Intra-parasequence architecture of an interpreted asymetrical wave-dominated delta
Résumé
Although modern wave-dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface-shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave-dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high-resolution stratigraphic architecture and interpreted palaeo-geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave-dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south-trending, wave-dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave-dominated shoreface-shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial-dominated delta-front deposits, which have previously been assigned to a separate 'stranded lowstand parasequence' formed by a significant, allogenic change in relative sea-level. High-resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea-level. Interpreted normal regressive shoreline trajectories within the wavedominated shoreface-shelf deposits are marked by aggradational stacking of bedsets bounded by non-depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave-base scour. Fluvial-dominated deltafront deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave-dominated shoreface-shelf and fluvial-dominated delta-front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave-dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.