Geology and late Cenozoic history of the Elephant Butte area, Sierra County, New Mexico

Abstract

A 256-km2 area surrounding the Elephant Butte Reservoir, Sierra County, New Mexico, was mapped to interpret the late Cenozoic (Pliocene to Holocene) history. The interpretation is based on examining faults, geomorphic features, and upper Cenozoic sedimentary and volcanic units.Less than 10% of the study area is covered by Precambrian and Paleozoic units, ap - proximately 40% is covered by upper Cretaceous units, and more than 50% is covered by Cenozoic units. The major Cenozoic sedimentary deposits are mapped as the Palomas Formation of the upper Santa Fe Group, which is divided into the piedmont and the axial facies. Other Cenozoic units include pediment, alluvial-fan, terrace, and eolian deposits.Regional geomorphic surfaces include the erosional Cutter surface, located along the rift margin, and the constructional Cuchillo surface located within the rift. Five terr ace levels occurring along the Rio Grande and Cuchillo Negro Creek and three pediment levels occurring along the Mud Springs Mountains are recognized and postdate the Palomas Formation. The Rio Grande is interpreted to have left the basin where it cut bedrock in the marginal uplifts by laterally cutting meanders at Long Point and Rattle - snake Islands and by stream piracy around Long Ridge. Most structural features resulted from Laramide and Rio Grande rift tectonic activity.Palomas Formation deposition occurred from early Pliocene to middle Pleistocene. The Rio Grande became a through-flowing drainage system flowing within the basin by 3.8 m.y. B.P. Initial incision of the Rio Grande during middle Pleistocene ended Palomas Formation deposition, formed the Cuchillo surface, and marked the end of major episodic movement on the Hot Springs fault. Since the initial incision, the Rio Grande has undergone several incision episodes and has partly migrated out of the basin. Presently, the Rio Grande has the potential to reestablish its course within the basin.