EXHUMED CENOZOIC RIVER CHANNELS ON THE HIGH PLAINS USA REVEALED BY HIGH-RESOLUTION PHOTOGRAMMETRY AND LIDAR

Cenozoic river-channel deposits in the High Plains host important aquifers and they have long been recognized as crucial archives of terrestrial biotas and paleoenvironments. An understanding of the large-scale architecture and geomorphology of these ancient streams has been impeded by the limitations of 20th-century imagery and mapping techniques. We provide the first regional analysis of exhumed Oligocene−Pliocene channel networks in eastern Wyoming, northeastern Colorado, and western Nebraska (>70,000 km^2) through a GIS analysis of high-resolution aerial photography and recent LiDAR. We map ~1000 km of linear, positive-relief landforms composed of sandstones and conglomerates. These landforms consist of: (1) linear rock exposures and trains of fitted, in-situ boulders, which typically form sharp-crested ridges 1 to 120 m in height; (2) narrow (30 − 300 m) chains of rounded hills and hummocky topography 40 – 80 m in height; and (3) low (1 – 10 m), flat-topped ridges. These features are interpreted as the erosional remnants of coarse-grained facies in channel axes. Continuous segments of these landforms range in width from a few (3 – 4) meters to ~1,000 m and in length from 20 – 14,000 m. Segments commonly exist in discontinuous trains, ostensibly because they are the products of once-continuous channel threads. Significantly, the trains increase in length with decreasing age and presumed duration of exposure: from <5 km (Oligocene), to 25 - 30 km (Miocene), to a maximum of 108 km (Pliocene). Most channels have sinuosities of 1.0 to 1.2 (straight to slightly sinuous), although a few are meandering (S >1.5). Groups of channel remnants form parallel chains (interpretive channel belts), downstream-radiating threads (interpretive distributary systems), downstream-converging threads (interpretive tributary systems and valley fills), arcuate sets (interpretive point bars), and irregular clusters. Our analysis sheds light on the dynamics and evolution of fluvial sediment dispersal pathways during the formation of the High Plains.