TAFONI IN THE PENNSYLVANIAN SHARON FORMATION OF NORTHEASTERN OHIO: NEW DETAILS

The Sharon Formation in northeastern Ohio forms dramatic erosional ledges composed of clean-quartz sand to conglomerate beds. These vertical exposures commonly display the honeycomb pattern of tafoni, as well as depositional structures such as cross bedding. The Sharon Formation is a braided-stream deposit that forms the Early Pennsylvanian basal unit of Pottsville Group, and unconformably overlies the Mississippian Cuyahoga Formation.

We studied the tafoni over two summers (2010-2011), observing the ledge orientation, sunlight, plant cover, and sedimentological occurrence. We also collected samples for petrographic, SEM-EDX, and x-ray powder diffraction (XRPD) analyses. Our field sites were Thompson Ledges Park (Geauga Co.), Chapin Forest Ledges (Lake Co.), Twinsburg Ledges Park (Summit Co.), and Whipps Ledges (Cuyahoga Co.).

Our field and laboratory observations show: 1) both iron oxide and pyrite cement concentrated near the exposed outer surfaces of the rock; 2) a continuous black layer of clay coating the exterior surfaces; and 3) episodic occurrences of efflorescence.

Consistent with earlier findings, our petrographic work also showed case hardening by iron oxide and pyrite cement of the thin walls separating individual tafona. Iron is abundant at and below the basal contact of the Sharon Formation, and Foos (2003) has shown the pH of ground water in the Sharon Formation can be acidic (pH range of 5.1 to 8.2) allowing for its remobilization. A black coating similar to that reported by Mustoe (1982) was also present covering the walls separating the tafona. In thin sections cut normal to the rock surface, this black coating was a continuous layer that outlined the exposed outer boundary of the rock. XRPD and SEM-EDX determined that kaolinite (variety dickite) was a major component of this coating. The crystallization of salt has long been proposed as an agent for the creation of tafoni in coastal and arid regions. Our outcrops are in a temperate climate inland from fresh-water Lake Erie; salts were not seen at first, or expected. After several trips to each park throughout the summer we, by chance on a sunny day after a heavy rainfall, observed white filamentous efflorescence along some bedding planes and in the recesses of the tafoni. This salt was not present in chemical or mineral analyses.