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Stratigraphic Architecture of a Shallow-Water Delta Deposited in a Coastal Plain Setting, Neslen Formation, Floy Canyon, Utah

Abstract

Shallow-water, coastal plain deltas are commonly associated with broad, low-gradient coastal-plain settings and are an under appreciated architectural style in marginal marine systems. These deltas are located in wetlands and are detached from the coeval shoreline. The goal of this study is to document the evolution and 3D architecture of a coastal plain delta in the Neslen Formation, Floy Canyon, Utah. The delta was deposited in a ~ 6 m deep wetland located about 70 miles from the coeval shoreline. The outcrop is highly rugose and exposes a large portion of the delta: from the feeder channel to the delta front. Upward and longitudinal facies variations of the delta lobe are evident. The feeder channel contains fine to medium grained, cross stratified sandstone. These strata longitudinally transfer to foreset stratigraphy that is composed of generally fine grained, upward coarsening and thickening beds containing predominantly ripple laminations. These beds de-amalgamate and become thinner and finer grained in the down-current direction towards the margin of the delta. The beds are contorted and amalgamated along the axis of the system. The upward and lateral facies associations are distinctively different from deltas deposited in deep-water, open marine settings. For example, there is little to no bottomset aggradation, the clinoform angles are shallow, and the foresets are dominantly composed of ripples. The delta foresets were deposited primarily by tractional processes as opposed to the sediment gravity flows commonly associated with deltas in open marine settings. The delta also has a high proportion of amalgamated beds and a low proportion of contorted bedding.