Coral growth and adaptation in a storm influenced, level-bottom setting, Lobelville Member of the Brownsport Formation (Upper Silurian), west-central Tennessee

The Lobelville Member of the Brownsport Formation (Upper Silurian - Ludlow) is a part of a nearly continuous succession of Silurian and Devonian age limestones and shales exposed in a narrow outcrop belt in the western valley of the Tennessee River in Tennessee. The Lobelville is a 10.1-15.5 m thick sequence of interbedded echinoderm-bryozoan wackestone and gray shale, deposited in a level-bottom, tropical marine setting at depths below fairweather wave base, but still affected by episodic disturbances. Strong currents, associated with large storms, disturbed the slowly accumulating skeletal sediment and introduced terrigenous mud which blanketed areas of the seafloor. The frequency of disturbance and sediment influx increased over time, culminating in the deposition of 0.5-1 m thick sequences of gray shale, interbedded with thin limestone beds and lenses. Stratigraphically, the Lobelville is underlain by the Bob Member of the Brownsport Formation and overlain by the Decatur Formation. Both units are composed of massive-bedded echinoderm packstone, deposited in shallow water shoal environments at depths above normal wave base. Within the Lobelville an abundant and diverse coral fauna is present. Two species of tabulate coral, Favosites louisvillensis and Thecia minor exhibit a distinctive large, platey growth habit which is interpreted to be an adaptation to the episodically disturbed, soft substrate environment. Genetic factors, governing corallite ontogeny and colony astogeny, appear to have had the greatest control on the growth form of these species. Individual corallites of F. louisvillensis and T. minor, had small mature diameters (1-2 mm wide), suggesting progenetic development, which allowed the corallites to reach maturity sooner. The resulting rapid generation of offsets, primarily at colony margins, promoted the development of large colony sizes, a trait which increased colony and species survival in the Lobelville environment. The large, platey colonies were less likely to be overturned during a disturbance, as well as less likely to be totally buried by storm-derived sediment following a disturbance. When partially buried, the rapid generation of offsets promoted colony regeneration over sediment covered areas, resulting in colony reestablishment and further outward colony increase. Four ecologic factors were identified as having some degree of effect on F. louisvillensis and T. minor colony growth: 1) rate and pattern of sediment accumulation; 2) normal and storm generated currents; 3) substrate conditions; 4) competitive interactions. Although colony development was often altered by one or more of these ecologic factors, genetic controls had a much greater affect on the overall development and growth form of F. louisvillensis and T. minor colonies. Low velocity water currents often scoured small, ephemeral cryptic habitats beneath colony margins of large, platey tabulate corals living on the Lobelville mud-rich substrate. A low diversity, opportunistic assemblage, dominated by bryozoans and spirorbid worms, colonized these cryptic substrates. Cavity longevity was controlled by the rate of sedimentation, frequency of disturbance and host growth responses. These factors limited the duration of a cryptic habitat, which in turn limited the diversity and growth of colonizing epibionts.