Abstract

Superb exposures of the Upper Cretaceous Ferron Sandstone Member of the Mancos Shale in southern Castle Valley, Utah, have been utilized in a detailed study of the stratigraphic and paleoenvironmental setting of the major coal beds of the Emery coalfield. Strata of the Ferron Sandstone Member accumulated in a suite of deltaic environments along the western margin of the Interior Cretaceous seaway during late Turonian time. Prodelta, delta-front, delta-plain, distributary-channel, and alluvial facies are recognized. The Ferron consists of five major cycles of deposition. Each cycle records rapid northeast progradation of a high-constructive delta followed by phases of abandonment and destruction. With the exception of the first delta cycle, only the seaward part of which is exposed, each of the cycles contains one thick bed of coal. Thick, laterally continuous coals are present only in the delta-plain facies and are generally associated with brackish-water claystones and siltstones. Peat accumulation appears to have occurred primarily during phases of delta progradation, though two areas have been identified in the Emery coalfield where peat-producing swamps continued to exist through phases of delta destruction into the following phases of progradation. The thickest part of each of the major coal beds of the Ferron occurs in a position just landward (southwestward) of the pinchout of its associated delta-front sandstone. This genetic relation forms the basis of a predictive model that can be used to guide coal exploration programs in Cretaceous coal-bearing rocks of the Western Interior.

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