AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Re-Evaluation of the Platte River Facies Model for Sandy Braided Alluvium, Using New Surface and Subsurface Data
(1) Department of Earth & Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE.
The Platte River braided fluvial facies model of Miall (1977, Earth Sci. Rev., 13, 1-62) has never been fully validated, despite having been applied extensively for more than thirty years. We tested this model with integrated surface and subsurface investigations of the Platte River in southeast Nebraska, USA. A more accurate model of facies stacking patterns was thence constructed from the results of surface geomorphic and facies mapping, analysis of aerial imagery time series, subsurface core drilling, wireline logging, ground-penetrating radar surveys and Optically-Stimulated Luminescence (OSL) geochronology.
Our results demonstrate that the Platte River displays considerably more facies variability, both spatially along its course, and temporally over the period 1858-2009, than can be accommodated in a single vertical sequence model. The extant Platte River model invokes thick successions of planar cross-bedded sand, with lesser trough cross-bedded, ripple cross-laminated and flat laminated sand, minor gravel beds and rare, thin mud partings. Whereas the Miall (1977) model does not justify the 28 m hypothetical succession that it depicts, our studies indicate that Holocene channel belt sandbodies of the Platte River are only 6-8 m thick, and therefore the depicted model would have to be a multistory body. Furthermore, channel scours and mainly small-scale trough cross-bedding dominated the subsurface at most of our active channel sites, with large-scale planar cross-strata evident only in older Holocene deposits. In the central Platte River, where discharge has decreased markedly relative to the lower Platte River, the channel belt has contracted significantly since 1858, and large parts of the formerly active channel belt have become transient or permanent vegetated islands, separated by functional anabranches. In these areas and elsewhere, the subsurface sediment architecture shows a dominance of small-scale sedimentary structure recording shallow channel floor environments rather than the macroform bar structures that might be predicted by the extant model. This change is a complex response to anthropogenic influences such as gradually decreasing water and sediment discharge. Accordingly, the historical channel change (~150 a) on the central Platte River is an excellent modern example of the abandonment of a braided channel belt that might in other circumstances require orders of magnitude longer to complete.