GEOLOGY OF THE NORTHEASTERN MARGIN OF THE TULAROSA BASIN, NEW MEXICO

We present a 1:50,000-scale map compiled from four 7.5-minute quadrangles between Tularosa and Bent, NM. This map encompasses the western slope of the northern Sacramento Mountains and the NE Tularosa Basin, which are separated by the NW-striking, Quaternary Alamogordo fault. Mountain block strata dip N-NE and consist of the Pennsylvanian Holder Fm. and Permian Bursum, Abo, Yeso, and San Andres Fms. The Hondo and Fourmile Draw Mbrs. of the San Andres thicken toward the NW. Mafic-intermediate alkalic 36-38 Ma sills and small laccoliths related to the Sierra Blanca volcanic field (SBVF) preferentially intrude the Bursum and the Yeso Fms. This area includes the Ancestral Rocky Mtn. Bent dome and the southern margin of the Laramide Sierra Blanca Basin, the latter coinciding with the ENE-striking Salinas Draw fault near the northern map boundary. A N-striking, W-down fault/monocline that affects the San Andres Fm. is on the W edge of the basement-cored Bent dome. The dominant structures in the bedrock are NE-striking normal faults that parallel trachyandesitic dikes. The dikes cut the sills. These NE elements likely relate to local stress fields associated with southern SBVF intrusions and with rifting.

Tularosa basin-fill was deposited as west-sloping alluvial fans during Miocene-Quaternary extension. Sand and gravel dominate near the mountain-front but gravel content decreases notably 1-3 km to the W. The medial fan consists largely of clayey-silty sand, but finer material dominates the distal fans. An upper, coarser-grained unit (40-90 m thick), less obvious to the south and underlying Holocene sediment, may be related to deposition during Plio-Pleistocene climates. Pleistocene surfaces contain more gypsum than Holocene surfaces and are ill-suited for agriculture.

The alluvial fan surface has been dynamic during the Holocene. Widespread fine-grained deposition occurred during the middle Holocene, and was immediately followed by slightly coarser-grained deposition. Denudation occurred in the late Holocene, leaving topographic highs capped by the coarse-grained sediment. We mapped both the approximate aggradation surface and the modern eroded surface using surface polygon feature classes in GIS, which can be used to calculate the amount of erosion over the past 2,000-3,000 yrs.