New Mexico Geological Society Annual Spring Meeting — Abstracts

A recent compilation of NMBGMR-STATEMAP geologic mapping of the Bitter Lake, South Spring, and Bottomless Lakes 7.5-min. quadrangles in the Lower Pecos valley depict a late Pleistocene- Holocene floodplain, flanked to the west by Plio-Pleistocene alluvial terraces, dissected by the Arroyo Hondo. These are cut into Permian limestone of the San Andres Formation to the west, overlain by the thick evaporite sequence of the Artesia Group. The extensive gypsum beds of the Seven Rivers Formation, riddled with karstic depressions and sinkholes, underlie the valley and form the eastern bluffs of the valley margin. Many of these sinks serve as conduits for numerous springs derived from the underlying artesian aquifer, and are oriented roughly parallel to or orthogonal to the SW-NE trending regional structural buckles.

Three alluvial terraces are mapped west of the Pecos valley: remnants of the Plio–Pleistocene Blackdom Terrace, the broad mid-late Pleistocene Orchard Park Terrace, and the late Pleistocene Lakewood Terrace. The inset modern Pecos floodplain, reaching a maximum thickness of ~45 m along the western margin, is comprised of late Pleistocene braided stream deposits, and 3 Holocene meanderbelts. A series of collapse depressions extending roughly 14 km on the eastern valley margin, also contain late Pleistocene-mid Holocene Pecos River alluvium.

The Arroyo Hondo has built extensive fans onto the Orchard Park Terrace throughout the Pleistocene. Gravel pits in Arroyo Hondo fan deposits exhibit decreasing pedogenic carbonate development from south to north, implying a northward shift of the river to its present entrenched channel. A large late Pleistocene distributary fan pushed the Pecos River eastward into the collapse depressions, allowing the early-mid Holocene Arroyo Hondo to build its first two of three meanderbelts across the entire Pecos floodplain. Two Pecos River meanderbelts have since cut through these Rio Hondo alluvial deposits.