Carbonate and interbedded siliciclastic-carbonate rocks of the Upper Cambrian Gatesburg Formation represent deposition on, and proximate to, the great American carbonate bank (GACB), a broad, rimmed platform of low relief that was subject to periodic sea level changes. This environmental setting produced a series of complex mixed carbonate and carbonate-siliciclastic sequences (dominated by carbonates) with limited lateral continuity between outcrops and wells. Many of the rocks formed on the GACB are targets of active petroleum exploration in western and north-central Pennsylvania, as well as in other areas of the Appalachian Basin. Potential reservoir targets include the Upper Cambrian Upper Sandy member of the Gatesburg Formation (Rose Run Sandstone of Kentucky and Ohio) and paleotopographic highs and paleokarst in the Mines Member of the Gatesburg Formation below the Knox unconformity (considered to be part of the Beekmantown throughout much of the Appalachian Basin). Although the latter typically are seismic plays, with companies searching for both stratigraphic and structural traps, knowledge of the patterns of sedimentation on the GACB is also critical to exploration efforts. Mixtures of carbonate and siliciclastic rocks resulted from spatial and temporal variability in depositional systems across the GACB, creating acute and complex reservoir heterogeneities. The distribution of porous and permeable sandstone and carbonate facies within the Cambrian sequence, as well as the juxtaposition of sandstones and paleokarst beneath the Knox unconformity, undoubtedly influenced the migration of fluids, including brines that diagenetically altered many of the rocks, primarily through dolomitization. The spatial distribution of reservoir seals, reservoir compartmentalization, and diagenetically controlled pore geometry are partially or wholly sedimentologic features.