THE IMPLICATIONS OF DRAINAGE REORGANISATION FOR ‘SURFACE TILTING’ OF THE GREAT PLAINS, USA

The stratigraphy of the Great Plains is studied to test for the impact of surface tilting versus climate change on post orogenic landscape evolution. The fluvial stratigraphy records a punctuated history of incision and aggradation, with three major episodes of incision occurring over the last 15 Ma. It has been argued that all or parts of the succession have been structurally tilted towards the east since deposition (McMillan et al., 2002; Duller et al., 2012). These studies assume constant channel gradients during sedimentation. This project tests these assumptions against a range of stratigraphic and geomorphic observations and considers the role of drainage reorganisation, and the evolution from bedrock to alluvial channels during cycles of incision and aggradation.

This study concentrates on three distinct fluvial systems – the Ogallala paleochannels of Miocene age, the Broadwater paleochannels of Pliocene age and the modern North Platte River. The paleogeographies of these rivers evolved from predominantly west to east flowing in Miocene times to northwest to southeast in Pliocene and modern times (Swinehart et al., 1985). There has been up to 200m of incision by these ancient channels, and the modern North Platte has removed 600m of surface topography (McMillan et al., 2002). The evolving flow directions question the assumption that upstream drainage area has remained constant as drainage capture modifies drainage areas.

This study uses both field observations and topographic analysis to assess the evolving fluvial morphologies of these three major channel networks and the corresponding changes in grainsize, sediment type, and channel flow depths. Here we integrate provenance, flow depth and grainsize data to demonstrate drainage reorganisation in the Late Miocene, coincident with increased flow depths. The increase in flow depth suggests paleodischarge has not remained constant and that channel gradients have steepened with time. Changing provenance and evolving source areas suggest drainage reorganisation at this time, which is evident as the flow direction of the North Platte switched to its present day orientation. Hence, the theory of surface tilt and/or increasing discharge may not be the only way to explain the development of this stratigraphy.