Preliminary lithostratigraphy, interpreted geophysical logs and hydrogeologic characteristics of the 98th Street core hole, Albuquerque, New Mexico

Core samples, cuttings, and numerous geophysical logs obtained from the 1560 ft (475.5 m) core hole drilled at 98th Street on the west side of Albuquerque provide key stratigraphic and hydraulicproperty information for the upper clastic sediments of the Santa Fe Group, which form the principal aquifer in the region. The core hole and an adjacent water-level monitoring well were drilled cooperatively by the U.S. Geological Survey (USGS) and the City of Albuquerque and investigated in collaboration with the New Mexico Bureau of Mines and Mineral Resources and the New Mexico Office of the State Engineer to improve understanding of aquifer characteristics and controls on ground-water availability and quality. The 751.5 ft (229 m) of core samples recovered from the core hole are the only undisturbed samples of nonlithified sediments of the upper part of the Santa Fe Group that have been collected in this area. These samples have allowed us, for the first time, to directly observe and characterize the lithic and sedimentologic features of this part of the section, and to correlate the detailed geologic features with geophysical-log characteristics, magnetic susceptibility measurements, hydraulic variables, and trace-element geochemistry. The adjacent well was designed to be an areally representative ground-water level and water-quality monitoring well for the Santa Fe Group aquifer. This report chiefly addresses the lithologic, stratigraphic, and hydrogeologic features determined from the 98th Street core hole; other reports address related characteristics.

Previous geologic studies predicted the stratigraphy at the site to be, from the land surface downward: 1) Quaternary alluvial and eolian valley-border sediments; 2) fluvial sand and gravel of the upper unit of the Santa Fe Group (Ceja Member of the Santa Fe Formation of Kelly, 1978; equivalent to the Sierra Ledrones Formation of Machette (1978a); 3) downward-fining basin-floor silty clay deposits and 4) fluvial sandy and silty facies of the middle unit of the Santa Fe Group (the Middle Red Member of Bryan and McCann, 1937, and Lambert, 1968). New geologic interpretations indicate that the drill site is in a fault block bounded by east-dipping normal faults and the oblique Atrisco-Rincon fault zone.

Core-hole sampling recovered 760.6 ft (231.8 m) of core, in core segments 2.1-2.375 in. (5.3-6 cm) in diameter, and 0.2-10 ft (6.1 cm-3 m) long. The core hole was cased with centered 3-in. PVC casing, and is available for geophysical logging. The monitoring-well hole contains four piezometers at depths of 1544 ft (470.6 m), 1112 ft (338.9 m), 749 ft (228.3 m), and 458 ft (139.6 m).

Sediments in the core are loose to weakly cemented gravel, sand, silt, and clay, and lithified sandstone. Laboratory analyses of particle-size distributions of 28 channel samples show that most silty sand samples are uniformly graded and poorly sorted; medium sand samples are moderately sorted. Six principal sediment types are used to describe the core; these sediment types are repeated in various combinations throughout the core and are used to define 22 lithologic units in the cored interval. The six principal sediment types contain sequences of beds having similar modal grain size and sedimentary structure, and are listed in decreasing abundance:

1) Silty fine sand, poorly sorted, containing a coarse silt matrix. Geophysical logs show highly variable baselines with deflections that are related to clay beds and sequences of silt, clay, and sorted fine sand. Density values of 2.12-2.25 g/cc and porosity values of 30-35 percent are typical.

2) Medium sand, moderately to poorly sorted. Geophysical logs show baselines of low variability with deflections that are related to clay beds and sequences of silt, clay, and sorted fine sand. Density values of 2.05-2.20 g/cc and porosity values of 30-35 percent are typical.

3) Clayey sandy silt, poorly sorted, locally microlaminated clay and silt, generally nonplastic. Geophysical logs show highly variable baselines with deflections that are related to sequences of clay and fine sand. Density values of 2.1-2.2 g/cc and porosity values of 30-40 percent are typical.

