Evaporite Cycles and Cycle Boundaries in the Upper Part of the Paradox Member, Hermosa Formation of Pennsylvanian Age in the Paradox Basin, Utah and Colorado

RAUP, OMER B., and ROBERT J. HITE, U.S. Geological Survey, Denver, CO

The evaporites of the Paradox Member of the Hermosa Formation of Pennsylvanian age in southeastern Utah and southwestern Colorado are direct precipitates from marine brines and have been changed only slightly by subsequent events. Geophysical logs of deep wells indicate that the Paradox Member is composed of at least 30 evaporite cycles. Lithologies that make up the cycles, in order of increasing salinity, are organic carbon-rich carbonate shale (black shale), dolomite, anhydrite, and halite (with or without potash). Studies of core from two wells in the central part of the basin show that some of the cycles in the upper part of the Paradox Member are remarkably symmetrical, indicating regular changes in salinity. Detailed petrologic studies have revealed newly recognized lithologic t xtures and cycle boundaries in 11 evaporite cycles, indicating very regular cyclicity of subaqueous sedimentation in a basin in which salinity was probably controlled by Gondwana glaciation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)