The Extent of Individual Hyperpycnal-Flow Beds in the Cretaceous Panther Tongue Delta, Utah, USA

Cornel Olariu, Ron J. Steel, and Andrew L. Petter
Jackson school of Geosciences, University of Texas at Austin, Austin, Texas

The distance over which river-generated hyperpycnal flows travel on the shelf and the extent of the resultant deposits are not well documented. The geometry and thickness of the beds are a reflection of the hyperpycnal-flow plume behavior. River-generated hyperpycnal flows will expand off the river mouths because of the lack of confinement and bottom friction. In this paper we document the extent and thickness variation of the individual hyperpycnal beds of the Cretaceous Panther Tongue Delta, Utah. Our data show that thick (meter) sandstone beds thin to decimeters over a surprisingly short distance, typically less than 1 km. After the initial abrupt dipping and thinning, the cm- thin sandstone beds can be followed for longer distances (hundreds of meters) with no significant thickness variation.

Panther Tongue Sandstone is a regressive sandstone wedge of Late Campanian age that is cropping out in Central Utah, in the Book Cliffs area (Fig. 1).

The Panther Tongue succession has a coarsening- and thickening-upward trend and is about 30 m thick. Based on the sedimentary facies the deposits are interpreted as fluvial- dominated delta deposits. At the base of the succession the beds are composed of siltstones alternating with cm-thick very fine sandstones. The basal thin beds are overlain by dm to m thick sandstones alternating with thin (cm to dm) siltstones. The top of the succession has a series of thick (>1 meter) amalgamated sandstone beds. Erosional surfaces between sandstone beds, especially in the upper part of the succession, are common. Sedimentary structures that indicate hyperpycnal flows are: unusually thick intervals (>1 meter) of plane-parallel lamination in the sandstone beds, and thick beds containing rippled to parallel-laminated to rippled, or massive to parallel laminated to massive, in the same bed (Fig. 2).

The Panther Tongue outcrops have kilometers of lateral continuity and individual beds can be mapped laterally. Individual sandstone beds were mapped using photomosaics and LIDAR (Light Detection and Ranging) data. The bedding diagram oriented down the depositional dip indicates that the beds are continuous over the extent of the outcrop (> 1 km) and dip southwards, i.e., in the downcurrent direction. The thick sandstone beds dip basinward at about 2.5 degrees and thin at a rate of 0.5/100 from meters to decimeters over a distance of hundreds of meters. The cm-thick beds were difficult to map but these were observed to have relative constant thickness and gentle dips over hundreds of meters in both dip and strike depositional direction (Fig. 3).

The geometry and thickness of the Panther Tongue beds are indicators of the hyperpycnal flow plume behavior. The hyperpycnal flows that are river generated will expand off the river mouths because the lack of confinement and the bottom friction. A modern analog of Panther Tongue Delta might be the Yellow River Delta in China. In Bohai Bay hyperpycnal plumes were observed in front of the modern Yellow River. The Yellow River hyperpycnal flows extend down the delta front and stop at the base of the delta front where the gradient decreases. Our data on Panther Tongue beds suggest a basin setting analogous with Bohai Bay, with low water depth (tens of meters) where the hyperpycnal flows expanded and stopped in a short distance off the river mouth.

Figure1. Panther Tongue outcrop belt in central Utah, USA. The study area is indicated with a red square.