New Mexico Geological Society Annual Spring Meeting — Abstracts

Salt Creek drains the southern Oscura Mountains, Mockingbird Gap Hills, and northern San Andres Mountains southward to Big Salt Lake (playa), and rarely to Lumley Lake (playa). Salt Creek is perennial in an upper reach due to base flow, and is home to one of two endemic populations of White Sands pupfish (Cyprinodon tularosa) in the northern basin. Although incision of the valley of Salt Creek into older basin fill and through intermediate terraces is subtle, local exposures of cross-cutting relationships, stratigraphy, and paleontological remains allow interpretations of the valley’s Quaternary geologic history.

Tularosa Basin fill consists of many layers of reddish brown fine-grained alluvium, channels of pebble gravel and pebbly sand, and gypsiferous layers of pedogenic, eolian, and spring-evaporite origins. This Quaternary fill locally contains megafauna trackways and one known fragmentary proboscidean tusk. Some coarsegrained channels are exposed as meandering ridges in deflated blowouts south of 33° 13’.

Inset below the top of the basin fill are several discontinuous terraces capped with pedogenic gypsum crust containing reworked pebbles from the basin fill. Inset below these terraces is a longitudinally wide-spread dark brown, gray, and reddishbrown fine-grained unit of thinly bedded and crossbedded alluvium. Where this unit is exposed due to lateral stripping of overlying alluvium, megafauna trackways are evident. A partial articulated skeleton of the late Pleistocene and early Holocene bison Bison antiquus occidentalis was recovered from this alluvium. Very tiny hydrobiid snail shells and fish scales are found in this unit downstream from the bison locality.

Overlying this unit south of 33° 17’ is thicker-bedded reddish brown fine-grained alluvium and local cross-cutting pebbly channels containing two extinct members of Pleistocene megafauna, the horse Equus conversidens and the camel Camelops hesternus. An extensive marsh covering an area more than 50 km² deposited up to 3 m of gypsum above the alluvium. This marsh contains the aquatic snails Planorbella and Stagnicola, scales and bones of small fish and aquatic microorganisms. Three radiocarbon ages from vegetal remains and ostracode valves date this marsh from 10,900 to 10,160 radiocarbon years before present.

Salt Creek entrenched through this marsh and the underlying alluvium as much as 12 m. Entrenchment followed deflation of extensive blowouts and playas at the down-stream end of Salt Creek (e.g. Big Salt Lake) during Holocene time. At least two intermediate levels of alluvial terraces mark still-stands in erosion in both the blowouts and Salt Creek.