ARCHITECTURE OF MIOCENE OVERBANK DEPOSITS IN NORTHERN PAKISTAN

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 fluvi al Chinji Formation (Siwalik Group) can be divided into sequences 1-20 m thick defined by the alternation of stratified sediments and paleos ols. Preservation of stratified sequences does not support a gradual v ertical aggradation of overbank sediments over the extent of the flood plain, but instead indicates periods of rapid sediment deposition foll owed by long hiatuses and soil development. Rapid deposition of overba nk sequences is predicted by models that hypothesize: (1) episodic ver tical aggradation of the entire floodplain, (2) deposition controlled by the growth of alluvial ridges, (3) incision and filling of local va lleys, (4) rapid filling of localized low areas, or (5) rapid depositi on associated with river-channel avulsion. These models also have impo rtant implications for understanding fossil preservation and floodplai n paleoecology. Paleosol-bounded sequences in the Chinji Formation can pinch out laterally over kilometers, indicating that episodes of rapi d deposition were restricted to local areas on the floodplain. There i s no evidence that bases of sequences are erosional, and they do not a ppear to record valley incision and filling. Sequences generally do no t thin and fine systematically away from the terminating margins of ma jor channel deposits, suggesting they are not related to the growth of alluvial ridges along active channels. Lithologic variations and patt erns of sediment disruption within sequences appear to reflect the rap id filling 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 bein g 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.