Large-scale structure of Calamus River deposits (Nebraska, USA) revealed using ground-penetrating radar

The large-scale (i.e. bar-scale) structure of channel deposits of the braid ed, low-sinuosity Calamus River, Nebraska, is described using ground-penetr ating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium-scale, trough-cross-stratified (sets 3-25 cm thick), very coarse to medium sands, that are associated with relativel y high amplitude, discontinuous GPR reflectors. Overlying deposits are biot urbated, small-scale cross-stratified (sets <3 cm thick) and vegetation-ric h, fine to very-fine sands, that are associated with low-amplitude disconti nuous reflectors. Near-surface peat and turf have no associated GPR reflect ors. In along-stream profiles through braid and point bars, most GPR reflec tors dip downstream at up to 2 degrees relative to the basal erosion surfac e, but some reflectors in the upstream parts of bars are parallel to the ba sal erosion surface or dip upstream. In cross-stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have l ow-angle inclinations (up to 6 degrees) towards cut banks of adjacent curve d channels. Basal erosion surfaces become deeper towards cut banks of curve d channels. These structures can be explained by lateral and downstream gro wth of bars combined with vertical accretion. Convex upwards forms up to 0. 5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in cha nnel fills record a complicated history of erosion and deposition during fi lling, including migration of relatively small bars. A revised facies model for this type of sanely, braided river has been constructed based on this new information on large-scale bedding structure.