FLUVIAL SEQUENCE BOUNDARIES DRIVEN BY CHANGES IN RELATIVE WATER DISCHARGE: THE PLATTE RIVER OF NEBRASKA AS PROPOSED EXEMPLAR

Sequence boundaries are regionally extensive erosional surfaces that record periods of landscape incision and degradation related to changing environmental conditions. Relative sea-level fall (RSL) seems to have driven most sequence boundaries, but changes in the ratio of water discharge to sediment discharge, or relative water discharge (RQW), can also produce continental sequence boundaries. Although sequence boundaries produced by RQW share many characteristics with RSL-driven ones, they are distinguished from the latter by: 1. Updip increases in depth of erosion, as opposed to downdip increases in RSL-driven sequence boundaries, and 2. Flat cross-slope profiles, as opposed to valley-form erosion surfaces of typical RSL-driven sequence boundaries. Whereas RSL-driven sequence boundaries are common in the downstream portions of fluvial systems, RQW-driven sequence boundaries can occur anywhere along a fluvial profile, and dominate in parts of fluvial drainage basins isolated from marine influence. The cross-sectional geometries of Quaternary lithosomes in the alluvial fill of the far-inland Platte River are consistent with our hypothesis of an RQW-driven continental sequence boundary. Both lithosomes have broad, flat basal surfaces. The Holocene sandbody is particularly instructive: it is a steep-margined and presumably entrenched, multistory, multilaterally stacked, composite lithosome incised 8 m into a Pleistocene one. Discrete channels became entrenched and then cross-cut each other to form the composite Holocene sandbody. Geochronological data from the Grand Island, NE area indicate: (1) an initial phase of channel aggradation at 5-4 ka, (2) minimal aggradation at 4-1 ka, and (3) a second phase of aggradation after 1 ka. These limited data appear to be compatible with interpretive paleoclimate records derived from terrestrial and aquatic microfossils, and sedimentary archives, which indicate a period of humid climate from 6-4 ka that succeeded an early Holocene period of aridity.