--> Abstract: Significance of Neptunian Fossil-rich Dikes in the Capitan-Massive (Permian) Guadalupe Mountains, New Mexico and Texas, by R. J. Stanton, Jr. and L. C. Pray; #90994 (1993).

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

ABSTRACT: Significance of Neptunian Fossil-rich Dikes in the Capitan-Massive (Permian) Guadalupe Mountains, New Mexico and Texas

Neptunian (syndepositional-marine) dikes have been reported in Paleozoic to Holocene reef and platform carbonates. Proposed origins of fissuring include gravity-induced slumping (perhaps triggered by earthquakes), flexing or faulting, and/or solution. Despite extensive syndepositional cementation and appreciable basin-margin relief, neptunian carbonate dikes are uncommon in the shelf-edge Capitan massive ("reef"). Known dikes are mostly vertical, and parallel and close to the "reef" escarpment. Although dikes filled with encrusting carbonate laminae (cement and enigmatic micrite) have long been recognized, dikes with abundant macroscopic marine biota (algal and/or invertebrate) have escaped attention. Several occur in the Capitan massive along the Permian Reef Trail at McKittrick Cany n and undoubtedly occur elsewhere.

Fossil-rich dikes are mostly 0.3-0.5 m wide, and are diverse in lithology and biota. Progressive opening, filling by grains and/or cement, and reopening created vertical composite layering; horizontal layering is absent. Grains consist of intact and fragmented biota (mostly forams, bryozoa, brachiopods, bivalves and crinoids, calcareous algae and Tubiphytes sp., and clotted non-skeletal grains). Calcareous sponges are uncommon despite their abundance in the host rock. Archeolithoporella is absent in fossil-rich dikes. Most dike skeletals were loose sea-floor particles,

but some (whole and paired shells and large Tubiphytes) probably were fissure dwellers. Preburial cement filled most primary porosity, but dark silt infilled void space within a coarse biotic assemblage on the open (basin) side of one fracture.

Neptunian dikes can aid in understanding this famous "reef." Their relative scarcity in the Capitan massive as compared to other early cemented shelf margins testifies to the general absence of faulting and flexing during Capitan progradation. The paucity of calcareous sponges in the dikes compared to the host rock indicates efficient sea-floor holdfasts, and fissure conditions inimical to sponges. The scarcity of micritic internal sediment in the fossil-rich dikes suggests little free micrite (sediment) on the sea floor, bolstering the cemented sea-floor concept of the Capitan massive.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.