New Mexico Geological Society Annual Spring Meeting — Abstracts

The upper Santa Fe Group in south-central New Mexico was formally defined as the Palomas Formation over 30 years ago and is interpreted as representing alluvial fan and axial-fluvial depositional environments (Lozinsky and Hawley, 1986). The Palomas Formation was previously subdivided into 3 to 5 member-rank lithostratigraphic units based on their general provenance (derived from the hanging wall or footwall of surrounding uplifts versus ancestral Rio Grande). Through recent mapping efforts in the Engle and Palomas Basins, we have established a more detailed Palomas Formation stratigraphy that reveals up-section trends in lithology and texture. These trends have significant implications for depositional setting, tectonics, paleoclimate, and groundwater hydrology in the arid basins of the southern Rio Grande rift. Our revised Palomas Formation stratigraphy uses provenance and texture to differentiate 5 member-rank units whose respective contacts are typically gradational or interfingering. These 5 units are mappable at 1:24,000 across an area of ~3,000 km² in Sierra and Socorro Counties, and can locally be expanded to as many as 15 subunits.

The 5 primary lithostratigraphic units and their estimated ages are: lower (~5-3.6 Ma), middle (~3.6-2.2 Ma), transitional (~2.5-2.0 Ma), upper (~2.0-0.8 Ma), and axial-fluvial (~5-2 Ma, where preserved). Age ranges are from radiometric ages, magnetostratigraphy, and biostratigraphy. The lower Palomas Formation consists of cemented pebbly channel-fill gravel with subordinate massive, sandy to silty hyperconcentrated flow deposits. Fine sand and silt, including hyperconcentrated flow deposits, and <20% pebbly, clay-free channel-fills characterize the middle unit. The transitional unit features silty sand, prominent carbonate ledges, and more channel-fill gravels than the middle unit but fewer than the upper unit. The upper unit consists of laterally extensive channel-fills and sand lenses interbedded with subordinate silty to clayey deposits. Interfingering with the lower and middle units is another member-rank unit, the axial-fluvial facies, consisting of cross-stratified sand with lesser gravel (including extra-basin lithologies) and finer floodplain deposits.

A newly recognized coarsening-fining-coarsening pattern in alluvial fan facies of the Palomas Formation indicates subdued deposition of coarse debris-flow or channel-fill gravel during the latest Pliocene to early Pleistocene (3.6 to 2.0 Ma), followed by significant fluvial deposition in transverse Rio Grande tributaries. We interpret this trend to reflect paleoclimate-modulated clastic input from the Black Range to the west. Lower sediment flux from these highlands coincided with a slightly wetter middle-late Pliocene (4-2.6 Ma). Progressive drying trends in the Pleistocene corresponded with piedmont progradation, perhaps due to more intense summer monsoons coupled with sparser hillslope vegetation. Other potential research avenues from our updated stratigraphy include testing models of axial-transverse sedimentation in extensional basins and incorporation into hydrostratigraphic units containing or confining aquifer systems in the Engle and Palomas Basins.

References:

1. Lozinsky, R.P., and Hawley, J.W., 1986, The Palomas Formation of south-central New Mexico—a formal definition: New Mexico Geology, v. 8, p. 73–78, 82.