Sedimentology and Distinction of Carbonate Buildups: Middle Ordovician, East Tennessee

Abstract

ABSTRACT Investigation of extensive skeletal grainstone deposits that comprise much of the Middle Ordovician sequence in East Tennessee has resulted in the recognition of two distinct carbonate buildup complexes. The Rockdell Formation comprises abundant pelmatozoan-bryozoan grainstones and packstones and lesser deposits of lime wackestone. These deposits represent carbonate sand shoals, consisting of a mixture of in situ and transported skeletal grains, which surround relatively small, isolated to anastomosing, lime-mud mounds. The Rockdell buildup was developed in shallow-water, high-energy conditions on a relatively stable platform. The Holston Formation is dominated by bryozoan-pelmatozoan packstones that surround roughly elliptical masses of bryozoan wackestone/boundstone. The Holston rep esents a series of lime-mud bryozoan patch-reefs that were surrounded by piles of in situ skeletal sand produced in pelmatozoan-bryozoan thickets. The Holston buildup developed in deeper water on the outer platform or perhaps downslope. Recognition and distinction of these two buildups is based on consideration of a number of criteria including 1) color, 2) overall composition and distribution of flora and fauna, 3) composition and distribution of bryozoan fauna, 4) types and distribution of component lithofacies, and 5) character of associated lime-mud deposits. Distinction of these buildups has major importance in the determination of East Tennessee paleogeography during the Middle Ordovician and also has implications for possible hydrocarbon exploration in these deposits. The criteria applied here may be useful in making distinctions among other carbonate buildups in the geologic record.