EARLY TERRESTRIAL TRACE FOSSILS FROM THE UPPER ORDOVICIAN, BALD EAGLE SANDSTONE OF PENNSYLVANIA

Dense accumulations of subvertical burrows are present near the top of the Centennial School Member of the Bald Eagle Sandstone, throughout much of the outcrop belt in central Pennsylvania. These trace fossils are at least 500' stratigraphically below terrestrial trace fossils reported from the Juniata Formation at Potters Mill, Pennsylvania. The Centennial School Member contains interbedded sandstone and shale and is overlain by the Lost Run Conglomerate (eastern portion of outcrop belt) and the laterally correlative Spring Mount Sandstone (western part of outcrop belt). The Centennial School Member records an upward-shallowing succession with marginal marine deposits near the base grading upward to fluvial and floodplain deposits. The uppermost floodplain deposits contain a succession of 30 to 100 cm-thick, red, mottled, poorly-sorted, mudstone beds that weather to angular ped structures. These mudstone beds, interpreted as paleosols, are separated by 8 to 15 cm-thick tabular beds of crosslaminated fine sandstone. Sandstone beds have relatively sharp bases, but the tops of beds are gradational into overlying mudstone. We interpret these sands as representing major flooding events. This interval of red mudstone and interbedded sandstone forms a stratigraphic marker and is recognizable through much of the Bald Eagle outcrop belt in Pennsylvania. The highest density of burrows occurs in the mudstone. In the mudstone, burrows are typically green in color and stand out in epirelief, suggesting that the burrows may have had an organic lining. Burrows reach diameters of nearly 2 cm and lengths of over 20 cm. Although less common than in the mudstone, some burrows completely penetrate the tabular sandstone beds and terminate in underlying mudstone. To date, we have recognized this burrowed paleosol horizon at outcrops as far east as Carroll, PA (near the northeast terminus of the Bald Eagle outcrop belt) and as far west as Loysburg Gap, PA (near the southwest terminus of the outcrop belt), indicating that the burrow-forming fauna was quite abundant on Late Ordovician flood plains.