Grain Size Distribution in Fluvial Dominated Delta Front Deposits; Case Study from the Upper Cretaceous Panther Tongue, Utah, USA
Sandstone beds of the delta front clinoforms of the Cretaceous Panther Tongue delta in Utah, have been sampled for grain size analysis. The grain size trend is more complicated than simply fining in basinward direction, or simply coarsening upward as is common in delta models. The implications of the grain size variability on individual delta-front sandstone beds will influence fluid flow in such reservoirs.
The Panther Tongue Sandstone is Campanian in age, represents the lower member of the Star Point Formation, and is generally described as a west-east oriented sandstone tongue. The eastward progradation is consistent with the overall paleogeography and the progradation of the underlying and overlying regressive clastic wedges. The Campanian shoreline along the west side of the Cretaceous Interior Seaway was generally north-south oriented. However, paleo-currents of the Panther Tongue indicate a southward transport of the sediments despite an overall north-south orientation of the shoreline.
The Panther Tongue deposits have long been interpreted as fluvial-dominated delta lobes with hyperpycnal flow deposits dominating the delta front. The delta-front clinoform have 1-3 degrees dips relative to a top ravinement surface. 37 samples collected along five sandstone delta front beds have been analyzed for the grain size distribution using a camsize counter. Basinward-thinning sandstone beds show variable grain size variability, from fine lower to medium lower sandstone, along the same bed. The eight vertical grain size transects show coarsening (fine lower to medium lower sandstone) as well as fining upward (medium lower to fine lower sandstone) changes from one sandstone bed to the next. The vertical variations have been explained through different flow intensities or lateral lobes (parasequences?) switches. An unexpected result is the lateral (basinward) grain size coarsening and fining of the same sandstone bed (for example from upper fine to lower medium to lower fine to upper fine to lower fine to upper fine sandstone) over a distance of about 400 m. The basinward down-dip grain size changes along the same delta front bed can be explained through (1) pulses in hyperpycnal flow, (2) effect of the waves to the hyperpycnal flows or (3) lateral amalgamation of different channels outflows.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California