Stratigraphy and depositional environments of the Middle Cambrian Maryville Limestone (Conasauga Group) near Thorn Hill, Tennessee

The Conasauga Group is a major part of the thick lower Paleozoic miogeoclinal sedimentary sequence in the over-thrust Valley and Ridge Province of the Southern Appala-chians. The Conasauga Group consists of six formations (alternately shale and carbonate) which represent an interfingering relationship between the Conasauga Shale to the west and the Honaker and Elbrook dolomites to the east. During the Cambrian, a pericratonic platform fringed eastern North America. During the middle Cambrian, the Maryville Limestone, the principal focus of this thesis, and the equivalent upper Honaker Dolomite formed within and around an intrashelf basin on this platform. Lateral facies transitions from cyclic peritidal dolomites to deeper water facies of the intrashelf basin include lagoonal pellet carbonates, oolitic and oncolitic grainstones, Renalcis boundstones, lime mudstones, and siltstones or silty carbonates.

Excellent exposure along U.S. highway 25E in the Copper Creek allochthon near Thorn Hill, Tennessee, provides an opportunity for study of the Maryville Limestone which is 168.9 meters thick at this section. The formation has been subdivided into 38 informal lithostratigraphic units composed of ten separate rock types as determined by field criteria. Four of these rock types consist of intermixed or interlayered limestone and dolostone.

The Maryville was divided into eight discrete facies based on analysis of 125 slabs, 112 thin sections, and 20 acetate peels, as well as on field observations. Based on these data, and on analysis of vertical contacts between lithologies, the following facies model is proposed. The general depositional environment for the Maryville Limestone is inferred to have been open marine, subtidal (2 to approximately 30 meters in water depth), distal carbonate ramp. This general depositional environment is composed of 5 laterally contemporaneous genetic environments within the distal ramp setting. The open lagoonal facies is composed of very fine and fine grained terrigenous clastics, mixed terrigenous clastic and carbonate grainstone, carbonate mudstone and grainstone, and limestone conglomerate. The oolitic and oncolitic facies were formed within oolite shoals (active zone) and lagoonward of them. This facies contains mudstone with interbedded storm deposited oolitic grainstone, oncolitic oolite, and oolite. The ooid-shoal stabilized-zone facies contains moderately to poorly sorted oolitic grainstone, oncolitic grainstone, and interlaminated mudstone and oolitic grainstone/packstone. A Renalcis boundstone facies formed in relatively protected settings shoreward of the ooid shoal stabilized zone. This facies contains Renalcis boundstone, mudstone, Renalcis grainstone, and calcarenite (undifferentiated). A proximal "restricted" lagoonal facies is composed of abundant mudstone with interbedded oncolitic packstone, wackestone, and grainstone.

The Maryville is composed of three carbonate ramp extension sequences and three interposed onlapping shelf lagoon sequences. These sequence pairs are asymmetric (in terms of thickness of net deposition within a complete cycle). Carbonate ramp extension sequences are usually thicker than onlapping shelf lagoon sequences. However, the uppermost Maryville facies is very thick, and the third onlapping shelf lagoon sequence includes a portion of the overlying Nolichucky shale.