Time Stratigraphy of River-Dominated Delta Deposits

Abstract

Physics-based morphodynamic depositional models predict facies patterns of a fluvial-deltaic system preserved during varying sea level, contrasting sand to mud input ratio, and forward- or backward-tilted subsidence profiles. The model had a single inlet through which sediment was supplied to a 5 by 13 km basin. Preserved successions are up to 60 m thick. The modelled stratigraphic record is 100% complete (deposits are preserved during every time step in all models) but preserved deposits are not omnipresent. The spatial organization of preserved deposits is examined in two types of 4D wheeler diagram (defined by the physical map view dimensions, time on the vertical axis, and colors showing either time-internal-deposit thickness or grain-size variations). Local gaps in the stratigraphic record occur at a hierarchy of spatial and temporal scales related to a range of depositional processes like lateral shifts in deposition during channel migration and mouth bar progradation, jumps in deposition following channel avulsions, and basin-scale shifts in deposition during shoreline regressions and transgressions. If local stratigraphic completeness is defined by the preservation of deposits more than a few millimeters thick during a given time interval (a designation that marks bypass and hiatus surfaces, as well as erosion surfaces, as time discontinuities), then local completeness is greatest where delta front deposits are thickest and declines landward into areas with abundant fluvial erosion and seaward as deposits become thinner. The back-tilted basin with muddier sediment supply creates the most complete vertical sections, on average, due to greater vertical stacking of the deltaic successions (more than 30% of sections are more than 30% complete). On the other hand, the fore-tilted basin with sandier sediment supply creates the least complete vertical sections due to greater large-scale proximal-distal shifts in deposition and greater channel avulsion rates (only a few percent are more than 30% complete). The muddier, fore-tilted basin deposits had the most distinct stratigraphic patterns related to sea level variations, with regressive-transgressive sands defining separate reservoirs and distinct regional bypass surfaces were formed during sea level lowstands. In contrast, in the sandier, back-tiled basin reservoir bodies are more poorly organized and lateral separation of deposits associated with sea-level falls and rises are less distinct.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019