--> Abstract: Subaqueous Deposition of the "C" Zone, Red River Formation (Upper Ordovician), Williston Basin, North Dakota, by J. S. Fox and P. E. Videtich; #91017 (1992).

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ABSTRACT: Subaqueous Deposition of the "C" Zone, Red River Formation (Upper Ordovician), Williston Basin, North Dakota

FOX, JON S., and PATRICIA E. VIDETICH, University of North Dakota, Grand Forks, ND

The "C" zone of the Red River Formation (Upper Ordovician) in the Williston basin has been interpreted by many as a shoaling-upward carbonate sequence, culminating with the formation of anhydrite in a supratidal environment. Recent petrographic analysis of cores from 28 wells, along with log analysis of 119 wells in Divide County, North Dakota, suggest that the entire sequence is more representative of subaqueous deposition in an increasingly evaporitic environment.

The "C" zone consists of three members: burrowed (lowermost), laminated, and anhydrite. The burrowed member is a mottled, dolomitic mudstone/wackestone. The laminated member is a fine-grained, laminated to massive dolostone, and commonly contains crystallotopic anhydrite and gypsum pseudomorphs. The anhydrite member consists of alternating beds of fine-grained dolostone with bedded massive to mosaic ("nodular") anhydrite. The contacts between all members are gradational.

Although commonly cited as evidence for supratidal conditions, mosaic anhydrite is not diagnostic of sabkha deposition and has been shown previously to be produced by diagenetic alteration of bedded sulfates. The primarily bedded nature of the anhydrite and its uniform distribution suggest subaqueous rather than continental deposition, as proposed by others in studies of the anhydrite in eastern Montana and southern Saskatchewan. This interpretation is supported by the lack of sedimentary structures suggestive of supratidal and intertidal deposition, particularly mudcracks, fenestral fabrics, intraclasts, deflation surfaces, and tidal channel deposits. The environmental interpretation for the "C" zone has important implications regarding models of dolomitization and other diagenetic e ents, which ultimately control reservoir porosity.

 

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)