4) Silt and clay, characteristically red to reddish brown and medium to high plasticity, massive to indistinctly microlaminated. Geophysical logs show variable baselines with broad, high-amplitude compound spikes that are related to sequences of silt and fine sand. Density values of 2.12-2.25 g/cc and porosity values of >45 percent are typical.

5) Sand and gravel, poorly sorted. Geophysical logs show variable baselines with deflections that are related to sequences of silty and sorted fine sand.

6) Sandstone, fine-to-medium grained, poorly sorted, cemented chiefly by calcite, which fills the original pore space. Geophysical logs show density values >2.25 g/cc and porosity values <30 percent.

The 22 lithologic units are correlated with recognized basin-floor fluvial lithofacies (Hawley, 1996), which include sand and gravel (lithofacies I), sand with lenses of pebbly sand, silt, and silty clay (lithofacies II), and interbedded sand, silt, and silty clay (modified lithofacies III, IV, IX).

The sediments in the core hole are correlated with three informal lithostratigraphic units. The top unit, 0-19 ft (0-5.8 m) depth, consists of Quaternary eolian sand and valley-border alluvium. Coarsegrained deposits in the 19-97 ft (5.8-29.6 m) interval are correlated with the upper unit of the Santa Fe Group. The fine-grained section in the 97-787 ft (29.6-239.9 m) interval is correlated tentatively with the middle unit of the Santa Fe Group. This section contains thick sequences of laminated red and olivebrown clay and silt overbank deposits (441-787 ft) in the distinctive Atrisco member of Connell and others (1998). The Atrisco is correlated with fine-grained zones in numerous wells throughout the central Albuquerque metropolitan area, and is recognized as a zone that separates the upper Santa Fe aquifer from underlying middle Santa Fe deposits. The lower section of the middle unit of the Santa Fe, 787-1500 ft (239.9-457.2 m) depth, includes an upper sequence of moderately sorted channel-fill medium sand, and a lower sequence of sand, silt, and clay overbank deposits. The age of the cored interval is not known precisely. The upper Santa Fe gravel is related regionally to a through-flowing river system that was established in the Rio Grande rift valleys in Early Pliocene time, >4.5 MA. The middle Santa Fe unit is dated tentatively by correlation with a fossiliferous section, in which sandy beds that directly underlie the upper Santa Fe are Late Miocene (Hemphellian), 4.6- 8.9 MA. Further, the middle Santa Fe unit, with dominantly normal magnetic polarity, may have been deposited during closely spaced normal magnetic chrons 5.9-8.3 Ma.

Four hydrostratigraphic units summarize the hydrogeologic framework for the 98th Street site: 1) Quaternary valley-border deposits, 2) upper Santa Fe sand and gravel deposits, 3) middle Santa Fe overbank deposits, and 4) middle Santa Fe channel-sand deposits. Empirical values of horizontal hydraulic conductivity estimated from core samples reveal a previously unknown contrast in hydraulic conductivity in the lowest two hydrostratigraphic units. Correlations among numerous wells show that the distinctively fine-grained Atrisco member, with estimated hydraulic conductivities (K) of <0.02-17 ft/day, is a laterally extensive barrier to vertical ground-water flow. The underlying unit that contains moderately sorted medium sand is a potential aquifer production zone that should be investigated further.

Laboratory determination of vertical hydraulic conductivity values for fine-grained core samples range from 10^-2 to 10^-7 ft/day; recompacted sandy samples have K values of 1 to 10^-2 ft/day. Results of tests conducted with increasing effective stress show that K values of all samples decrease with decreasing porosity. Comparison of K values from laboratory, empirical, and calculated geophysical values shows discrepancies of 1-3 orders of magnitude (ft/day), indicating that additional analyses of core samples and geophysical data are necessary for future characterization of the Santa Fe Group aquifer.