters the Pajarito Plateau by underflow through the rocks that underlie the Sierra de los Valles (Griggs, 1964; Purtymun, 1984). This underflow is supplemented by recharge from drainages that cross the plateau (Kwicklis et al., 2005). Hydrogeologic conditions beneath the Sierra de los Valles west of the Pajarito fault zone are largely unknown because there are no deep wells in this area. Groundwater probably flows through Tschicoma lavas and underlying geologic units at depth. The geologic units beneath the Tschicoma Formation are poorly constrained but probably consist of Keres Group volcanics, Santa Fe Group sediments, Eocene sedimentary rocks, Paleozoic and Mesozoic sedimentary rocks, and Precambrian crystalline rocks (Smith et al., 1970; Goff and Gardner, 2004; Smith, 2004). In the western part of the plateau, in the vicinity of Pueblo and Water Canyons, the water table is straddled by two lobes of down-faulted Tschicoma lavas that extend up to 3 km (2 mi) east of the Pajarito fault zone. Groundwater flow through dacite most likely occurs as fracture flow in the lava interiors and as porous flow in interflow zones and interbedded clastic deposits.

View larger version (78K): [in this window] [in a new window]   Fig. 19. Map showing distribution of geologic units at the top of the regional saturated zone beneath the Pajarito Plateau. The wells that provided geologic control for this map are indicated by dots using the same color scheme as Fig. 3. The LANL boundary is shown by the green outline and the Pajarito fault zone is shown in blue. The map portrays the dominant rock unit in the upper 15.2 m (50 ft) of the regional saturated zone.

Basalt straddles the water table in two areas. The most extensive is located in the south-central part of the plateau where as much as 60 m (195 ft) of saturated Cerros del Rio basalt occurs at the top of the regional zone of saturation in Well DT-10 and 90 m (290 ft) occurs in Well R-22 (Fig. 19). A smaller region of Miocene basalts straddles the water table in a north-trending zone extending between Wells R-12 to R-5. The southern extent of this zone is poorly constrained.

The Tesuque Formation is the primary rock unit making up the regional aquifer in the eastern part of the plateau and in the Buckman well field east of the Rio Grande (Fig. 3 and 19). Miocene basalts are interbedded with the Tesuque Formation beneath parts of the plateau, but are absent in wells drilled to depths of 300 to 575 m (1000–1900 ft) in the Buckman well field (Shomaker, 1974). Most of the production from municipal supply wells in Guaje Canyon and in lower Los Alamos Canyon comes from the Tesuque Formation.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 
The hydrogeology of the Pajarito Plateau is probably typical of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their recharge from adjacent mountainous areas. The plateau overlies the deepest part of the west-tilted Española basin adjacent to the highlands of the Jemez volcanic field. Hydrogeologic units consist of Miocene and Pliocene basin-fill deposits and interfingering volcanic rocks from the Jemez and Cerros del Rio volcanic fields. Miocene and Pliocene sedimentary and volcanic rocks are covered by Pleistocene ash-flow tuffs making up the Pajarito Plateau.

Groundwater of the Pajarito Plateau occurs as shallow groundwater in canyon-floor alluvium, as moderately deep perched groundwater in bedrock units of the vadose zone, and as deep groundwater associated with the regional saturated zone. The vadose zone is between 200 (600 ft) and 375 m thick beneath mesas of the plateau. Vadose zone hydrogeology is dominated by thick deposits of Bandelier Tuff in the surface and near-surface environment and by Cerros del Rio basalt and the Puye Formation at depth.

The regional aquifer consists of the Miocene Tesuque Formation, older fanglomerates, and pumice-rich volcaniclastic sedimentary rocks, Mio-Pliocene river deposits, and the Pliocene Puye Formation. Together, the older fanglomerate, Mio-Pliocene river deposits, pumice-rich volcaniclastic rocks, and Puye Formation form a westward-thickening wedge of relatively coarse-grained deposits that represent the most productive part of the regional aquifer in the western Española basin (Purtymun, 1995). The Tesuque Formation is also an important groundwater producer, but it is a relatively fine-grained unit that is not as productive as the coarse-grained volcaniclastic and riverine deposits.

