Abstract

Coal beds most commonly occur in stratigraphic units characterized by rapid lateral facies changes for which determination of time relationships and correlation of facies are difficult. The Ferron Sandstone Member of the Mancos Shale in central Utah is such a unit. The Ferron records the activity of a delta system that prograded in a northeasterly direction into the Interior Cretaceous seaway during late Turonian time. The Ferron consists of five delta cycles, each of which includes one coal zone. Each of the coal zones contains at least one, and usually several, laterally persistent kaolinitic claystone partings. Laboratory study of the partings demonstrates that they represent altered volcanic ash falls. These partings have proven particularly useful in reconstructing the depositional history of the C coal bed of the Emery coal field. They permit division of the C coal bed into four isochronous units. The coal bed accumulated in a basin that developed concurrently with subsidence of the delta plain during both the constructive and destructive phases of the third delta cycle of the Ferron. The area of peat accumulation expanded in both seaward and landward directions during the interval of time represented by the coal. Peat accumulation was terminated by transgression of the sea across the seaward part of the peat deposit, where prodelta and delta-front strata of the fourth delta cycle now disconformably overlie the coal. In its landward part, the C coal bed is overlain, with no evidence of erosion, by delta-plain strata of the fourth cycle.

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