EBTAG Annual Workshop and Field Trip
May 13, 2014

¹New Mexico Bureau of Geology & Mineral Resources, 801 Leroy Place, Socorro, NM, 87801, dkoning@nmbg.nmt.edu

²Emeritus Senior Environmental Geologist; N.M. Bureau of Geology & Mineral Resources, P.O. Box 4370, Albuquerque, NM, 87196-4370

We summarize the lithologic character of lithosomes A and S in the Santa Fe area. A lithosome is a body of sedimentary rock, exhibiting a relatively uniform and distinctive lithologic character, that intertongues with different sedimentary rock types. Both lithosomes A and S were deposited on a piedmont by streams flowing westward out of the Sangre de Cristo Mountains between 26 and 13 million years ago. Tectonism associated with the Rio Grande rift has tilted these deposits 10-15° to the west.

Lithosome A was deposited by streams whose headwaters drained granitic rocks. It contains lenticular to ribbon-like channel-fills of pebbly sandstone interbedded with medium to thick, tabular beds of poorly sorted, clayey-silty sand. The proportion of the latter lithofacies increases to the west, where it becomes better sorted and finer-grained. The sand is arkosic and the gravel fraction contains granite with <10% quartz, quartzite, and yellowish Paleozoic siltstone. Evaluation of data from 13 pumping tests of wells screened in this unit indicate low hydraulic conductivities of 0.03-0.9 ft/day.

In contrast, lithosome S was deposited by a larger drainage associated with an ancestral Santa Fe River. Its sediment was deposited on a fluvial fan which was likely comparable to modern, large fluvial fans in the eastern San Joaquin Valley in California. This unit interfingers with lithosome A sediment to the north and south. Lithosome S is overall coarser and redder than lithosome A. Strata consist of sandstone and pebbly sandstone channel-fills interbedded with floodplain deposits of fine-grained sandstone to mudstone. Channel-fills typically display very thin to thin, horizontal to cross-stratified beds. Channel-fills are generally amalgamated (stacked on top of each other) and the widths of the resulting channel-fill complexes are greater than those of lithosome A, commonly 10s to 100s of meters. Overall texture of lithosome S is coarsest near the axis of the fluvial fan, which is approximately between Tano Ridge and the Santa River, and fines to the north, west, and south of the axis. Although relatively arkosic, its pebble and sand fraction contains appreciable (1-30%) Paleozoic and Proterozoic detritus derived from what is now the upper Pecos River drainage -- including sand grains and clasts of quartzite, sandstone, siltstone, and limestone. Lithosome S exhibits relatively high hydraulic conductivity values for coarser strata near the fan axis (0.4-23 ft/day) but values are comparable to lithosome A for finer-grained strata to the west and north of the fan axis.

Inspection of well data indicates that lithosome S coarsens up-section, in contrast to lithosome A, but there is a lower, relatively coarse tongue between the Santa Fe city well field and Tano Ridge. Lithosome S gradationally overlies a light gray to tan, 200-400 ft-thick, lower tongue of lithosome A. Basal strata of lithosome S and lithosome A are notably fine-grained near the city well field. They are interpreted to have been deposited near the axis of the Santa Fe embayment ca. 25 Ma, on a topographically low basin floor, and intertongue westward with volcaniclastic alluvial fan deposits of lithosome E. Subsequently, lithosome S advanced westward over lithosome E fan deposits until it extended to the Santo Domingo Basin. This westward progradation probably accounts for the upward coarsening of lithosome S. Between 13.5 and 13.0 Ma, both lithosomes A and S coarsen, and the northern boundary of lithosome S shifted southward. Between ~13 and 3 Ma, piracy of the Pecos River captured what was the headwaters of the Santa Fe River, so that the drainage area of the Santa Fe River was smaller by the time of deposition of the Plio-Pleistocene Ancha Formation.