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AAPG ACE 2018

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Estuarine Deposits of the Neslen Formation Near Harley Dome, Previous HitBookNext Hit Cliffs, Utah: An Outcrop Analogue for Heterolithic, Tidally Modified Estuarine Channels and Tidal Flats

Abstract

The Neslen Formation (Fm) near Harley Dome, Previous HitBookNext Hit Cliffs, UT, contains tidally influenced deposits that are heterolithic at many scales, and provides an outcrop analogue for such reservoir systems as the Viking Fm. of Alberta, Canada, which contains heterolithic estuarine deposits that fill incised valleys.

Our investigation identified deposits of proximal to distal tidal-fluvial channels, sandy-muddy tidal flats, and floodplains. Ichnology was critical to distinguish between deposits of distal and proximal tidal-fluvial channels, and tidal flats versus floodplains. The Teredolites Ichnofacies was identified at several Previous HitstratigraphicNext Hit levels and used as an indicator of marine incursions. Stromatolites and ooids (ooliths) were identified in the Neslen Fm, suggesting distal, restricted environments consistent with a backbarrier-embayed estuarine setting.

Most sandbodies at Harley Dome are interpreted as point bar deposits of estuarine tidal-fluvial channels. Sandbodies are dominated by inclined heterolithic stratification and form laterally extensive heterolithic sheets separated by finer grained, highly carbonaceous intervals. Tidal indicators include abundant single-double mud drapes, mud rip-ups, mud balls, sigmoidal beds, combined-flow ripples, and rare herringbone cross-stratification. The internal structure of the channel deposits is highly complex, and includes inclined heterolithic stratification comprising sandstone interbedded with variably thick, bioturbated siltstone-mudstone containing Rhizocorallium, Teichichnus, and Planolities. Recurring incisions within sandbodies, some of which are draped with bioturbated siltstone-mudstone, are common. Internal mud drapes and mud-draped incisions would act as baffles-barriers to flow in analogous reservoirs, whereas finer grained intervals between sandbodies act as local seals.

In contrast to recent studies of the Neslen Fm along the Previous HitBookTop Cliffs, bayhead deltas were not identified near Harley Dome, and most channels contain marine trace fossils. The predominance of what we interpret to be distal to proximal tidal-fluvial channels and tidal flats, abundant brackish water-marine trace fossils, the recurrence of the Teredolites Ichnofacies, rare carbonates, and the lack of bayhead delta or wave-modified deposits suggests deposition within a distal, yet protected estuarine environment. Modern analogues might include Baffin Bay and other bays along the Texas Gulf Coast, as well as the Gironde Estuary of France.