Fluvial and Alluvial Depositional Systems in Rift Basins
The Palomas basin in the Rio Grande rift of southern New Mexico is a Neogene half graben, whose Pliocene-lower Pleistocene fill (Palomas Formation) has been partly exhumed, providing a nearly three-dimensional view of basin-fill architecture. The distribution of Palomas Formation depositional systems and their lithofacies is strongly asymmetrical. Footwall-derived sediment was deposited on small (radial length < 2 km), steep (slopes 1-4°) fans and consists primarily of turbulent-flow, channel conglomerates and hyperconcentrated-flow, pebbly sand, whereas hanging wall-derived sediment was deposited on broader (radial length 15 km), gentler (slopes 0.7-1°) fans and displays a greater proportion of channels with lateral accretion sets and overbank mudstones, suggesting that medial and distal hanging wall-derived fan channels behaved like gravel-bed streams. Axial-fluvial sediment, deposited by the ancestral Rio Grande, occupies a narrow (< 5 km) belt near the footwall block and consists of multistory pebbly medium-coarse channel sand, mottled overbank mudstone, and fine sand deposited as crevasse splays or in small channels. One to three excursions of the axial-fluvial system toward the footwall, each lasting 105-106 years, were accomplished by toe cutting of the footwall-derived fans and resulted in deposition of multistory channel sands, while the western edge of the floodplain was dominated by deposition of overbank mudstone and crevasse-splay/small-channel fine sands. Tectonic tilting of the half graben is the probable cause of the fluvial incursions toward the footwall, because toe cutting of the fans is largely restricted to the footwall side of the basin and the number of incursions correlates to distinct segments of the border fault system.