Abstract

The Upper Cretaceous (Campanian) Panther Tongue is a well-exposed progradational deltaic member of the Star Point Formation in the Wasatch Plateau, Utah. Unlike the eastward-thinning, wave-dominated deltaic system of the overlying Blackhawk Formation, the Panther Tongue exhibits predominantly southwestward-thinning, fluvial-dominated deltaic sandstones which intertongue with shelfal shales of the Mancos Shale.

Five major lithofacies are recognized in the Panther Tongue: 1) prodelta burrow-mottled siltstone, gradational with the underlying Mancos Shale; 2) lower delta front interbedded siltstone and very fine- to fine-grained sandstone; 3) upper delta front medium-grained laminated sandstone; 4) distributary mouth bar medium-to coarse-grained, low angle stratified sandstone; and 5) distributary channel medium- to coarse-grained, trough cross-bedded sandstone. Graded sandstone beds (Bouma ABCDE) are present in the delta front deposits and contain numerous groove, flute and other sole marks. These turbidites are inferred to represent density flows off the delta front during periods of river flooding. Paleocurrent measurements indicate southwest transport. Similarly, distributary mouth bar sandstones show seaward stepping or shingling in the southerly direction, due to delta progradation.

Panther Tongue lithofacies are commonly arranged in a vertical coarsening-upward sequence, approximately 82 ft (25 m) thick, consisting of (from the base up): prodelta, delta front and distributary mouth bar. Distributary channel sandstones may or may not be present, capping or cutting into the top of the sequence. Overall, the Panther Tongue is interpreted to be a parasequence deposited during a eustatic lowstand. The unconformity at the top of the Panther Tongue is interpreted to have been formed during a slow sea-level fall. A coarse-grained lag at the top of the Panther Tongue was probably reworked during the transgressive rise of the Mancos Sea (represented by the overlying Mancos Shale).

The Panther Tongue rapidly pinches out to the east, where only silty vestiges of the lower delta front deposits interfinger with the Mancos Shale. The persistence of the deltaic sandstones in the southerly direction indicate that the Western Interior shoreline may have had a rough east-west orientation during Panther deposition. Lateral facies relationships of distributary channel deposits indicate geometries on the scale of several miles (km) in width, and up to 65 ft (20 m) depth. These lateral and vertical relationships may have useful applications in determining geometries and reservoir characteristics of similar fluvial-dominated delta deposits.