Basalts are intercalated with the Tesuque Formation, older fanglomerates, and the Puye Formation. The Tesuque and older fanglomerate basalts are extensively altered and appear to be relatively poor producers of groundwater, although data are limited. The youngest basalts, belonging to the Cerros del Rio volcanic field, form part of the regional aquifer in the south-central part of the plateau. These basalts are mostly unaltered, and highly porous interflow zones may act as important groundwater pathways locally.

	ACKNOWLEDGMENTS

 
We would like to acknowledge the work of Bill Purtymun whose geological and hydrological investigations over three decades defined many fundamental aspects of the hydrogeology of the Pajarito Plateau. David McGraw at the New Mexico Bureau of Geology and Mineral Resources provided the LANDSAT base map used in Fig. 2. Jamie Gardner and Claudia Lewis of LANL provided the current fault map for the Pajarito fault zone that was modified for use in Fig. 3. Our ideas about the geology of the Pajarito Plateau have greatly benefited from discussions with our colleagues at LANL, especially Scott Baldridge, Bill Carey, Greg Cole, Jamie Gardner, Fraser Goff, Alexis Lavine, Claudia Lewis, Steve Reneau, Rick Warren, and Giday WoldeGabriel. The Groundwater Protection Program and the Environmental Restoration Project at Los Alamos National Laboratory supported this work. Thorough reviews by Fraser Goff and David Sawyer substantially improved the manuscript and are greatly appreciated.

	REFERENCES

TOP ABSTRACT INTRODUCTION REGIONAL GEOGRAPHIC SETTING REGIONAL GEOLOGIC FRAMEWORK RELATION BETWEEN GEOLOGIC AND... GEOLOGIC SETTING OF GROUNDWATER... CONCLUSIONS REFERENCES

 

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				K. H. Birdsell, B. D. Newman, D. E. Broxton, and B. A. Robinson Conceptual Models of Vadose Zone Flow and Transport beneath the Pajarito Plateau, Los Alamos, New Mexico Vadose Zone J., August 16, 2005; 4(3): 620 - 636. [Abstract] [Full Text] [PDF]

				B. A. Robinson, D. E. Broxton, and D. T. Vaniman Observations and Modeling of Deep Perched Water Beneath the Pajarito Plateau Vadose Zone J., August 16, 2005; 4(3): 637 - 652. [Abstract] [Full Text] [PDF]

				E. H. Keating, B. A. Robinson, and V. V. Vesselinov Development and Application of Numerical Models to Estimate Fluxes through the Regional Aquifer beneath the Pajarito Plateau Vadose Zone J., August 16, 2005; 4(3): 653 - 671. [Abstract] [Full Text] [PDF]

				E. Kwicklis, M. Witkowski, K. Birdsell, B. Newman, and D. Walther Development of an Infiltration Map for the Los Alamos Area, New Mexico Vadose Zone J., August 16, 2005; 4(3): 672 - 693. [Abstract] [Full Text] [PDF]

				P. H. Stauffer and W. J. Stone Surface Water-Groundwater Connection at the Los Alamos Canyon Weir Site: Part 2. Modeling of Tracer Test Results Vadose Zone J., August 16, 2005; 4(3): 718 - 728. [Abstract] [Full Text] [PDF]

				B. A. Robinson, G. Cole, J. W. Carey, M. Witkowski, C. W. Gable, Z. Lu, and R. Gray A Vadose Zone Flow and Transport Model for Los Alamos Canyon, Los Alamos, New Mexico Vadose Zone J., August 16, 2005; 4(3): 729 - 743. [Abstract] [Full Text] [PDF]

				P. H. Stauffer, K. H. Birdsell, M. S. Witkowski, and J. K. Hopkins Vadose Zone Transport of 1,1,1-Trichloroethane: Conceptual Model Validation through Numerical Simulation Vadose Zone J., August 16, 2005; 4(3): 760 - 773. [Abstract] [Full Text] [PDF]

				S. G. McLin, B. D. Newman, and D. E. Broxton Vadose Zone Characterization and Monitoring Beneath Waste Disposal Pits Using Horizontal Boreholes Vadose Zone J., August 16, 2005; 4(3): 774 - 788. [Abstract] [Full Text] [PDF]

				E. P. Springer Statistical Exploration of Matrix Hydrologic Properties for the Bandelier Tuff, Los Alamos, New Mexico Vadose Zone J., July 18, 2005; 4(3): 505 - 521. [Abstract] [Full Text] [PDF]

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