Stratigraphic implications of the spatial and temporal variability in sediment transport in rivers, deltas and shelf margins
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Sediment delivery to a basin exerts a first-order control over sedimentation, and therefore study of sedimentary rocks can reveal information about the nature of sediment delivery in the past. This dissertation examines several aspects of this problem using experimental, outcrop, and subsurface data. Flume experiments were undertaken to test the combined effects of autogenic alluvial aggradation and forced regression on the development of fluviodeltaic stratigraphy. Alluvial aggradation occurred in response to steady relative sea-level fall, and eventually consumed the entire sediment budget as the river lengthened in response to forced regression. The Campanian Lower Castlegate Sandstone (Utah) was studied as a potential ancient analog resulting from similar autogenic behaviors as observed in the experiments. Extensive measurement of grain-size distributions and paleo-flow depths from outcrop were utilized to explore downstream changes in paleo-hydraulics of the ancient fluvial systems in the Lower Castlegate in response to extensive alluvial aggradation and consequent loss of sediment from transport. An interesting finding was the stratigraphic signature of backwater hydraulic conditions in the distal reaches of the Lower Castlegate paleo-rivers. Finally, a simple and novel inversion scheme was developed for estimating paleo-sediment flux from ancient shelf-margin successions. An advantage of the methodology is that it allows for both spatial and temporal reconstruction of paleo-sediment flux patterns. The inversion scheme was applied to shelf-margin successions in the Washakie-Sand Wash Basin of Wyoming, the New Jersey Atlantic margin, the North Slope of Alaska, and the Zambezi margin of East Africa using published subsurface datasets. The Neogene passive margins within the studied datasets were found to consistently deposit around one-third of their total sediment budget on the shelf-margin topset, and bypass two-thirds of their budget beyond the shelf edge. The implications of this finding on the flux of terrestrial-derived particulate organic carbon (POC) from rivers to the ocean were explored, and a long-term average flux of POC to deepwater storage was estimated. The sediment-flux inversion scheme was also applied to derive input parameters for stratigraphic modeling of the Ebro margin. The modeling results indicate that the autostratigraphic behavior of the margin may have been previously underestimated.