Intra-parasequence architecture of an interpreted asymmetrical wave-dominated delta

Abstract

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 wave‐dominated 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 delta‐front 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.

Distribution of discontinuous mudstone beds within wave-dominated shallow-marine deposits: Star Point Sandstone and Blackhawk Formation, Eastern Utah, USA

“…However, the delta observed in the NNE-SSW-trending shoreline of the Kenilworth K4 parasequence contains clinoforms that dip locally towards the north (Figures 7a, 10b), indicating deflection opposite to regional longshore drift. This is contrary to what is commonly observed in the Star Point Sandstone and Blackhawk Formation (Charvin et al, 2010;Hampson et al, 2011), and it is possible that the relatively small delta in the Kenilworth K4 was deflected by a beach ridge, not by longshore drift. Plots of horizontal shale density in the investigated outcrops show that deposits of wavedominated deltas contain approximately three times more shales than shoreface deposits (Figures 10c, 10d, 16).…”

“…The similarity of the clinoform dips indicates that the outcrop of the delta in the Kenilworth K4 parasequence is viewed in a down-dip cut. Deltas in wavedominated settings are often deflected towards the direction of longshore drift (Wright, 1977;Dominguez, 1996;Bhattacharya and Giosan, 2003;Charvin et al, 2010), and previous workers have determined the longshore drift direction in the Book Cliffs to be towards the south (Taylor and Lovell, 1995;Slingerland and Keen, 1999;Charvin et al, 2010). However, the delta observed in the NNE-SSW-trending shoreline of the Kenilworth K4 parasequence contains clinoforms that dip locally towards the north (Figures 7a, 10b), indicating deflection opposite to regional longshore drift.…”

“…Taylor and Lovell, 1995;Howell and Flint, 2003;Hampson and Howell, 2005). Shoreface deposits (SF) are believed to have received sediment mostly from longshore drift of sand derived from wave-reworking and winnowing of mouth-bars of locally developed wave-and river-dominated deltas (DF), which developed around localized fluvial input points (Bhattacharya and Giosan, 2003;Charvin et al, 2010). The subaerial expression of the shoreface deposits are believed to have been strandplains with linear beach ridges, while the coastal plain behind the DF are believed to have consisted of a distributary channel and arcutate beach ridges (e.g.…”

Towards a sequence stratigraphic solution set for autogenic processes and allogenic controls: Upper Cretaceous strata, Book Cliffs, Utah, USA

“…Detailed architectural analysis of selected parasequences in the Blackhawk Formation suggests that they contain evidence for deltalobe switching during progradation of a wave-dominated deltaic shoreline (Hampson & Storms 2003;Hampson & Howell 2005;Charvin et al 2010). In each case, a wave-dominated shoreface succession that records eastward progradation of a north-southtrending shoreline is onlapped by a southward-prograding fluvialdominated delta-front succession near the downdip pinchout of the parasequence (e.g.…”

“…Fig. 5; Charvin et al 2010). Both the wavedominated shoreface succession and fluvial-dominated delta-front succession are capped by the same coal seam (e.g.…”