Upper Ordovician Tempestites from Swatara Gap, Pennsylvania: Depositional Processes Affecting the Sediments and Paleoecology of the Fossil Faunas

The Upper Ordovician, Reedsville Formation at Swatara Gap, Lebanon County, Pennsylvania contains two benthic fossil faunas. The stratigraphically lowest strata of the Swatara Gap exposures (interbedded siltstones and shales) contain a shallow subtidal fossil fauna which includes diverse brachiopods, trilobites, mollusks, echinoderms, and a eurypterid. Although diverse, the fauna is dominated by organisms well suited to live on and in a muddy substrate, such as small sowerbyellids (Sowerbyella and Thaerodonta), flat strophomenids (Rafinesquina), and the trilobite Cryptolithus. The stratigraphically highest strata, which contain shales and lack siltstones, include the burrowing, inarticulate brachiopod Pseudolingula, infaunal pelecypods, and the trilobite Cryptolithus. Other fossils from the stratigraphically lower Swatara Gap rocks are noticeably absent from these upper shales. Both the silty and the muddy substrate faunas were preserved by storm sedimentation. The silt-rich rocks record distal storm processes-but not as distal as the siltpoor rocks (mud tempestites). Although the result of distal storm processes, these mud tempestites indicate high sedimentation rates, and the fossil fauna buried by the mud tempestites includes organisms well adapted to living in, or escaping from, thick muddy deposits. The benthic fossils found in the silty and muddy Reedsville rocks do not readily fit published paleobathymetric models. This is probably because factors other than absolute depth-such as substrate and sedimentation ratewere primary controls on organisms' occurrences.