Stratigraphic architecture of a shallow-water delta deposited in a coastal-plain setting: Neslen Formation, Floy Canyon, Utah

Abstract

Shallow-water, low-accommodation, coastal-plain deltas are deposited in wetlands detached from the coeval shoreline associated with broad, low-gradient settings and are an underappreciated architectural style in marginal marine systems. The goal of this study is to document the stratigraphic architecture and evolution of a shallow-water delta in the Neslen Formation, Floy Canyon, Utah and compare it to modern analogs deposited in a similar water depth. A further goal is to contrast the shallow-water delta documented to a comparatively deeper-water delta to test the implications of the numerical model developed by Edmonds et al., (2011). The delta analyzed in this study was deposited in a ~ 4 m deep wetland that was located about 65 kilometers landward of the coeval shoreline. The outcrop of the delta is highly rugose and exposes, in three dimensions, a large portion of the delta: from the feeder channel to the delta front. Upward and lateral facies variations of the delta stratigraphy are evident. The feeder channels contain fine-grained, cross-stratified sandstone. These channels transfer longitudinally to the axial parts of mouth bars that are composed of amalgamated fine-grained, upward-coarsening and thickening beds that contain predominantly ripple laminations. The beds de-amalgamate and become thinner and finer-grained towards the lateral and distal margins of the delta. Stratigraphically adjacent elements in the Neslen delta stack highly compensationally. Within the elements, stratigraphically adjacent stories stack laterally and rotate relative to one another to form a radially dispersive pattern. Younger stories stack progradationally down-current. The shallow-water delta documented in this study has similar longitudinal lithofacies trends and stacking patterns to modern analog deltas deposited in similar water depths (such as the Okavango and Wax Lake deltas). When compared to a relatively deeper-water delta, such as the delta documented by Matthew Andresen in the Iles Formation, the lithofacies trends and stacking patterns are different. For example, the shallow-water delta (Neslen delta) has bottomset aggradation only at the distal part of the system, the clinoform angles are shallow (1 to 3 degrees), the foresets were deposited primarily by tractional processes, and the mouth bars stack more compensationally. Conversely, the comparatively deeper-water delta has bottomset aggradation throughout the deposit, the clinoform angles are steeper, the foresets were deposited primarily by gravitational processes, and the mouth bars stack less compensationally.