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Planform asymmetry and stratigraphic variability of river delta deposits: Holocene examples and case studies of the Ferron Sandstone (Cretaceous), Utah, U.S.A.
River delta deposits are important geological archives and they host petroleum, groundwater, and mineral resources. Their scientific and economic value necessitates interpretive and predictive models, but extant models are hindered by limited understanding of planform asymmetry and stratigraphic variability. This study examines controls on spatial complexity from a literature review of Holocene deltas worldwide and field studies of Cretaceous deltaic rocks in Utah, U.S.A. Review of 27 deltas formed during the Holocene sea-level highstand focuses on recent studies with high-resolution datasets across the spectrum of subaerial and subaqueous deposits. Upcurrent vs. downcurrent sediment distributions reveal complex patterns of asymmetry controlled by discharge partitioning, lobe abandonment and localized transgression, plume deflection, longshore currents, and variable subsidence. These processes result in alongshore variations in clinoform geometries, sediment caliber, and stratal lapping relationships. The superbly exposed Ferron Sandstone offers a rare opportunity to study the internal architecture of an ancient asymmetric delta complex formed during relative fall of sea-level. Detailed examination of lithofacies and trace fossils along a 16-km, depositional strike transect reveals evidence for enhanced bioturbation of delta front facies downdrift of contemporaneous mouth-bars and distributary channels. This finding contradicts extant models which predict that downdrift environments are unfavorable for marine organisms. An alternative model is presented in which animal-sediment interaction was augmented during seasonal reversal of seaway circulation or during normal circulation when river-derived nutrients were sufficiently concentrated relative to fresh water and sediment input. A major sequence boundary (SB) in the Ferron is examined along a 20 km depositional dip transect. Lithofacies and bounding surfaces change such that the SB varies from clearly discernable subaerial unconformity (SU) to cryptic surface alternating between SU and transgressive ravinement surface in several locations. The variability of this surface reflects stepped transgression over a series of falling stage deltas marked by localized incisions and amalgamated or aggradational topsets. This work provides a broadened view of the controls on delta morphology and internal architecture. It provides alternative models for stratal geometries and facies patterns once thought to be unique to specific controls. Finally, it offers critical field constraints on the fundamental surface of sequence stratigraphy.
Korus, Jesse T, "Planform asymmetry and stratigraphic variability of river delta deposits: Holocene examples and case studies of the Ferron Sandstone (Cretaceous), Utah, U.S.A." (2015). ETD collection for University of Nebraska - Lincoln. AAI3689344.