New Mexico Geological Society Annual Spring Meeting — Abstracts

Well-exposed Pliocene-lower Pleistocene strata adjacent to the intrabasinal Mud Springs Mountains (MSM), near Truth or Consequences, New Mexico provide a nearly three-dimensional view of basin-fill and records the interplay between small alluvial fans derived from the MSM, large alluvial fans derived from the Black Range (BR) and smaller fault blocks, and the axial Rio Grande. The MSM is a large (10x2 km) northwest-trending intrabasinal fault block located within the northern part of the eastward-tilted Palomas half graben. The BR constitutes the western hanging wall and the Caballo Mountains (CM) constitute the eastern footwall.

Deep canyons and badlands allow mapping of nearly 115 m of strata at the scale of 1:10,000. Alluvial-fan sediment derived from the MSM footwall extended between 50-1000 m from the mountain front before interfingering with and being replaced by BR-derived alluvial-fan sediment. Hanging wall-derived alluvialfan sediment derived from the MSM extends further into the basin (~2.5 km) from the mountain front, where it interfingers with and is replaced by both BRderived alluvial-fan sediment and by sediment of the axial Rio Grande. The relative abundance of Rio Grande fluvial-channel deposits increases away from the MSM and toward the footwall of the Palomas basin (CM), probably in response to contemporaneous active faulting and eastward tilting of the half graben during most of the deposition. Spread of MSM-derived alluvial fans into the basin was inhibited by low and intermittent sediment yield from small (<1 km2 ) catchments in the MSM, as well as toe-cutting by the axial Rio Grande and by BR-derived fan channels, whose erosive power was related to high relief and high-elevation catchments with areas on the order of ~500 km² .