AAPG Foundation 2019 Grants-in-Aid Projects

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Impacts of Backwater Effect on Fluvial-Deltaic Stratigraphy: An Example provided from the Tullig Sandstone, Western Irish Namurian Basin


Despite the importance of backwater effect in shaping channel morphology and sediment dispersal patterns in fluvial-deltaic systems, impacts on the stratigraphic record remain a subject of active debate. The hydrodynamics of rivers approaching a basin, influenced by the onset of backwater conditions, give rise to decelerating flow velocity and decreasing boundary shear stress. These changes trigger morphodynamic responses include decreasing sediment transport capacity and sediment deposition at the transition from uniform to backwater flow. As a consequence, channel depth increases and bed sediment grain size decreases downstream. However, the identification of these hydraulic signatures in the rock record is not as straightforward as in modern systems. Therefore, few existing studies has quantitatively established the connection between backwater effect and the stratigraphic record. This represents a significant, unresolved gap between morphodynamic concepts established in geomorphology and fluvial-deltaic stratigraphy. This proposal seeks to identify linkages between strata and morphodynamics by measuring variability in fluvial deposits across the backwater zone for the Western Irish Namurian Basin. The results of this analysis will bolster analytical models that seek to link observed stratigraphy with predicted sediment accumulation patterns. Furthermore, the proposed research provides a basis for a direct assessment of paleohydraulics by combining time-and-space variable changes in grain size and channel dimension. This research will produce quantitative metrics to evaluate the dimensions, connectivity, and grain size variation of channel bodies of ancient fluvial-deltaic systems, and thereby provide valuable geological insights for hydrocarbon bearing rocks.