--> Intra- and Intersequence Process Changes of Mixed Energy Deltas in Outcrop and Subsurface: Maastrichtian Fox Hills, Washakie Basin, Wyoming

AAPG ACE 2018

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Intra- and Intersequence Process Changes of Mixed Energy Deltas in Outcrop and Subsurface: Maastrichtian Fox Hills, Washakie Basin, Wyoming

Abstract

Mixed energy deltas are complex systems built up by river, wave and tidal processes. Despite previous focus on one single dominant process acting at the time of deposition, most of the deltas show mixed signals because during progradation from inner to outer shelf the amount of relative importance of the three main processes changes. This paper documents depositional process changes within individual regressive sequences (intra-sequence) and between successive sequences (inter-sequences) using kilometer-long outcrops and about 1000 well logs in the subsurface of the Washakie basin in Wyoming. The topset compartments of three early Maastrichtian clinothems are described, with focus on the role they played in building the Lance-Fox Hills-Lewis shelf-margin prism. Subsurface regional well log correlations allowed recognition of regressive transits and shifts of individual deltaic lobes. Migration and shifting of deltaic lobes describe the behavior of the shoreline trajectory over short-time scales (forth-order cycles). The aggradation and progradation pattern of deltaic complexes indicate variability in depositional processes both in time and space. The regressive transits of deltas started at about 40 km inland from the precedent shelf edge and preserved river-dominated, tide-influenced and wave-modified deltaic deposits. Sandstone maps of the individual clinoform topsets show that (1) numerous deltaic lobes coexist within each clinothem, implying multiple rivers, (2) delta lobes have an autogenic compensational stacking pattern, (3) there is variation in the dominant depositional processes acting on the shelf at short time scales (about 100 k.y.) and (4) wave-dominated deltas are to be expected closer to the shelf margin. Comparison of the Maastrichtian Fox Hills with Holocene deltas allows distillation of some new basic rules on deltas: (1) all deltas are shaped by mixed processes – there are significant differences between different lobes of the same deltaic system, (2) each delta complex has a different architecture, (3) less process variability is encountered on the outer shelf, and (4) wave processes dominate “late-regression” stages.