Lithofacies and Fauna of the Loyalhanna Limestone (Chesterian, Upper Mississippian) and Their Implications for its Origin in the Appalachian Basin

Abstract

ABSTRACT The Loyalhanna Limestone is one of the most recognizable rock units in the Appalachian Basin. Differential weathering of its large-scale cross-bedding and massive character make this unit a widely used dimension and quarry stone. Diverging thoughts on the origin of the iconic cross-bedding have led to a long-standing debate over whether the genesis of the Loyalhanna is eolian or marine. Diagnostic bedding characters exhibited by the Loyalhanna are equivocal with most features possible in environments of both subaerial or subaqueous dune formation. Other diagnostic subaerial textures such as exposure surfaces, translatent ripple laminates, tracks, and trails are unknown in the unit. However, the Loyalhanna does contain a number of features that can be attributed to subaqueous deposition. For example, endolithic algal borings and coatings are ubiquitous. The Loyalhanna Limestone also displays a north-to-south change in lithofacies characteristics. Along its northern and eastern boundaries, the typically massive Loyalhanna is interbedded with rippled sandstone and red to variegated shale. Near its southern geographic limits, the Loyalhanna is interbedded with nodular, argillaceous, fossiliferous limestone intervals. Between these two interbedded lithofacies, the unit consists of the typical massive, cross-bedded, sandy limestone. Petrographically, the Loyalhanna contains a common microfauna of endothyrid foraminifers, and fragmented brachiopods, bryozoans, and echinoderms. A macrofauna was noted at eight locations. This fauna consists of articulated brachiopods, gastropods, bivalves, and trilobites. At other locations, comminuted megafauna assemblages are widespread. These fragmental faunas are known even in north-central and northeastern Pennsylvania. The presence of marine micro- and megafossils from throughout the Loyalhanna depositional basin and the equivocal nature of the eolian features indicate that this unit is largely if not completely marine in origin.