Silicification of Trace Fossils in Carbonates: Evidence from Permian Kaibab Formation, Southwestern Utah

Katherine J. Whidden, David J. Bottjer

Silicification in carbonates, particularly silicified trace fossils, has received relatively little previous study. Chert comprises a significant percentage of the upper Fossil Mountain Member of the Kaibab Formation, a Permian epicontinental limestone. Distribution and origin of this chert were studied from outcrops in southwestern Utah. The origin of much of this chert is believed to be as silicified Thalassinoides burrows. Field evidence for trace fossil silicification includes (1) silicified cylindrical tubes with Y-shaped branching patterns as well as hollow tubes, and (2) polygonal box-work patterns of tubes. In addition, brachiopods, bryozoans, and abundant specimens of the sponge Actinocelia maendrina Finks are also silicified.

Petrographically, microcrystalline quartz is found within the matrix of chert nodules and in void spaces within fossil skeletons. Quartz can also partially to completely replace echinoderm plates, and more rarely, brachiopods and bryozoans. The order of silicification appears to be matrix and skeletal void spaces, echinoderms, brachiopods, and bryozoans. Much of this silicification probably took place passively, with silica precipitating in void spaces within the matrix and fossil debris. Variable permeabilities combined with the presence of silicasaturated pore waters within the burrows may account for the matrix and void space silicification. Dissolution of biogenic silica from abundant sponges could have provided the silica.

Recognition of silicified trace fossils in carbonates provides a different approach to the study of early diagenetic silica precipitation. These silicified trace fossils also represent new information on bioturbation in ancient carbonates, a subject that has, until recently, been relatively unstudied.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.