Shelf construction in a foreland basin: storm beds, shelf sandbodies, and shelf‐slope depositional sequences in the Upper Cretaceous Mesaverde Group, Book Cliffs, Utah

Abstract

The Upper Cretaceous (Campanian) Kenilworth Member of the Blackhawk Formation (Mesaverde Group) is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of North America. Analysis of this section at a combination of small (sedimentological) and large (stratigraphical) scales reveals the dynamics of progradation of a shelf‐slope sequence into a subsiding foreland basin. Four major lithofacies are present in the upper Mancos and Kenilworth beds of the Book Cliffs. A lag sandstone and channel‐fill shale lithofacies constitutes the thin, basal, transgressive sequence, which rests on a marine erosion surface. It was deposited in an outer shelf environment. Shale, interbedded sandstone and shale, and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave‐dominated delta and its flanking strand plains prograded seaward. Analysis of grain size and primary structures in Kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies. The fine‐ to very fine‐grained graded strata of the interbedded facies are tempestites, deposited out of suspension by alongshelf storm flows (geostrophic flows). There is no need to call on cross‐shelf turbidity currents (density underflows) to explain their presence. Very fine‐ to fine‐grained hummocky strata are likewise suspension deposits created by waning storm flows, but were deposited under conditions of more intense wave agitation on the middle shoreface. Cross‐strata sets in this region are bed‐load deposits that accumulated on the upper shore‐face, in the surf zone. Lag strata are multi‐event, bed‐load deposits that are the product of prolonged storm winnowing. They occur on transgressive surfaces. While the graded beds are tempestites in the strict sense, all four classes of strata are storm deposits. The distribution of strata types and their palaeocurrent orientations suggests a model of the Kenilworth transport system driven by downwelling coastal storm flows, and probably by a northeasterly alongshore pressure gradient. The stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth, but also because of progressive sorting, which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments. The Kenilworth Member and an isolated outlier, the Hatch Mesa lentil, constitute a delta‐prodelta shelf depositional system. Their rhythmically bedded, lenticular, sandstone and shale successions are a prodelta shelf facies, and may be prodelta plume deposits. Major Upper Cretaceous sandstone tongues in the Book Cliffs are underlain by erosional surfaces like that beneath the Blackhawk Formation, which extend for many tens of kilometres into the Mancos shale. These surfaces are the boundaries of Upper Cretaceous depositional sequ...

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

“…12b) and shelf-turbidite sandstones Fig. 12c) in the mass-balance framework, which is consistent with net along-strike sediment import or export by shallow-marine processes (Q shin , Q shout ) (see Parrish et al 1984;Swift et al 1987;Hampson et al 2008;Hampson 2010). The shoreline thus represents a key moving boundary within the Star Point-Blackhawk-lower Castlegate sediment routing system.…”

Modern sedimentary processes along the Doce river adjacent continental shelf

“…Continental shelves under an accommodation regime are subject to strong variations in sea level and/or high sediment dispersion rate, i.e., continental shelf that is subjected to marine transgression, with the presence of erosive processes and estuarine environments. Supply regime continental shelves are those with a high rate of sediment input or higher sediment grain size that would not be mobilized by the hydrodynamic energy at the site, or regressive environments with progradation and/or presence of deltaic systems (Swift et al 1987, Swift & Thorne 1991. In the case presented herein, the muddy deltaic lobe in the adjacent inner shelf indicates a supply regime.…”

The origin and preservation of a major hurricane event bed in the northern Gulf of Mexico: Hurricane Camille, 1969

“…The sandy beds are called event beds because their erosive bases and sedimentary and biogenic structures indicate that they were emplaced by £ows at the event scale (Seilacher, 1982), on the order of a few hours or days duration. Storms have been shown to be the mechanism for formation of these beds on modern shelves (Hayes, 1967;Swift et al, 1981;Morton, 1981;Nelson, 1982;Snedden et al, 1988;Figueiredo et al, 1982;Gagan et al, 1990), and have been suggested as the origin of sandstone event beds (Brenchley et al, 1986;Snedden and Nummedal, 1990;Swift et al, 1987).…”