GEOLOGIC MAP OF QUATERNARY DEPOSITS ON THE CAPITOL PEAK SE AND SHEEP MOUNTAIN QUADRANGLES WITH ILLUSTRATIONS OF UNCOMMON SURFICIAL FEATURES, NORTHERN TULAROSA BASIN, SOUTH-CENTRAL NEW MEXICO

The quadrangles straddle the northernmost floor of the Tularosa Basin where three drainages influence the landscape at three different levels. The Three Rivers fan dominates the southeastern quadrant and provides a high, nearly planar surface against which the drainage of the Carrizozo valley aggrades, and below which the upper Salt Creek-Mound Springs drainage is incised. Sulfate-precipitating wetlands at different levels created low-gradient moist surfaces. One such surface stalled the progress of the Carrizozo lava flow. Burned grasses buried ~280 cm at the snout of the lava flow yielded an age of 4,680 ± 40 radiocarbon yrs BP (Beta-277075; Cal BP 5,480 to 5,310 with 95 % probability). The lava flowed over alluvium and a complex environment of evaporite deposition and eolian features. These contrasting depositional processes buried the lava’s margins and kipukas by up to 3 m of alluvial, evaporite, and loessal sediments. Post-lava alluvium from the Carrizozo Valley accumulates on low margins of the lava flow, on elevated gypsum features, and on older dissected alluvium.

To the west, the broad valley of Salt Creek is incised up to 13 m below the level of maximum basin fill. The valley borders of Salt Creek consist of wind-deflated basin-fill exposures of fine-grained clastic and gypsic beds and cross-bedded pebbly sand channels, and moist fine-grained alluvial/eolian slopes where groundwater seeps just below the surface. At least three distinct levels of inset terrace deposits mark aggradation after episodes of valley incision. The most extensive terrace 2-11 m above Salt Creek is a 1-3-m-thick gypsum-wetland deposit yielding three radiocarbon ages of 10,900, 10,600, and 10,130 yrs. Bones of extinct megafauna are rare in deposits older than the marsh.

The SW quadrant consists of a complex string of marshes, playas, blowouts, eolian dunes, and alluvial channels with discontinuous outcrops of basin fill. Subsurface dissolution indicators are karst features and tilted basin-fill. The uncommon surficial features related to accumulation by gypsum precipitation include gypsum spring mounds, megamounds, meandering raised-levee streams, platform marshes, raised-rim marshes, and hummocky rolling plains that appear to represent modification of previously deposited gypsum by eolian and/or dissolutional processes.