Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara River, Nebraska: effect of base-level rise

Base-level rise of approximate to 2.35 m on the Niobrara River has resulted in aggradation of the channel belt and a recent avulsion. Overbank areas h ave become flooded by rising groundwaters, and more than eight crevasse spl ays have formed between 1993 and 1997. Two crevasse splays, situated on the west and east sides of the Niobrara, have been studied using ground-penetr ating radar (GPR), shallow boreholes and topographic surveys. The vibracore s and GPR profiles provide a nearly three-dimensional view of the architect ure of crevasse splay deposits. The east splay was initiated in the winter of 1993/94 and has expanded to cover an area approximate to 200 m by 1000 m , with sediment up to 2.5 m thick. The west splay, which was initiated by t he opening of a crevasse channel through a levee in the autumn of 1995, cov ers an area approximate to 150 m by 250 m, with up to 1.2 m of sand deposit ed in a single year. The Niobrara splays are sand dominated and characteriz ed by bedload deposition within channels, 5-30 m wide and 0.5-2 m deep, wit h the development of slipfaces where splays prograde into standing bodies o f water. Sedimentary structures in cores include horizontal lamination, rip ple lamination and sets of cross-stratification. There is a slight tendency for splays to coarsen up,but individual beds within the splays often fine up. The abundance of crevasse splays on the Niobrara River contrasts with o ther braided river floodplains. In the Niobrara, crevasse splay formation f ollowed aggradation within the channel belt, which occurred in response to base-level rise. The link between crevasse splays, channel aggradation and base-level rise has important implications for the interpretation of ancien t braided river and floodplain sequences. It is suggested that crevasse spl ay deposits should be an important component of aggrading fluvial sediments and, hence, should be preserved within the rock record. In this case, the aggradation and crevassing have been tied to a rise in base-level elevation , and it is suggested that similar deposits should be preserved where braid ed rivers are affected by base-level rise, for instance during transgressio n and filling of palaeovalleys.