Siliceous Spiculites as Shoreline Indicators in Deltaic Sequences
Lithic and faunal properties and stratigraphic relationships of three Pennsylvanian silica–cemented spiculites in the Appalachian Plateau lead to inferences about the time and specific environmental conditions of spiculite formation. Fauna within spiculite and associated cherty beds indicate a normal Pennsylvanian marine environment, whereas excellent preservation of shells, as well as associated uncompacted silicified coal, indicate early introduction of silica cement. Coal and seat rock closely overlying and underlying the spiculites suggest that the silicified beds represent transgressive elements along a swampy shoreline receiving little detritus. Rapid lateral disappearance of spicule material into thick marine–brackish equivalent detrital strata indicates that sponge colonies were excluded from areas of rapid detrital influx. In the landward direction from paleo–shorelines, spiculites grade into coal and desilicated seat rock which may have provided an immediate nearby source for silica. Absence of spiculite and cherty sediments from Pennsylvanian shorelines where a wide brackish zone separates marine environments from the shoreline suggests that abundant sponge colonies could develop only where marine water closely approached the strand.