AAPG ACE 2018

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Morphodynamics and Stratigraphic Architecture of Shelf-Edge Deltas Subject to Constant vs. Dynamic Environmental Forcings

Abstract

When deltas dock at the edge of continental margins they generally construct thick stratigraphic intervals and activate channelized continental slope systems. Present day highstand sea-level conditions have pushed most deltaic systems well inbound of their shelf-edges, making it difficult to study their space-time dynamics and resulting stratigraphic products. Several competing theories describe how deltas and their downslope environments respond to sea-level cycles of varying magnitude and periodicity. We explore these hypotheses in a physical experiment where the topographic evolution of a coupled delta and downdip slope system was monitored at high temporal and spatial resolution. In the first stage a delta aggraded at the shelf-edge under constant water and sediment supply, in addition to a constant generation of accommodation through a sea-level rise. In the second stage the sediment transport system responded to low magnitude and high frequency sea-level cycles. Finally, in the third stage the transport system responded to a high magnitude and long period sea-level cycle. In each stage, fine sediment from the input grain size distribution and dissolved salt in the input water supply promoted plunging hyperpycnal flows. Specifically, we compare the mean and temporal variability of the sediment delivered to the slope system between stages. In addition, we compare stratigraphic architecture and sediment sizes delivered to the slope system in each stage. A key finding is that sea-level cycles with large amplitudes and long periods pump less sediment to the deep marine relative to smaller and shorter cycles. In addition, the presence of a shelf-edge enhances the time-scales of deltaic autogenic processes as channel tip progradation is slowed due to the rapid water depth increase. These results are used to improve inversion of slope deposits for paleo-environmental forcings and forward stratigraphic prediction.