--> ABSTRACT: Panther Tongue Sandstone Outcrop Case Study II: Evolution of Delta Types Within a Forced Regression, by W. R. Morris, H. W. Posamentier, J. P. Bhattacharya, K. B. Loomis, J. A. Kupecz, C. Wu, M. Lopez-Blanco, P. R. Thompson, D. B. Spear, B. Kendall; #91020 (1995).

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Panther Tongue Sandstone Outcrop Case Study II: Evolution of Delta Types Within a Forced Regression

W. R. Morris, H. W. Posamentier, J. P. Bhattacharya, K. B. Loomis, J. A. Kupecz, C. Wu, M. Lopez-Blanco, P. R. Thompson, D. B. Spear, B. Kendall

The Upper Cretaceous Panther Tongue sandstone, Star Point Formation, central Utah, consists of two types of inertia-dominated braid-delta complexes. The older complex comprises interbedded sandstone and mudstone, and is characterized by turbidites with mudstone drapes. Delta-front facies dip at less than 5° Beds thin and fine basinward and interfinger with burrowed muddy prodelta deposits. The upper-delta front within this complex comprises traction deposits and rare loaded to massive distributary-mouth-bar sandstones interpreted as inertia flows related to river floods. This complex is interpreted as a mixed load, turbidite-dominated delta.

The younger complex is sand-prone, and is characterized by steeply dipping (25-27°) foresets. This delta front is interpreted to be deposited by avalanche processes, and by rapidly deposition of high-density turbidites. Traction deposits are limited to the numerous distributary mouth bars. Prodelta facies comprise bioturbated-sandstone and siltstone turbidites. This complex is interpreted as a bed-load-dominated, Gilbert-type delta.

It is interpreted that grain size exerts the primary control on the stratal architecture, with lower slopes in the muddier mixed-load systems and higher slopes in the sandier systems. This is due to the greater capacity for downslope transport in the muddier inertial flows, as well as a decrease in slope from suspension settling of mud. The sandy lobes in the Gilbert deltas are also smaller than in the mixed-load deltaic lobes resulting in more internal downlap surfaces in the Gilbert delta complex.

The evolution of deltaic complexes is interpreted to be related to a decrease in mud within associated fluvial systems. The seaward increase in sand content characterizes forced regressions and can result from the progressive cannibalization of older deltaic and fluvial deposits during a drop in relative sea level.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995