EXPLORING DEPOSITIONAL HISTORY AND POTENTIAL CYCLICITY IN THE ALLENTOWN FORMATION, EASTON, PENNSYLVANIA

Following Late Precambrian rifting of Rodinia, the east coast of North America was a tectonically passive margin. Marine transgression throughout the Cambrian resulted in thick strata of Late Cambrian carbonate shelf sediments, some as the Allentown Formation in eastern Pennsylvania and New Jersey. Previous studies have noted a variety of carbonate lithofacies with lateral variation in the dolomitic Allentown and have interpreted the unit as reflecting a very shallow carbonate shelf environment. Rare quartz sand-rich layers are interpreted as having entered the system by wind transport processes. A few studies have noted a rhythmic character to the Allentown’s stratigraphic changes in color, lithology, and bed thickness, and these have been interpreted in a sequence stratigraphic context and with time series analysis to show eustatic cycles related to Milankovitch cycles.

In this study, detailed stratigraphic descriptions and measurements were taken of the Allentown Formation at two outcrops in Easton, PA. These columns, totaling ~64 meters of section, represent the most detailed descriptions of the Allentown succession to our knowledge. The lithological variation was generally similar to that noted in the literature: primarily ooids, stromatolites, rip-up clasts, laminated micrite, quartz sand lenses and beds, and carbonate intraclasts. Yet, rip-up clasts and erosional bedding surfaces were observed at a greater frequency in the Easton outcrops as compared to other previously studied localities. One rip-up clast bed includes quartz clasts reaching over a centimeter in diameter, potentially refuting an eolian transport mechanism for the quartz-rich sediments. These observations suggest a shallow sub-tidal, and possibly inter- or supra-tidal, environment at these field sites.

Recent literature on the water depths and the lateral and temporal environment changes of carbonate shelf sediments in the modern analog of the Bahamas will allow for the use of the Allentown stratigraphic column to construct a history of the depositional environments and explore depositional cycles. If it is assumed that the sea level changes of the passive margin are reflective of eustatic changes, we may be able to connect periodicity with climate patterns.