Shifts in sediment provenance across a hierarchy of bounding surfaces: A sequence-stratigraphic perspective from bulk-sediment geochemistry

Geochemical differentiation of sediment packages framed by surfaces of chronostratigraphic significance represents an effective tool to unravel the stratigraphic architecture of multi-sourced sediment-supply systems on a variety of temporal scales and through a wide spectrum of lithofacies assemblages. Sediment provenance shifts were examined across three-orders of bounding surfaces, arranged in a hierarchy of 105 to 103 years sediment packages, from late Quaternary alluvial, deltaic, coastal and shallow-marine strata of the Po Basin, where controlling factors of sedimentary evolution are firmly constrained by strong age control. A total of 150 samples were analyzed for bulk-sediment geochemistry by X-ray fluorescence spectrometry (XRF). Geochemically unique catchment lithologies, such as ultramafic rocks and dolostones, were used as end-member grain assemblages to assess clear geochemical signatures, and chemostratigraphic correlations were generated on the basis of key element abundances and ratios. Sequence boundaries originating in response to tectonic uplift and related to 105 years sediment packages are associated with marked, basin-wide changes in sediment composition that reflect phases of substantial basin rearrangement, with strong modification of the drainage patterns. On an intermediate temporal scale (104 years depositional units), large magnitude sea-level fluctuations had a major impact on sediment pathways, producing sharp provenance shifts on a systems tract scale. The Holocene transgressive surface records a substantial change in clastic sediment flux, as the alluvial system turned into a wave-dominated estuary. On the other hand, no notable changes in composition were observed across the maximum flooding surface. Finally, identifiable provenance variations developed on a short-term, facies tract scale (103 years sediment packages) and revealed by lower-rank bounding surfaces denote dramatic changes in sediment dispersal patterns in response to simple autogenic processes, such as channel avulsion and delta lobe switching. In the stratigraphic context provided by the ancient record, such provenance shifts could easily be misinterpreted as the sedimentary expression of higher-rank bounding surfaces. This study supports the robustness of a comprehensive geochemical approach to an improved interpretation of the subsurface geological record across a hierarchy of depositional elements. It shows, however, that provenance shifts at chemofacies boundaries alone cannot be used to infer allogenic or autogenic controls on sedimentation. High-resolution facies analysis within stratigraphically constrained time intervals needs to be considered before promoting predictive relationships between provenance shifts and basin evolution.