Abstract

The Pliocene-Pleistocene Camp Rice and Palomas Formations in the Rio Grande rift of southern New Mexico provide an excellent test of the role of basin symmetry in the distribution of piedmont and axial-fluvial facies. In the asymmetrical Palomas and northern Mesilla basins, the axial-fluvial facies is characterized by multistory channel sands and sandstones and is concentrated near the locus of maximum subsidence within a few kilometers of the footwall scarp. Fanglomerate derived from the footwall uplift extends only a few kilometers or less from the footwall scarp, whereas alluvial-fan and alluvial-flat conglomerate, sand, and mudstone deposited on the hanging-wall dip slope occupy a much wider outcrop belt. In the symmetrical Hatch-Rincon basin, the axial-fluvial facies extends to within a few kilometers of both the northern and southern basin margins and has a much higher percentage of fine-grained overbank deposits, some of which contain calcareous paleosols. In all three basins, a tongue of fanglomerate as much as 30 m thick prograded over the axial-fluvial facies near the end of Camp Rice and Palomas deposition.

The distribution of facies of the Camp Rice and Palomas Formations supports previously published models that predict a two-stage history of asymmetrically subsiding basins. During tectonically active periods, axial-river channels preferentially avulse into the topographically lowest area of the alluvial plain, which is directly above the axis of maximum subsidence. During the postorogenic stage, when erosion rate exceeds subsidence rate, coarse, transverse-dispersed sediment progrades toward the center of the basin.

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