Petrology and depositional history of a Middle Ordovician carbonate platform: Chickamauga Group, northeastern Tennessee
The Middle Ordovician of northeastern Tennessee comprises two stratigraphically and paleoenvironmentally discrete sedimentary packages. In the northwest, the Chickamauga Group, which can be further subdivided into nine formations, is primarily composed of shallow-water carbonate-platform deposits formed during the transgression of the early Middle Ordovician sea. Water depths normally were no greater than about 50 m, usually less. Greatest depths were probably present in the southeastern parts of the platform, where deeper-water (below wave base) reef complexes developed. Farther southeast, partially equivalent Middle Ordovician deposits, comprising at least five distinct formations, exhibit considerably more lithologic and paleoenvironmental diversity. These deposits include a relatively thin basal sequence of shallow-marine carbonates overlain by deep-water, hemipelagic carbonates and turbiditic shales and siltstones. Maximum depths of water in this region were on the order of 700 m. The earliest recognizable stage in the history of this paleoenvironmentally complex sequence is associated with the development of a shallow-water carbonate platform in the southeastern-most part of the area (Whiterockian time). Subsequent rapid downwarping in this area resulted in the drowning of the platform and the formation of a starved, deep-water, foreland basin in which hemipelagic carbonates and related slope deposits began to accumulate. The final phase of tectonic movement in the area was associated with the nearly isochronous flooding of exposed Lower Ordovician sediments (Knox Group) to the northwest. This event, which marked the beginning of deposition of the Chickamauga Group, was associated with the first introduction of eastwardly derived turbidites into the basin. Continued infilling of the basin was initially coupled with a gradual deepening trend on the carbonate platform to the northwest. Eventually, the basin was filled to the point at which terrigenous material began to be effectively transported across the basin and onto the platform. Associated with this terrigenous influx, an initial deepening, related to a reduction in carbonate production, was followed by gradual shallowing on the platform. Continued terrigenous input and shallowing ultimately resulted in the development of terrigenous-carbonate tidal flats throughout the area. The Tennessee Middle Ordovician contains a sequence of depositional facies that detail the developmental history of a foreland basin in an active margin setting. Recognition of similar sequences elsewhere may be used to infer analogous depositional settings and tectonic styles.