Holocene Stratigraphic Framework of the Central Platte River, Nebraska
This paper summarizes results from an ongoing investigation into the three-dimensional character of the Holocene Platte River alluvium in Nebraska. Over the period of time spanning the Late Pliocene though the Holocene, the central Platte River has incised a valley with ~100 m thick deposits of sand and silt near Grand Island, Nebraska. Channel bodies within the Platte River Valley have been quantified using borehole cuttings, sediment cores, 10 m resolution DEMs, Soil Survey data, 7½ minute topographic maps and aerial photography. GPR was used to determine stratigraphic architecture of the fluvial features. OSL techniques and relative age dating have provided a sequence of depositional events that have shaped the surface in the modern Platte Valley as well as the adjacent northern uplands. The Holocene fill can be divided into two units, labeled Qap2 and Qap1. The mid-Holocene Qap2 unit is highly fragmented and obscured by the Qap1 unit. Qap 1 can be subdivided into the North Channel and Main Channel units. The North Channel surface consists of several inactive channels that were active from the Late Holocene to the 19th century. The main channel has been active since the Late Holocene and comprises an inactive and an active system divided into several anabranches. Channels in the active surface have a barform stratigraphy, comprising nested channels with shallow laterally and vertically accreted bedforms, possibly unique to the Platte River. The effects of anthropogenic developments, such as reservoir dams and diversion canals have led to the stabilization of bars into channel islands by vegetation; giving the active system a more anastomosing character. The changes in the nature of this stream are artificial, but resemble those of progressively abandoned braided systems. The dimensions and character of each channel unit can be used as a model for similar deposits in the subsurface to determine the controls on some braided fluvial reservoirs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009