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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

View larger version (42K): [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 a new window]   Fig. 2. Map of the Pajarito Plateau and surrounding region showing significant geographic features.

View larger version (78K): [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 (47K): [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.

View larger version (43K): [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.

View larger version (86K): [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 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.

View larger version (57K): [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.

View larger version (100K): [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.

View larger version (76K): [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.

View larger version (89K): [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.

View larger version (98K): [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 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 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.

View larger version (87K): [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 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 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.

View larger version (36K): [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.

View larger version (78K): [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.