290-10 Grain Size Distribution in Fluvial Dominated Delta Front Deposits; Case Study from the Clinoforms of the Upper Cretaceous Panther Tongue, Utah, USA

Session: Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

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Authors:

Abstract:

Delta sedimentary deposits forming parasequences have been shown to have a coarsening up grain size trend explained by the overall progradation of the shoreline so that the shallower overlying, higher energy deposits ride above the more distal finer deposits. In a plan view, the delta sediments are predicted to fine outward in the delta front area from the delta distributary channels into the basin because the fluvial current velocity is decreasing as the flow transitions from channel confinement to unconfined in the basin. To test these well-known observations about river deltas grain size trends, we sampled a series of sandstone beds from the Cretaceous Panther Tongue delta in Utah, USA, for grain size analysis.

The Panther Tongue Sandstone is the lower member of the Campanian Star Point Formation. The Campanian shoreline along the west side of the Cretaceous Interior Seaway was generally north-south oriented. The Cretaceous Panther Tongue deposits were interpreted as fluvial dominated delta that has 20 to 30 m thick delta front built of hyperpycnal flow deposits with palaeoflows southward oriented.

Delta front clinoforms dip at 1 to 3 degrees and sandstone beds show a thinning basinward but a complex grain size variability along the same bed. The results suggest that grainsize trends in river-dominated delta-front deposits are more complicated than a simple coarsening upward or fining basinward trend, as many existing models suggest. The vertical grain size trends show coarsening as well as fining upward changes from one bed to the next. The vertical variations have been explained through different river outflow intensities or parasequences switches (lobes avulsions). A somewhat unexpected result is the intra-parasequence lateral (proximal to distal) grain size coarsening and fining of the sediments in the same bed. The basinward down-dip grain size changes can be explained through (1) pulses in hyperpycnal flow, (2) effect of the waves reworking the hyperpycnal flows or (3) lateral amalgamation of different channels outflows. The non-monotonous variability of grainsize trends has implications to sediment transport, delta architecture, and subsurface reservoirs.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10587

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