PALEOENVIRONMENTAL SIGNIFICANCE OF INVERTEBRATE FAUNAS FROM THE LATE MISSISSIPPIAN LOYALHANNA FORMATION OF THE NORTH-CENTRAL APPALACHIAN BASIN

One of the most easily recognized and enigmatic sedimentary units of the Appalachian basin is the late Mississippian Loyalhanna Formation. Characterized by large-scale, festoon cross-bedding composed of a mixture of quartz and carbonate sand, this unit has long perplexed sedimentary geologists regarding its origin. Based upon its large-scale cross-bedding the Loyalhanna was initially believed to be an aeolianite. It was later reinterpreted as a shallow marine sand wave complex on the basis of textural attributes, bedding characteristics, and paleocurrent evidence. More recently, detailed reevaluation of the cross-bedding characteristics and frosted sand grains has prompted researchers to again interpret the genesis of the Loyalhanna as aeolian. Recent recovery of a brachiopod-dominated fauna from six localities of the Loyalhanna Formation in western Maryland, northern West Virginia and southwestern Pennsylvania has significant bearing on the environmental interpretation of the origin of this unit. The fauna is dominated by four brachiopod orders represented by the genera, Pugnoides, Cleiothyridina, Composita, Anthracospirifer, Kitakamithyris. The genus Dielasma occurs less commonly, and the gastropod Straparollus and indeterminate bryozoan fronds are also present. The brachiopods are commonly articulated and unabraided and are scattered throughout the cross-bedded intervals. Furthermore, in some areas the festoon cross-bedded units are interbedded with dark-gray lime mudstone. The widespread presence of these articulated faunal components within the Loyalhanna, in conjunction with its interbedding with limestones, suggests that the shallow marine interpretation is the more reasonable for the studied area.