New Mexico Geological Society Annual Spring Meeting — Abstracts

Throughout most of its course in New Mexico, the Rio Grande has incised a channel into erodable basin-fill sediments of the Rio Grande rift. However, in the Elephant Butte area the river crosses the basin-margin Hot Springs fault at three localities to cut into more resistant bedrock of a marginal uplift . At the two northern localities, the Rio Grande has migrated 1 km or less east of the Hot Springs fault. At the southern locality, the river has migrated more than 2 km east of the fault and follows a complicated course around a prominent sandstone ridge, Long Ridge.

The two northern crossings probably resulted from laterally-cutting meanders where the Hot Springs fault juxtaposed Santa Fe Group sediments with erodable shales of the McRae Formation.

A stream piracy model is proposed for the southern crossing. In this model, the Rio Grande originally flowed west of the Hot Springs fault. East of the fault, a tributary of the Rio Grande eroded northward between Long Ridge and Elephant Butte and eventually captured the river north of Long Ridge. Plugging or partial plugging of the Rio Grande channel by debris aggrading at the mouth of a major western tributary, Cuchillo Negro Creek, may have aided in diverting the river into its new course, Elephant Butte Canyon.

Evidence supporting piracy includes; 1) Long Ridge is topographically higher than any ot her basin-fill deposits, 2) no fluvial deposits occur on Long Ridge, 3) resistant nature of sandstone comprising Long Ridge hinders migration. 4) absence of the two oldest terrace levels in Elephant Butte Canyon, and 5) Cuchillo Negro terraces shift south in conjunction with piracy. Piracy probably occurred during middle to late Pleistocene time.