FACIES ARCHITECTURE OF PLIOCENE-LOWER PLEISTOCENE ALLUVIAL-FAN AND AXIAL-FLUVIAL STRATA ADJACENT TO THE INTRABASINAL MUD SPRINGS MOUNTAINS, SOUTHERN RIO GRANDE RIFT

The Mud Springs Mountains (MSM) near Truth or Consequences, New Mexico, is a large (10X2 km), north-trending, intrabasinal fault block located within the northern part of the eastward-tilted Palomas half graben. Along the southern tip of the MSM, Pliocene-lower Pleistocene basin fill up to 90 m thick records the interplay between small alluvial fans derived from the footwall and hanging wall of the MSM, the axial Rio Grande, and large alluvial fans derived from the Black Range (BR) and smaller fault blocks that constitute the western hanging wall of the Palomas basin. BR-derived alluvial fans extended 15-30 km into the basin before encountering the MSM fans and the axial Rio Grande. Extensive badlands and deep canyons allow mapping strata of the four dispersal systems at the scale of 1:12,000. Alluvial-fan detritus derived from the MSM footwall extends only 50-600 m from the mountain front before interfingering with and being replaced by BR-derived alluvial-fan sediment. Hanging wall-derived alluvial-fan detritus of the MSM spreads farther into the basin (~1.0 km), where it interfingers with and is replaced by both BR-derived 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 (Caballo Mountains), probably in response to active faulting and concomitant eastward tilting of the floor of the larger half graben. 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 by toe-cutting by the axial Rio Grande and by BR-derived, cobble-bed fan channels, whose erosive power was related to high-relief and high-elevation catchments with areas on the order of ~500 km².