--> Evidence of Thrust Sheets and Décollement Within the Sedimentary Sequence of Northern Ohio; Clues for Possible Hidden Oil and Gas Pools

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Evidence of Thrust Sheets and Décollement Within the Sedimentary Sequence of Northern Ohio; Clues for Possible Hidden Oil and Gas Pools


Recent observations in Ohio have identified potentially new trapping mechanisms for oil and gas. Geologic mapping, core analysis, and high-resolution aerial imagery has revealed evidence of thrust sheets and décollement within sedimentary sequences in northern Ohio. This study examines four sites in Ohio where the exposed geologic units exhibit these structural features. Southwest of Toledo, on the Maumee River east of the Bowling Green Fault, small-scale, low-relief anticlines show Silurian-age units folded above the Ordovician-age Utica Shale. The anticlines trend NE – SW, indicating SE – NW movement of the strata overlaying the Utica. A horizontal thrust fault might be present in the Utica layers to enable these folds to form. Underneath Lake Erie, the Cargill Salt Mine is the second site examined as part of this study. Exposed in the mine, the Silurian-age Salina salt beds display clear décollement in the salt layers. The overlaying formations horizontally thrusted toward the west. Along Chappel Creek in Erie County, the Ohio Shale, Bedford Shale, Berea Sandstone, and the Cuyahoga Formation are exposed and display numerous small-scale folds. These small-scale folds are mainly low relief, with limbs dipping 10º–30º, but occasionally the folds are very intense and overturned. Analysis of these NW – SE or N – S trending folds indicates they formed by tectonic compressional stress. Horizontal thrust faults within the Devonian-age Chagrin Shale facilitated the creation of these folds. This fault zone is noticeable in core OHEE-2, just south of Chappel Creek. At Old Woman Creek in Erie County, Bedford Shale exhibits internal minor folds and faults, and displays a clear décollement zone between the Ohio Shale and Berea Sandstone. Horizonal thrust faults are present at the top of the Ohio Shale and the base of the Berea Sandstone. The study concludes that the incompetent Utica Shale, Salina salt beds, Chagrin Shale, and Bedford Shale acted as a décollement zone and enabled westward movement of the overlaying layers during the Paleozoic. A horizontal thrust sheet must be present within these décollement zones to facilitate movement of the overlaying formations. The presence of over 60 small-scale anticlines, observed only in a few streambeds and riverbeds, highly suggests the existence of similar anticlines under glacial deposits in northern Ohio. These anticlines, if confirmed, could be considered as potential structural traps for oil and gas pools.