New Mexico Geological Society Annual Spring Meeting — Abstracts

The Mound Springs quadrangle encompasses the northern edge of the Tularosa Basin and southern exposures of the Oscura Mountains. We mapped gypsum-depositing springs and related extinct gypsum-spring deposits in the central and southern part of the area because one of the active springs provides a home for the endangered White Sands pupfish (Cyprinodon tularosa). The northern part of the quadrangle includes east-sloping cuestas of highly faulted Pennsylvanian and Permian Madera, Bursum, Abo, and Yeso formations. The high-angle Laramide (?) faults trend northwest. These are cut by east-west-trending, vertical, mid-Cenozoic diabasic dikes. The bedrock is offset by Quaternary faults at the south end of the Oscura Mountains. One east-west trending normal fault parallels a diabasic dike and offsets (with a fault scarp) mid-late Quaternary piedmont deposits. Another Quaternary normal fault trends northnortheast and truncates (with a scarp) Quaternary pediments, piedmont gravels, and east-dipping cuestas.

Piedmont landforms and gravelly deposits from the Oscura and San Andres Mountains are classic alluvial aprons with three or more inset levels of channel, fan, and eolian sand-loesssheet development. Modern channels gather in the north-central part of the quadrangle and continue southwestward toward Salt Spring at the head of the Salt Creek drainage, a perennial saline stream in the northern Tularosa Basin. The gravel-poor Holocene deposits adjacent to the channels are inset below older gravelly piedmont deposits along the medial parts of dissected fans, but partially bury distal piedmont gravel bars farther out in the basin.

The present-day Mound Springs are crater-topped conical hills of gypsum deposited around the margins of calcium-sulfate-dominated brackish springs. These cratered mounds reach up to 5.5 m high and 50 to 250 m across. Dozens of extinct cratered mounds occupy the western flank of an earlier and much larger accumulation of discharge-related gypsum, covering an area of at least 16 km². Similar extensive fossil discharge deposits are present to the northeast and southwest of the Mound Springs area. These older deposits are pitted with aligned sinkholes more than 8 and perhaps as much as 13 m deep.

Lewis Gillard and Leo Gabaldon helped with the graphic presentation.