Evidence of across‐shelf transport of fine‐grained sediments: turbidite‐filled shelf channels in the Campanian Aberdeen Member, Book Cliffs, Utah, USA

Abstract

The shore-normal transport of fine-grained sediments by shelf turbidity currents has been the focus of intense debate over the last 20 years. Many have argued that turbidity currents are unlikely to be a major depositional agent on the shelf. However, sedimentological, architectural, stratigraphic and palaeogeographic data from the Campanian Aberdeen Member, Book Cliffs, eastern Utah suggests otherwise and clearly demonstrates that storm-generated and river flood-generated underflows can transport a significant volume of fine-grained sediments across the shelf. These across-shelf flowing turbidity currents cut large subaqueous channel complexes up to 7 m deep, tens of kilometres basinward of their time-equivalent shoreface. The shelf channels were filled with organic-rich siltstones, mudstones and very fine-to finegrained Bouma-like sandstone beds, including wave-modified turbidites, hyperpycnites and classical turbidites. Deposition was above storm wave base. Palaeocurrent data reveal an overwhelmingly dominant across-shelf (east-south-east), offshore-directed transport trend. Tectonic activity and/or concomitant palaeogeographic reorganization of the basin may favour the generation of these turbidite-rich shelf deposits by altering the relative balance of wave versus fluvial energy. Increased erosion and sediment supply rates, because of tectonic uplift of the hinterland, may have increased the probability of fluvial dominance along the coastline and, hence, the possibility of submarine channelization in front of the river mouths. Additionally, the coastline may have become more sheltered from direct wave energy, thus allowing the fluvial processes to dominate. Seasonal increases in rainfall and storm activity may also favour the generation of across-shelf underflows. On wave-dominated shorelines, isolated shelf channels and lobes are most likely to be found down-dip of fluvial-feeder systems in relatively high sediment supply settings. These features are also most likely to occur in systems tracts that straddle a sequence boundary, especially those which are tectonically generated, as these would enhance the potential for altering basin morphology and, hence, the balance of fluvial and wave energy. Isolated shelf channels are recognized in older and younger strata in the Book Cliffs region, implying that wave-supported gravity flows were a recurrent phenomena in the Campanian of Utah. It is probable that isolated shelf bodies are preserved in other stratigraphic intervals in the Cretaceous Western Interior of North America, and other basins worldwide, and are currently being overlooked or misidentified. Shoreface-to-shelf facies models should be revised to incorporate turbidite-rich shelf deposits in some shelf settings.

Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

“…Furthermore, large numbers of interpreted turbidite shelf lobes and turbidite filled channels have recently been identified in the basin (Pattison et al 2007;Hampson 2010). While it has been suggested that gutter casts may be a useful proxy for predicting down-dip occurrence of turbidites on the inner shelf (Pattison et al 2007), no detailed descriptions of models for their genesis exist. The recognition and predicted distribution of OTZe, over the more common OTZt, is important because there is a sharp permeability contrast between the better reservoir quality sandstones and the poor reservoir quality mudstones.…”

“…The lateral accretion surfaces indicate sustained flow. In the context of a prograding, wave-dominated shoreline fed by multiple deltas developed around fluvial input points (Charvin et al 2010;Eide et al 2014), and significant amounts of gravityflow deposits basinwards (Hampson 2010), erosively based channels within OTZ deposits with evidence for sustained flow are likely to be the deposits of subaqueous channels, cut and filled by hyperpycnal flows fed from distributary channels up-dip, and feeding shelf turbidite systems downdip (Pattison et al 2007). These channels could also be interpreted as incised valley fills, but this seems unlikely due to that there are no evidence for this interval being subject to subaerial exposure, and no evidence for contemporaneous lowstand deposits.…”

“…The OTZ is a heterolithic succession comprising sandstone beds deposited by occasional storms interbedded with mudstones deposited from suspension during fair weather periods (Dott & Bourgeois 1982;Myrow & Southard 1996;Dumas & Arnot 2006). The beds are commonly described as tabular (Elliott 1978;Howell & Flint 2003), but several workers have reported localized intervals where the offshore transition contains abundant erosive gutter casts, and where sandstone beds pinch and swell significantly both along strike and down depositional dip (Brenchley et al 1986;Plint 1991;Hadley & Elliott 1993;Van Wagoner 1995;Pattison et al 2007). In this paper, two types of OTZ are distinguished: "Tabular OTZ" (OTZt) in which the beds are tabular and laterally continuous, and "erosive OTZ" (OTZe) in which the sandstone beds are lenticular to undulating and associated with numerous, erosive gutter casts.…”

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

“…The Mancos Shale contains isolated sandstones and sandy intervals that were deposited as river-fed, storm-wave-modified gravity flows supplied from the west (e.g. Cole & Young 1991;Pattison 2005;Pattison et al 2007) and as nearshore deposits that were reworked towards the south by tides and waves (e.g. Boyles & Scott 1982;Hampson et al 2008).…”

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

“…The flanks of these channels are in some places seen to erode into with the surrounding offshore transition deposits, indicating that they are younger than the Kenilworth K3 parasequence. These channelized incisions are interpreted to be subaqueous channels, cut and filled by hyperpycnal flows from distributary channels updip and feeding shelf turbidite systems down-dip (Pattison et al, 2007). Further discussion about these channels is beyond the scope for this paper.…”