SEQUENCE STRATIGRAPHIC FRAMEWORK OF THE CAMBRO-ORDOVICIAN TRANSITION IN THE ILLINOIS BASIN: IMPLICATIONS FOR DEFINING THE CHRONOSTRATIGRAPHIC POSITION OF THE GUNTER AND ROSE RUN SANDSTONES
The Gunter Sandstone is up to 25 feet thick and consists of fine- to medium-grained, mature quartz sandstone that can be correlated from western Illinois to eastern Missouri. It overlies, with a sharp contact, the Upper Cambrian Eminence Formation and grades upward to the Oneota Dolomite. The Gunter is interpreted as the basal part of the Lower Ordovician transgressive package deposited on a maximum regressive surface, the Cambro-Ordovician boundary. The Eminence Formation (up to 2000 feet) constitutes a southeastward-thickening, mixed carbonate-siliciclastic succession in the Illinois Basin and grades laterally into the Jordan Sandstone in the extreme northwest part of Illinois. Dolomite layers in the Eminence Formation are dense and contain relics of bioclasts, ooids, intraclasts, and peloids. The Eminence consists of two sequences superimposed by small-scale shallowing-upward cycles capped by a sandy dolomite or cemented quartz sandstone layer. The Rose Run Sandstone (over 400 feet) is correlated with the Upper Cambrian Eminence Formation. As with the Eminence, the Rose Run consists of interlayered dolomite and quartz sandstone. In contrast to the Gunter Sandstone, base of the Rose Run is gradational with the underlying Upper Cambrian Copper Ridge Dolomite, but its upper contact with the overlying Lower Ordovician Beekmantown Dolomite is sharp. The Rose Run Sandstone is interpreted as a highstand deposit within the upper sequence of the Upper Cambrian Copper Ridge Dolomite. Results of this study indicate that, in the absence of biostratigraphic data, sequence stratigraphy provides a strong tool for correlation, and aids in recognition and correlation of potential reservoirs and seals.