--> Abstract: Neptunian Fossil-Rich Carbonate Dikes in the Capitan-Massive, (Permian) Guadalupe Mountains, New Mexico and Texas, U.S.A., by L. C. Pray and R. J. Stanton, Jr.; #91012 (1992).

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ABSTRACT: Neptunian Fossil-Rich Carbonate Dikes in the Capitan-Massive, (Permian) Guadalupe Mountains, New Mexico and Texas, U.S.A.

PRAY, LLOYD C., University of Wisconsin, Madison, WI, and ROBERT J. STANTON, Jr., Texas A&M University, College Station, TX

Syndepositional (marine-neptunian) carbonate dikes are uncommon in the Capitan-massive, despite extensive cementation and syndepositional fracturing of this submerged shelf-edge ("reef") unit. Vertical dikes near and parallel to the shelf-edge, composed of encrusting carbonate laminae (cement and enigmatic micrite) have long been recognized. Dikes with abundant macroscopic marine biota (algal and/or invertebrate) have rarely been reported. Several occur in the Capitan-massive along the Permian Reef Trail at McKittrick Canyon and undoubtedly occur elsewhere in the Capitan-massive. Dike interpretation can contribute toward better understanding the genesis of the famous Capitan-massive.

The fossil-rich dikes vary in biota and texture. Most are one-half to one-third of a meter wide, nearly vertical, and parallel the shelf margin. Horizontal layering is absent. Changes across dikes in texture, skeletals, internal sediment, and cement record episodes of progressive filling, cementing, and refracturing. In one dike the size of skeletal constituents increases as biotic diversity decreases toward the younger (basin) side of the dike (horizontal inverse graded bedding). Biotic changes accompany the lithologic changes: Early stages of fracture fill contain abundant calcareous algae and Tubiphytes sp. with sponges, bryozoan, crinoid, and other grains; bivalves and gastropods with some brachiopods are late stage biota; some of the latest biota, of whole and paired shells of on dike and large Tubiphytes sp. of another, may be fissure dwellers. Two dikes of uniform texture (1-2 mm grainstone) consist largely of skeletal grains of Tubiphytes sp., dasycladacean algae and nonfusulinid forams. Archeolithoporella appears absent throughout.

Early cement fills most primary porosity. In one dike later dark silt filled remaining fracture space and infiltered the biotic assemblage facing the open (basin) side. Micrite dikes, rarely more than a few centimeters wide, also occur but scarcity of micritic internal sediment in dikes suggests little free micrite (sediment) on the sea floor, bolstering the cemented sea floor concept of the origin of the Capitan-massive.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)