ARCHITECTURES AND PROCESSES OF DELTAS ON THE SHELF: THE IMPORTANCE OF OUTCROP DATA TO RECOGNIZE PROCESS CHANGE IN THE REGRESSIVE AND TRANSGRESSIVE PHASES OF THE DELTA

When deltas transit forth and back across the shelf they produce gradual aggradation on the shelf platform. The dominant process regime can change between the regressive and transgressive phases of any cycle of delta transit and consequently the relative amount of river, wave or tidal energy during a single regressive (or transgressive) transit. Here we document three cases in which deltas are wave-influenced during their regressive transit but become tide-influenced during transgression. These three examples correspond to: 1- The Eocene of West Spitsbergen, Norway, in which the deltas transited a narrow shelf during rising sea level delivering large amount of sand to the outer shelf and partly to the shelf-margin. During the regressive phase, waves were the main process regime; however some fluvial influence is still preserved since deltas were originally driven by fluvial processes and later reworked by waves. Common features indicative of wave influence are swaley and hummocky cross-bedding and wave ripple lamination. As deltas transited back across the shelf tides became more important with significant tidal inlet/estuary deposits developed. 2-The second example corresponds to the Late Cretaceous Rock Spring Formation, Wyoming-Utah, in which the deltas were located within an inner shelf position but in a ramp setting. Here, waves were the dominant process regime; swaley cross-bedding and parallel bedding are the main sedimentary features identified in the outcrops. During transgression, tidal-influence was dominant, with landward-oriented cross-strata found within the transgressive deposits. 3-The third example corresponds to the Late Pliocene Orinoco delta in Trinidad. Here, the deltas transited the entire shelf platform during conditions of relative sea-level fall. They became mostly wave-influenced through the entire regressive transit, with some fluvial influence at the beginning of the regression. Wave-generated structures like swaley and hummocky cross-bedding are common along the outcrop exposure. Fluvially-generated features of the base of the deltaic succession are mainly flat-laminated and graded sandstones beds. The corresponding transgressive facies are absent in this outcrop; however they are abundant (estuarine sands with large ophiomorpha burrows) in the upper succession