Factors Controlling Trace-Metal Distribution in Alluvial and Coastal Deposits: Implications for Hydrocarbon Exploration
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Reliable estimates of sediment transport and storage from a multi-sourced system across a variety of depositional environments require accurate identification of the sediment sources and their role in modulating sediment supply to the down-dip components of the dispersal system. Through a modern example from the Po Plain, involving three segments (drainage area, alluvial plain, and coastal plain/delta) of the system, this study provides insights into the usefulness of a combined sedimentological and geochemical approach, which should also be applicable to the ancient record. Unlike ancient successions, where individual causes of compositional variability are generally very difficult to unravel, recent (late Holocene) sedimentary successions provide an excellent opportunity to understand the relative impact on trace-metal distribution of source-rock composition and changes in grain size, helping to predict whether and to what extent these two variables might be combined or superposed in the rock record. Among the variety of facies associations that compose the sediment routing system, channel-related deposits, for which connection between river transport and hinterland sediment supply can reasonably be inferred, represent the building block for a comprehensive approach to sediment transfer and storage. For trace-metal characterization, levee and crevasse fine to medium sand appears to be more suitable for provenance analysis than does its coarser-grained (bar sand) counterpart. Identification of end members for individual catchment-river systems with sufficient precision may help disentangle provenance mixing from lithologically homogeneous alluvial (floodplain), deltaic (bay, lagoon, delta front), and nearshore (beach) facies associations, for which the relative contributions from individual detrital sources are virtually unknown.