Abstract

Understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition. Overbank deposits of the fluvial Chinji Formation (Siwalik Group) can be divided into sequences 1-20 m thick defined by the alternation of stratified sediments and paleosols. Preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain, but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development. Rapid deposition of overbank sequences is predicted by models that hypothesize: (1) episodic vertical aggradation of the entire floodplain, (2) deposition controlled by the growth of alluvial ridges, (3) incision and filling of local valleys, (4) rapid filling of localized low areas, or (5) rapid deposition associated with river-channel avulsion. These models also have important implications for understanding fossil preservation and floodplain paleoecology. Paleosol-bounded sequences in the Chinji Formation can pinch out laterally over kilometers, indicating that episodes of rapid deposition were restricted to local areas on the floodplain. There is no evidence that bases of sequences are erosional, and they do not appear to record valley incision and filling. Sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits, suggesting they are not related to the growth of alluvial ridges along active channels. Lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel. This infilling of floodplain topography may reflect a continuous process, whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels. Alternatively, the filling of local flood-plain topography may have been caused by widespread but short-lived events associated with river-channel avulsion.

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