Tectonic Significance of Lithicwacke-Polymictic Conglomerate Petrofacies Association Within Upper Cretaceous Torchlight Sandstone, Big Horn Basin, Wyoming

Nazrul I. Khandaker, Carl F. Vondra

The Torchlight Sandstone belonging to the Upper Cretaceous Frontier Formation in the Big Horn basin, Wyoming, shows a distinctive lithicwacke-polymictic conglomerate petrofacies association. The polymictic conglomerate is composed of granule-cobble-sized clasts of quartzite, chert, andesite, argillite, and phyllite. The survival of phyllite, argillite, and neovolcanic andesite clasts indicate that the detritus underwent very little subaerial transport before it was deposited along the proximal margin of the foreland basin. A petrologically heterogeneous upland source of high to moderate relief is indicated by the clast size and composition. Hydrodynamic structures, in conjunction with textural attributes, and compositional data indicate that detritus moved southeast from ts source terrane and was deposited by a high-energy distributary complex. The lithicwacke petrofacies is dominated by higher chert and quartz content with a subordinate amount of labile components including paleovolcanic clasts and fine-grained matrix. The development of phyllosilicate matrix around quartz and chert grains preserved the primary porosity and permeability of the sandstone. Absence of any noticeable quartz overgrowth apparently contributed to the preservation of good reservoir quality in this petrofacies.

Considering its (Torchlight Sandstone) close proximity to the thrust belt and to the locus of andesite volcanism in the northwest and west, it is suggested that the extrabasinal detritus within the foreland basin can provide significant clues as to the timing of the thrust events and volcanicity in the adjacent region. New perspectives for hydrocarbon exploration and regional correlation may be gained by employing this petrofacies association.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.