New Mexico Geological Society Annual Spring Meeting — Abstracts

Research into deposits of the Pecos River Valley has raised questions about the timing of valley incision and drainage-reorganization that disrupts the continuity between the north-flowing ancestral drainage system and modern south-flowing Pecos River. Deciphering the internal stratigraphy of the Gatuña Fm. and age of the overlying Mescalero paleosol is central to understanding the upper age limit of the formation and the subsequent entrenchment of the Pecos River. Solution subsidence from dissolution of evaporitic upper Permian rocks has caused variable accommodation and tilting that affected deposition and the stability of surfaces modulating petrocalcic horizon formation. This dynamic basin setting convolutes outcrop exposures and has caused discrepancies in geologic mapping. Variable field interpretations either: (1) place any post-Ogallala deposits as part of the Gatuña Fm. capped by the Mescalero or (2) frame it as a two stage package of Ogallala and post-Ogallala equivalent sediments where Pleistocene-aged deposits capped by the Mescalero are inset into Neogene-aged deposits overlain by an Ogallala-caprock equivalent termed the Pierce Canyon Caliche.

New uranium-series dates from this study returned minimum ages of 328 ±21 ka (n=4) and 208 ± 18 ka (n=4) building on evidence for a period of formation-capping petrocalcic soil development limited to the middle-Pleistocene. This variably aged soil would have experienced diachronous development including punctuated calcite precipitation during the warm and dry marine-isotope-stage 9 (MIS-9) and MIS-7 interglacial periods. The beginning of surface stability is bracketed between the deposition of the Lava Creek B ash at around 630 ka and the start of calcic development between ~570 ka (MIS-15) and ~420 ka (MIS-11) based on existing u-series dates. Optically stimulated luminescence dates place the end of petrocalcic development between ~143 and 90 ka (MIS-5). However, fossils tentatively dating to the late middle-Pleistocene (MIS-6) are found in deposits inset into the Mescalero at Nash Draw. If the paleosol is considered to represent the soil development between the end of Gatuña Fm. deposition and the entrenchment of the Pecos River, then soil development during the MIS-5 interglacial is genetically related to the contemporary south-flowing system. We argue that discrepancies in interpretations of these deposits and geomorphic surfaces are a result of diachronous calcic development related to solution subsidence throughout the middle-Pleistocene causing perceived stratigraphic distinctions rather than separate Neogene and Quaternary soil horizons.