THE SALONA-COBURN TRANSITION (TRENTONIAN, MID-ORDOVICIAN) IN THE WIDENED SPRING MILLS ROADCUT (CENTRAL PENNSYLVANIA)

Widening of the roadcut on the N side of PA 45, 0.1 km E of Penns Creek (Centre County) has further exposed an interval of thinly interbedded micrite, fine calcarenite, and shale, somewhere in the combined thickness of the Salona plus Coburn Limestones. Those are lithologically and faunally distinct, but gradational; this exposure permits examination of the transition between them. We examined rocks and fossils throughout the outcrop, plotted abundances of fossils against their horizons, and looked for trends.

Taxa seen include hemispherical bryozoans (Prasopora), branching trepostomes (Batostoma, Parvohallopora, Eridotrypa, Bythopora), bifoliates (Stictopora), strophomenids (Sowerbyella, Rafinesquina, Leptaena, Strophomena), orthids (Paucicrura/Dalmanella, Plectorthis, Dinorthis, Platystrophia), trilobites (Cryptolithus, Bathyurus, Flexicalymene, Isotelus), crinoids (small columnals; Dendrocrinus), and ichnofossils (cylindrical burrows, Cruziana).

The most conspicuous trend observed is the great increase in abundance of Sowerbyella in the upper quarter of the succession. (Paucicrura/Dalmanella remains rare throughout.) A more obscure trend is that cylindrical burrows are more abundant low in the section. Another is that micrite/calcilutite beds are more dominant in the lower part while shale becomes more prevalent up-section. Very thin and discontinuous shell-lags (calcirudites) gradually become more prominent in the upper part, but none unequivocally marks the formational contact, traditionally placed at the lowest “significant” shell-lag. Previous workers (Thompson 1963) noted two such and interpreted one as a tongue coming in prematurely from the overlying Coburn. No taxa drop out or come in partway through the succession.

Most taxa counted were rare and fluctuated irregularly upward, instead of showing clear trends, possibly because of small-scale patchiness on the ancient sea floor and preservational vagaries. The fossil assemblage is close to that of the Sowerbyella-Dalmanella community (McKerrow 1978). Our findings are consistent with the transition from the restricted shallow marine environment of the Salona to the deeper, more openly circulating waters of the Coburn.