Role of Wave-Modified Underflows in the Across-Shelf Transport of Fine-Grained Sediments: Examples from the Book Cliffs, Utah, U.S.A

Simon A.J. Pattison
Department of Geology, Brandon University, Brandon, Manitoba, R7A 6A9, Canada, [email protected]

At least two main types of hyperpycnal flows have been identified from studies of modern and ancient shelves: river flood-induced underflows and storm wave-induced underflows or oceanic floods. Even though oceanic floods occur over a shorter time period, they have the capacity to transfer a much greater volume of fine-grained sediments onto the shelf. Wright et al. (1988) estimated that the storm wave-induced underflows on the Huanghe delta had a down slope sediment flux that was two orders of magnitude greater than those generated by river flood events. Oceanic floods are more common along active margins that are drained by small to moderate-sized rivers (Wheatcroft 2000). In these settings, storm waves are coupled with an increased delivery of river-derived fresh water and fine-grained sediments, creating high density underflows which transport sediments from the shoreface to the shelf.

This study addresses the question as to why there is an apparent paucity of storm wave-induced underflow deposits in modern and ancient shelf settings given the ubiquity of storm deposits in many shallow marine sections worldwide. The western margin of the Cretaceous Western Interior Seaway was characterized by small-to-moderate-sized rivers and was storm-dominated, which are near perfect conditions for the generation of storm wave-induced underflows. This is an ideal place to study the question of sediment delivery on the shelf and to examine the role of wave-modified underflows in transporting sediments onto the shelf.

Early results from the Upper Cretaceous strata of the Book Cliffs region, eastern Utah and western Colorado have revealed an unusual suite of isolated shelf sandstone bodies which have channelized and lobate geometries. These bodies are concentrated along four specific horizons including the Turonian Lower Ferron Sandstone/Juana Lopez Member (Mancos Shale), Campanian Storrs Member (Star Point Formation), Campanian Aberdeen to Kenilworth members (Blackhawk Formation), and Campanian Prairie Canyon Member (Mancos Shale), and consist of wave-modified turbidites, hyperpycnites and classical turbidites (Fig. 1). Paleocurrent data reveals a dominant across-shelf, offshore-directed transport trend. These observations demonstrate that wave-supported gravity flows are capable of transporting large volumes of fine-grained sediment (i.e. clay, silt and fine sand) from the shoreface to the inner shelf (Fig. 2). Most hyperpycnites are wave-modified, with interbedded combined flow ripples (i.e. symmetrical to weakly asymmetrical crests; convex-up sigmoidal profiles), quasi-planar laminations, and low-amplitude HCS (Fig. 1), indicating deposition above storm wave base during oceanic flood events. Wave-modified hyperpycnites and wave-modified turbidites require further investigation before a genetic facies model can be constructed for hyperpycnites.

References

Wheatcroft R.A., 2000, Oceanic flood sedimentation: a new perspective: Continental Shelf Research, v. 20, p. 2059-2066. Wright L.D., et al. 1988, Marine dispersal and deposition of Yellow River silts by gravity-driven underflows: Nature, v. 332, p. 629-632.

Figure 1. Wave-modified turbidites and hyperpycnites, Book Cliffs, Utah, U.S.A.