--> ABSTRACT: Woodford Shale (Upper Devonian-Lower Mississippian) and Associated Phosphate Nodules, by Suzan Siy; #90996 (1990).

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ABSTRACT: Woodford Shale (Upper Devonian-Lower Mississippian) and Associated Phosphate Nodules

Suzan Siy

A deep-water marine environment is inferred for the Woodford Shale formation (Upper Devonian-Lower Mississippian) in the Arbuckle and Ouachita mountains of southern Oklahoma. Black shales and associated phosphate nodules were deposited peripherally to the carbonate shelf adjacent to the North American craton. Analysis indicates that shales and interbedded cherts lack siliciclastic detritus and shallow-water fauna, and that they have an abundance of siliceous organisms. Depletion of Ce in the Woodford Shale relative to average shale compositions suggests the influence of deep oceanic water. Carbon isotope compositions of shales, cherts, and phosphates for ^dgr13C range from -27.5 to -30.2 ^pmil. These values are comparable to Devonian-Mississippian values report d for distal marine sediments that get successively lighter farther from shore.

In-situ phosphate nodules occurring in the top of the formation are early diagenetic features that formed in the upper few centimeters of organic-rich sediment in a poorly oxygenated subaqueous environment. A hierarchy of apatite morphologies in nodules indicates stages of diagenesis. Structureless collophane probably is representative of primary marine apatite. Higher order apatite phases successively include globular collophane, botryoidal apatite, and apatite crystallites. The latter commonly is associated with silicified nodules. Silicification of phosphate nodules represents the most advanced stage of diagenesis prior to sediment lithification, and it implies that silica diagenesis within the Woodford occurred after nodules formed. Preserved radiolarians and sponge spicules obser ed in nodules also indicate that apatite precipitated before biogenic silica dissolved.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990