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Palaeoceanographic Issues |
1 Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK
2 Department of Geology, University of Southampton, Southampton S09 5ND, UK
Sediments recovered during Ocean Drilling Program Leg 116 (Bay of Bengal deep-sea fan) fall into three mineralogically and geochemically distinct groups. The first of these groups (Group I), characterized by quartz-mica-rich turbidites, constitutes the largest proportion of distal fan sediments. Trace element patterns are similar to modern River Ganges suspended sediment and are consistent with a Himalayan (meta)sedimentary/granitic source. The second group of sediments (Group II) is represented by organic carbon-rich, smectite-kaolinite turbidites. Trace element data reveal significant enrichment in compatible and ferromagnesian elements consistent with a significant contribution from a basaltic crustal source. Although mixing of a basaltic source with granitic crust can account for specific geochemical relationships, mixing of these components cannot account for observed rare earth element (REE) patterns. REE data are best explained by mixing of basaltic detritus (e.g. Deccan Trap basalts of central India) with Precambrian tonalitic crust of the Indian subcontinent. A third group of carbonate-rich sediments (Group III), containing a low-latitude marine fauna, is characterized by a clastic component similar in composition to smectite-kaolinite turbidites. Although carbonate-rich sediments are superficially similar to Group II turbidites, geochemical data indicate a reduced contribution from basaltic crustal sources compared with smectite-kaolinite turbidites. A possible southern India/Sri Lankan provenance has been assigned to Group III turbidites on the basis of faunal content and geochemical composition, although insufficient information exists to substantiate this using geochemical data alone.
The distribution of lithofacies and provenance-sensitive geochemical signatures suggests that the relative contribution of Himalayan and Indian subcontinent sources to distal fan sedimentation varied with time. Controls on sediment supply include variations in uplift, weathering and erosion rates, eustatic sea-level changes, and switching of major distributary fan channels. Although a degree of correlation is observed between the occurrence of coarser-grained Himalayan-derived turbidites and relative low-stands, the relationship is patchy. Assuming that the rate of sediment supply from the Himalayas to the Bay of Bengal remained relatively constant, the most likely controls on distal fan sedimentation are thought to be related to an interplay between sea-level change and channel switching. As a consequence attempts to reconstruct major Himalayan tectonic and climatic events based on data obtained from a record of distal fan sedimentation may be unreliable due to the discontinuous nature of Himalayan sediment supply to distal fan sites.
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