The stratigraphy, carbonate petrology, and depositional environments of the Chickamauga Group (Middle Ordovician) of northern East Tennessee

Examination of more than 250 thin sections and nearly 300 polished slabs and acetate peels along with detailed field studies at nine principal and several supplementary sections has revealed the precise nature of the stratigraphy, petrology, depositional environments, and bryozoan distribution in the Middle Ordovician Chickamauga Group in northern East Tennessee.

The Chickamauga can be subdivided into nine formations in the study area: Five Oaks, Lincolnshire, Rockdell, Holston, Benbolt, Wardell, Bowen, Witten, and Moccasin. While the Chickamauga Group (and its included formations) is restricted in occurrence to areas west of the Saltville Fault (at least in northern East Tennessee), equivalent Middle Ordovician strata occur east of the fault: Mosheim, Lenoir, Whitesburg, Blockhouse, and Sevier formations. Biostratigraphic evidence suggests that the Chickamauga Group ranges from middle Chazyan (early Llandeilo) to middle Trentonian (late Caradoc), while the equivalent Middle Ordovician deposits east of the Saltville Fault are older, ranging from probably early Chazyan (late Llanvirn) or even Whiterockian (early Llanvirn).

The Chickamauga Group contains a diverse assemblage of mostly carbonate lithofacies representing a complex mosaic of shelf environments developed in a relatively shallow water (0-50 meters) setting. The Holston Formation is a lone exception, representing sedimentation in a deeper subtidal (perhaps 50-l00 meters) shelf edge environment. In

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contrast to the relatively stable, although complex, environmental regime suggested for the Chickamauga Group, the equivalent strata to the east depict an extremely diverse and rapidly changing environmental scenario, including shallow shelf (Mosheim and Lenoir formations), slope (Whitesburg Formation), and basin (Blockhouse and Sevier formations) environments.

Field study of the Chickamauga Group reveals the presence of abundant stylolites. In thin section, numerous other pressure solution features such as stylocumulate, reactate, and stylobreccia are apparent. These have been produced by simple overburden pressure and probably account for the nodular bedding observed in some of the Chickamauga formations.

Three types of stromatactis structures have been recorded in the Holston Formation of this study. Type I consists of short, flat-bottomed patches of radiaxial fibrous or drusy spar. These are apparently produced through precipitation of cement in a void perhaps formed by uneven sediment compaction or burrowing. Type II is a very irregular form of stromatactis composed of radiaxial fibrous spar and invariably associated with bryozoans. The presence of relic floating grains in these spars and Fe-rich coatings on contacts with the surrounding grains suggest that solution and/or dissolution-reprecipitation may have been active in the formation of these structures. Type III stromatactis is the most common, yet difficult to explain type. It occurs as alternating subparallel layers of white, radia: ial fibrous spar and red lime mudstone. Cathodolumenescence studies suggest that these structures formed through a two stage infilling of voids by cement, followed by neomorphic inversion of the cements to radiaxial fibrous calcite.

Investigation of the bryozoans of the Rockdell and Holston formations has revealed characteristic faunal differences between the two units, which along with lithofacies and environmental features, serve to differentiate these two outwardly similar formations. . The Holston represents a complex of bryozoan reefs and flanking skeletal sands which accumulated in a relatively deep water, low energy environmental setting characterized by an absence of calcareous algae. The Rockdell sediments indicate much shallower water, higher energy conditions in which transported skeletal sands and lime mud mounds accumulated in near shoal water conditions. . Unlike the Holston, the Rockdell deposits contain an abundant and unique algal flora. The Holston contains two distinct bryozoan communities: the reef community of encrusters- Constellaria antheloidea-Bythopora-Pachydictya, and a reef flank community of Batostoma-Diplotrypa-Nicholsonella-Ottoseetaxis- Constellaria-Bythopora. The Rockdell is characterized by a community of Stictopora-Ottoseetaxis-Nicholsonella-Coeloclema-Helopora. Bryozoan faunas recovered from the Rockdell-Holston deposits show a close association with the Trenton Group of New York State. Conodont faunas of these same deposits implies an early Black Riveran age for the sediments, however.