--> ABSTRACT: More Evidence for a Barrier Reef: Microfacies Analysis of the Tansill Outer Shelf, Permian Capitan Reef Complex, by B. L. Kirkland, C. H. Moore; #91020 (1995).

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More Evidence for a Barrier Reef: Microfacies Analysis of the Tansill Outer Shelf, Permian Capitan Reef Complex

B. L. Kirkland, C. H. Moore

Did the Capitan reef grow to sea level? The answer lies in the sediments deposited just shelfward (leeward) of the reef.

In Walnut Canyon the transition between the massive boundstones of the Capitan reef and the bedded bioclastic grainstones and packstones of the Tansill Formation is abrupt. These grainstones and packstones contain diverse, angular to sub-angular bioclastic debris and are interbedded with dasyclad-dominated grainstones, which suggest rapid deposition as a back reef apron. Further shelfward, peloid, dasyclad, foraminifera packstones and peloid packstones dominate. Here percentage of mud, degree of micritization, and rounding of bioclasts all increase. This suggests a decreasing rate of sedimentation and decreasing environmental energy -- conditions typical of a lagoon. Still further shelfward, the transition from these peloid packstones to micrite-rich pisoid packstones and wackstones i gradational. These micrite-rich pisoid packstones are associated with an even greater proportion of mud, abundant fenestral fabrics, occasional stromatolites, a few very fine quartz grains, and a limited biotic assemblage (ostracods, dasycladacean algae, stromatolites, and two types of foraminifera). These factors suggest an increase in salinity. Continuing shelfward the micrite-rich pisoid facies is transitional to pisoid (walnutoid) grainstone/packstone and tepee structures.

We propose that during deposition of the Tansill in Walnut Canyon, the reef grew to sea level and formed a protected lagoon across the outer shelf. Similar microfacies and relationships were recognized between the Capitan Formation and the Yates and Seven Rivers formations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995