Paleogeographic controls of coal accumulation, Cretaceous Blackhawk Formation and Star Point Sandstone, Wasatch Plateau, Utah

Abstract

Analysis of sequences, associations, and relationships of facies in the Upper Cretaceous Blackhawk Formation and Star Point Sandstone in the Wasatch Plateau, Utah, leads to inferences regarding the paleogeographic controls affecting the accumulation of coals in delta–barrier-island complexes. Progradation, lateral shifting, and abandonment of these complexes created four major landward-thinning tongues of the upper sandstone bed of the Star Point Sandstone that extend into the lowermost Blackhawk Formation. Each tongue of the Star Point Sandstone trends northwest to southeast, subparallel to the general orientation of the Late Cretaceous paleoshoreline, and represents accretion-ridge, distributary-channel, and mouth-bar sandstones. The accretion-ridge or barrier-island sandstones grade landward into coastwise lagoonal and back-barrier swamp deposits, which, in turn, merge with bayhead deposits of short-headed streams. These deposits grade laterally into delta-plain deposits that consist of major distributary channels which crosscut the coastwise lagoons. Two coal populations were formed in this paleogeographic setting: delta-plain and back-barrier coals. Delta-plain coals formed as thin to thick, discontinuous bodies. Back-barrier coals are thicker and more laterally continuous. They parallel depositional strike and accumulated in swamps on surfaces of infilled lagoons. Here, the swamps were sites of accumulation of thick coals and were influenced by abandonment of associated deltaic barrier-island complexes. Successive offlap events resulted in accumulation of the Hiawatha coal, not as a single continuous bed, but as a series of discontinuous, related coal bodies.