Processes and Architectures of Deltas in Shelf-break and Ramp Platforms: Examples from the Eocene of West Spitsbergen (Norway), the Cretaceous Western Interior Seaway (SW Wyoming & NE Utah), and the Pliocene Paleo-Orinoco Delta (SE Trinidad)
Carlos A. Uroza
The University of Texas at Austin, Department of Geological Sciences Austin, Texas
The transit of deltas forth and back across the shelf produces gradual aggradation on the shelf platform. The dominant process regime may change between the regressive and transgressive phases of any delta transit cycle and consequently the relative amount of river, wave or tidal energy. Here, three different cases in which deltas are wave-dominated during their regressive transit but become tide-influenced during transgression are documented, plus a case in which deltas are tide-influenced during their entire regressive transit. The first example from West Spitsbergen, Norway, includes deltas that transited a narrow shelf during rising sea level delivering large sand volumes to the outer shelf and shelf-margin. During the regressive phase, waves were the main process regime. As deltas transited back across the shelf tides became more important with significant tidal inlet/estuarine deposits developed. The second example from the Rock Spring Formation, Wyoming-Utah, shows deltas deposited within an inner shallow-water ramp setting. Here, waves were the dominant process regime, but during transgression tidal-influence was dominant, with abundant landward-oriented cross-strata present. The third example from the Paleo-Orinoco delta in Trinidad includes deltas that transited the entire shelf platform during lowstand conditions. They became mostly wave-dominated through the entire regressive transit, with fluvial influence (hyperpycnal flows) at the beginning of the regression. The corresponding transgressive facies (estuarine sands with large ophiomorpha burrows) are seen in upper successions. The additional case from the Haystack Mountains Formation, Wyoming, shows lowstand deltas developed on an outer ramp setting that became tide-influenced through the whole regressive transit.
AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid