New Mexico Geological Society Annual Spring Meeting — Abstracts

Understanding the basin structure and Santa Fe Group (SFG) basin fill stratigraphy in the northeastern San Agustin Plains is critical for accurate estimations of groundwater volume and drawdown effects associated with a proposed groundwater withdrawal project. We present a preliminary basin model based on well analysis coupled with synthesis of previous geologic mapping, electrical resistivity soundings, and gravity data. These indicate that a 4-8 km wide, NNE trending graben with two sub-basins is present north of a south-down fault that extends WSW from the VLA headquarters.

Analysis of cuttings and geophysical logs for two deep wells allows partial characterization of the SFG. The northern well (SA-222, 1565 ft TD) is located 11 km north of Hwy 60 on a fault-bounded structural shelf. It encountered 724 ft of SFG underlain by 841 ft of interbedded basaltic andesites, cemented volcaniclastic sediment, and ignimbrites (Vicks Peak, La Jencia, and Hells Mesa Tuff). The well's location and sand+gravel composition is consistent with a piedmont SFG lithofacies assemblage. The southern well (SA-221, 3495 ft TD) was drilled in the middle of the southern sub-basin and did not encounter bedrock. Both wells exhibit coarsening-upward trends culminating in a 320-340 thick, upper interval consisting of sandy gravels interbedded with clayey sands. A lower interval (2075-3495 ft in SA-221; 507-724 ft in SA-222) consists of a fine-grained upper subunit and a coarser lower subunit.

SFG lithofacies assemblages interpreted in SA-221 include basin floor as well as western and eastern piedmont. Basin floor deposits are characterized by low neutron and resistivity signatures and variable sand composition. The petrology of western piedmont deposits consists of intermediate volcanic detritus with up to ~50% tuffs (proportion of latter increasing up-section). Inferred eastern piedmont deposits contain angular-subangular sand composed predominately of interpreted Vicks Peak Tuff (VPT). This sand gives particularly high gamma ray readings (170-200 API), collaborated by similar values for in situ VPT in well SA-222. Although a local footwall source cannot be ruled out, we favor an eastern provenance for VPT detritus because a 250-260 ft thick, sandy gravel interval composed primarily of VPT occurs in a well 9 km to the east. As would be expected per Walther's Law, in SA-221 fine-grained basin-floor deposits lie between the VPT sand and western piedmont intervals.

Preliminary hydrogeologic assessments are made for the SFG. In SA-221, we have estimated a porosity compaction curve based on density, neutron and sonic geophysical logs. The upper SFG shows high total porosities (10 to 40%), though clay content significantly lowers the effective porosity (consistent with relatively low resistivity values in the upper gravely interval of SA-222). The lower SFG interval has lower total porosities (5% to 15%) and higher clay content that likely translates to poor groundwater yields. Two sandy intervals in SA-221, at 680-1210 ft and 1555-2075 ft, exhibit relatively high resistivity and neutron values and total porosities of 24% to 48%. In general, the total aquifer storage should be lower than previously estimated both because of the higher clay fraction and the degree of compaction.