Stratigraphic, paleoenvironmental, and echinoderm distributional patterns in an unfaulted Middle Ordovician (Chickamauga Group) shelf-edge to on-shelf transect in East Tennessee

Previous workers studying Middle Ordovician carbonate sequences in East Tennessee have attempted to study changes in lithotypes and faunas across regional strike (northeast-southwest), however, regional thrust faulting has masked many important depositional features and boundaries. In the study area, the carbonate rocks are fortuitously oriented in such a manner as to reveal lithofacies characteristic of carbonate shelf-edge and shelf environments. These lithofacies grade laterally in an unfaulted sequence from near shelf-edge lithotypes in the northeast to on-shelf "lagoon" environments to the southwest along strike. The major importance of the area is that it is possible, for the first time, to analyze the lithofacies and biofacies transitions which take place across the ancient shelf-edge onto the "shelf-lagoon" environments without having the facies patterns developed by these environments dissected by regional thrust faulting.

Detailed field studies of four measured sections (measured from the Knox unconformity to the base of the Moccasin Formation averaging 450 meters per section) and examination of 426 thin sections and more than 400 polished slabs and acetate peels reveal the complex nature of the faural and lithofacies relationships in the Chickamauga Group. The Chickamauga Group is subdivided, in the study area, into 12 formations: Five Oaks, Tumbez, Fleanor, Lincolnshire, Rockdell, Chapman Ridge, Ottosee, Benbolt, Wardell, Bowen, Witten and Moccasin. . These units are listed in stratigraphically ascending order although there are many complex lateral interfingering relationships. . These formation names are applied to carbonate shelf, shallow water environments, with the iii

iv exception of the Chapman Ridge and Ottosee Formations, and represent a generally transgressive sequence. Determination of the time relationships of these units is based on environmental considerations, interfingering relationships and progradational cycles.

The line of measured stratigraphic sections oriented parallel to the regional southwest-northeast strike is interpreted to represent a transect across a portion of the Middle Ordovician shelf from near the shelf-edge in the northeast to the inner shelf in the southwestern portion of the study area. The complex lithofacies relationships observed in the shelf sediments are best summarized as lower, middle or upper stratigraphic packages. . The lower stratigraphic package is composed of the Five Oaks, Tumbez, Fleanor and Lincolnshire Formations and was deposited in peritidal to shallow marine environments. This lower package is strongly influenced by the relief on the underlying post-Knox unconformity. The middle stratigraphic package consists of the Rockdell Formation which represents a generally transgressive sequence of near shelf margin and on-shelf patch reef environments. . The lithofacies and biofacies transitions which take place near the shelf-edge and shelfward in this particular line of sections in the study area are well exemplified by the lateral facies changes within the Rockdell Formation. The upper stratigraphic package is dominated by carbonate rocks representing shallow marine (restricted lagoon) and peritidal environments of the Chapman Ridge, Ottosee, Wardell, Benbolt, Bowen and Witten Formations.

Bryozoan-red algal bound patch reefs observed in the study area have provided a means for recognizing and distinguishing on-shelf from shelf-edge carbonate buildups. The encrusting red alga, Solenopora sp.,

associated with these patch reefs was found to assume a morphology and a role in the build-up process that had previously not been observed in the Middle Ordovician carbonates of East Tennessee. It formed thin undulose layers and, along with encrusting bryozoans, bound muds of the reef core. Previously, Solenopora sp. was only known to be associated with shallow water lagoonal facies in the Middle Ordovician sequence (Moore, 1978).

Comparison of on-shelf patch reefs of the Rockdell Formation with the shelf-edge Holston reef complex has revealed some similarities in diagnostic lithofacies (e.g., reef flank and core), composition of reef flank material and mode of cementation and cement paragenesis. . The major differences observed are in the size and geometry of the reefs and their organic composition. . The Rockdell patch reefs are small in areal extent and circular whereas the Holston reef complex is large in areal extent with a linear geometry coincident with the shelf-edge. The binding and framework components of the Rockdell patch reefs are characterized by encrusting bryozoans and red algae in contrast to the bryozoans of the shelf-edge Holston reef complex.

Investigation of the different types of cements in the Rockdell patch reefs revealed a definite paragenetic sequence of cementation and internal sedimentation from which the diagenetic history of the patch reefs could be determined. The sequence of cementation was initiated by a low-iron content, radial fibrous marine cement which was generally followed by vadose silt and pendant cement. Primary and secondary pore space and cavities were finally filled with a late stage, high-iron content, blocky calcite cement. Initial partial cementation of sediments took place in the marine environment penecontemporaneously with sedimentation in the patch reef. The patch reef sediments were subsequently subaerially exposed for a period of time during which dissolution, reprecipitation and internal sedimentation took place in a vadose diagenetic environment. Following a period of subaerial exposure, the remaining primary porosity and the secondary porosity produced in the vadose diagenetic environment were sealed by cementation in a fresh water phreatic diagenetic environment.

Documentation of the occurrence and stratigraphic distribution of echinoderms in the paleoenvironments of the study area has shown that echinoderms are abundant and relatively diverse (i.e., Crinoidea, Rhombifera, Paracrinoidea, Parablastoidea, Edrioasteroidea, Edrioblastoidea and Eocrinoidea). Echinoderms played a significant sedimentological role as major sediment producers and, as trappers and binders of sediment, were capable of modifying the environment by influencing local hydrodynamic conditions.

Much of the prior information on Middle Ordovician echinoderms from East Tennessee has come from the Benbolt Formation and from the Hogskin Member of the Lincolnshire Formation. A diverse assemblage of echinoderms collected from the Rockdell Formation in the present study contributes significantly to the informational gap between the Lincolnshire and Benbolt Formations and suggests that formational names assigned to many echinoderm speciments in the region may be in error. The study indicates that the parablastoid Meristoschisma hudsoni. Sprinkle does not occur below the Benbolt and coeval units.