--> ABSTRACT: Petrographic Studies of the Evaporite Cycles in the Upper Part of the Paradox Formation of the Pennsylvanian Hermosa Group in the Paradox Basin, Utah and Colorado, by Betty M. Miller; #91020 (1995).

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Petrographic Studies of the Evaporite Cycles in the Upper Part of the Paradox Formation of the Pennsylvanian Hermosa Group in the Paradox Basin, Utah and Colorado

Betty M. Miller

The Paradox basin of southeastern Utah and southwestern Colorado contains a thick sequence of 29 known evaporite cycles within the Paradox Formation of the Pennsylvanian Hermosa Group. Numerous organic-rich black shales associated with a series of these evaporite cycles are the most important hydrocarbon source rocks in the basin. Recent studies have been conducted by the U.S. Geological Survey on the lithology, mineralogy, and geochemistry of evaporite cycles in two cores from the Delhi-Taylor Oil Company's Cane Creek No. 1 and Shafer No. 1 wells located in the central part of the basin in Utah. The results of the current petrographic studies are correlated with these earlier mineralogical and geochemical analyses.

Each of the evaporite cycles in the upper part of the Paradox Formation in both cores contain a halite bed and an underlying sequence of penesaline and siliciclastic rocks collectively referred to as "interbeds." Of the 11 evaporite cycles in the two cores, cycles 2 and 3 are the most completely developed in terms of lithologic representation and distribution. The interbeds of these two cycles are nearly vertically symmetrical and consist of a sequence of transgressive anhydrite and silty dolomite, black calcareous shale, and regressive dolomite and anhydrite overlain by halite beds ranging from 50 to nearly 200 feet in thickness.

Preliminary petrographic studies of the evaporite cycles found in the cores indicate the presence of relic textures of pelletoid and oolithic limestones within the dolomite and anhydrite interbeds, organic-rich anhydrite and halite couplets, organic-rich micrite and anhydrite laminations within the dolomite and anhydrite interbeds, pseudomorphic textures of anhydrite and halite after gypsum within interbedded dolomites and anhydrites and at the top of the anhydrites underlying the basal halites, organic-rich "shales" versus organic-rich silty dolomites, and anhydrite laminations draped over the top of coarse angular crystals of halite ("snow-on-the-roof" textures). These results raise important questions as to the nature of the syndepositional (primary) and postburial (secondary) phas s for calcite, dolomite, anhydite, and halite, and provide new insights that may alter prior interpretations of depositional environments for evaporite cycles and their subsequent diagenetic alterations.

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