High-Resolution Stratigraphic Architecture Within from Ancient Asymmetrical Wave-Dominated Deltaic Parasequence

Karl Charvin¹, Gary Hampson¹, and Kerry Gallagher^2
1Earth Science and Engineering, Imperial College, London, United Kingdom
²Geosciences Rennes, University of Rennes, Rennes, France

Although modern wave-dominated shorelines exhibit complex geomorphologies, their ancient counterparts are described in terms of shoreface-shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between different types of wave-dominated shorelines in the past and to misleadingly simplistic evaluations of the internal architecture of associated petroleum reservoirs. In this paper, we document the variability in palaeo-geomorphology and stratigraphic architecture within a parasequence that is interpreted to record the advance of an ancient asymmetrical wave-dominated delta. The studied deposits comprise the Standardville parasequence of the Aberdeen Member (Blackhawk Formation, Utah, USA), which records the easterly progradation of a north-south trending shoreline. Distal wave-dominated deposits are overlain across a downlap surface by southerly prograding fluvial-dominated delta-front deposits, which have previously been assigned to a separate “stranded lowstand parasequence”. High-resolution stratigraphic analysis reveals that they record a single episode of shoreline progradation characterised by alternating periods of normal and forced regressive shoreline trajectory. Normal regressive shoreline trajectories within the wave-dominated deposits are marked by aggradational stacking of bedsets. Forced regressive shoreline trajectories in the same deposits are characterised by shallow incision of fluvial distributary channels and progradational stacking of bedsets. Fluvial-dominated delta-front deposits record the regression of a bayhead delta into a sheltered embayment lacking wave influence. Wave-dominated shoreface and fluvial-dominated delta-front deposits are interpreted to represent respectively the updrift and downdrift flanks of a single asymmetrical wave-dominated delta, occurring within the same parasequence.

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