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.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.