ABSTRACT

Six steep (0.6-4.3°), short (radial length 1.5-5 km), deeply incised Pliocene to early Pleistocene paleofans and associated axial-fluvial strata are preserved adjacent to the footwall of the Palomas half graben in the southern Rio Grande rift. Two different scales of interbedding of lithofacies are evident in the exposed basin-fill sediment. The footwall-derived alluvial-fan strata are arranged into 3-10 m-thick cyclothems representing three events: (1) development of calcic paleosols that were contemporaneously or subsequently truncated by channels up to 4 m deep, (2) infilling of channels by turbulent-flow, horizontally bedded and cross-bedded conglomerates, and (3) deposition of hyperconcentrated-flow pebbly sandstones. These cyclothems probably resulted from paleoclimatic changes on the scale of 150 kyr. On a larger geographic and temporal scale, the axial river (ancestral Rio Grande), which primarily deposited cross-bedded pebbly sand, experienced kilometer-scale lateral movement toward the footwall block, resulting in one thick (80 m) axial-fluvial lithosome in the northern part of the basin and two or three thinner (8-25 m) axial-fluvial lithosomes in the south. A protracted period of fault movement and consequent basin tilting was the probable driving force behind lateral movement of the river, and toe cutting of the fans by the river was the dominant process.