Abstract

In Walnut Canyon of the Guadalupe Mountains the transition between the massive boundstones of the Massive Member of the upper Capitan Formation 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 is gradational. These micrite-rich pisoid packstones are associated with an even greater proportion of mud, abundant fenestral fabrics, occasional stromatolites, and a limited biotic assemblage (ostracods, dasycladacean algae, stromatolites, and two types of foraminifera). These features suggest an increase in salinity. Continuing shelfward the micriterich 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 in the lower and middle Capitan Formation and the Seven Rivers and Yates formations.