Ancient Deltas (Pre-Holocene)
The exceptional outcrops of the Book Cliffs, Utah, allow detailed reconstruction of 3D shoreline morphology and coeval facies relationships in a series of wave-dominated, shallow-marine parasequences of Late Cretaceous age. Each parasequence records progradation of a wave-dominated delta and/or strandplain system. These systems ar characterized in terms of vertical facies successions, trace-fossil assemblages, shoreline-shelf profiles, paleogeography, and paleo-geomorphic evolution. The ancient shorelines are compared to modern wave-dominated shorelines via three quantitative geomorphic measures: shoreline lobosity, areal proportion of river channels, and spacing of river channels. Our approach is driven by geomorphic insights gained from detailed, intra-parasequence facies architecture, and highlights the limitations of conventional analysis of outcrop and subsurface datasets using 1D logged sections and generalized 2D facies-trend and isopach maps.
Many of the wave-dominated shoreline deposits exposed in the Book Cliffs show clear evidence for river processes and river-derived sediment input, implying that fluvial sediment input was required for shoreline progradation. Along the most extensively exposed shoreline successions, broadly coeval rivers and/or valleys constitute 0-68% of the shoreline, and the spacing of rivers and/or valleys varies from 1 km to over 68 km. By comparison, rivers constitute 1-12% of modern wave-dominated deltas and strandplains, and river spacing ranges from 1 to 130 km. Detailed reconstructions of intra-parasequence facies architecture in two parasequences suggest that shoreline morphology, paleogeography, and evolution were controlled by wave-generated longshore drift and river-mouth locations. A combination of river avulsion and efficient alongshore redistribution of sediment by wave processes resulted in relatively linear shoreline paleogeographies for each mapped parasequence, but these trends obscure subtle lateral variations in facies and intra-parasequence stratigraphy. Autocyclic river avulsion and delta-lobeswitching did not generate localized flooding. Instead, parasequence-bounding flooding surfaces are inferred to have been caused by allogenic changes in relative sea level and/or sediment supply.
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Deltas are amongst the most environmentally and economically important coastal sedimentary environments. Studies of deltas lag behind research in both fluvial and deep-water depositional systems, as well as more geomorphologically oriented land studies. This knowledge lag reflects both a reorientation of the petroleum industry in the last two decades toward deep-water systems, as well as the difficulty of working across the shoreline with the traditional tools used for oceanographic or land-based work. However, deltaic studies are experiencing a renewed focus, because of their global importance in environmental and other societal concerns. This volume stems from a special session: Deltas: Old and New, held at the Annual Geological Society of America conference in October 2002, that was convened to highlight these new directions in deltaic research.