Revised Chronostratigraphy of the Cambrian System in Subsurface Ohio
Babcock, Loren E.; Baranoski, Mark T.
In recent years, the search for new hydrocarbon reserves, some of them in unconventional plays, and strata that could serve as targets for CO2 sequestration, have helped re-focus attention on the Knox and pre-Knox (Proterozoic-Cambrian) stratigraphy of the Appalachian Basin region of North America. A consideration central to interpretation of the economic potential of Cambrian strata in the subsurface of eastern and midwestern North America is the age and facies relationships of these rocks.
Historically, two problems have impaired our ability to discern age relationships of Cambrian strata in the subsurface Ohio and adjacent areas: 1, few biostratigraphic and other recognized chronostratigraphic indicators; and 2, reliance on outmoded time-stratigraphic models. With new information bearing on both of these issues we are now able to take a few steps forward in resolving the generalities of time-stratigraphic relationships in the Cambrian of Ohio.
The International Subcommission on Cambrian Stratigraphy (ISCS) has recommended a new global standard for Cambrian chronostratigraphic nomenclature that eliminates many of the ambiguities surrounding the assignment of North American rock units to various regional "standards." The global model subdivides the Cambrian into four series and 10 stages. Although not all series/epochs and stages/ages of the Cambrian System/Period have not yet been defined, those that have been ratified can be correlated precisely in most areas of the world using multiple chronostratigraphic guides.
So far, age relationships of Ohio's subsurface strata are based mostly on trilobite biostratigraphy, meaning that heavy reliance is placed on the chance discovery of recognizable remains in drill cores. Based on available evidence, most, if not all, of Ohio's subsurface Cambrian is limited to provisional Series/Epoch 3 through the Furongian Series/Epoch. A new, incomplete trilobite exoskeleton recovered from the Mt. Simon Sandstone is consistent with an age of provisional Epoch 3 (probably Guzhangian). The oldest previously recognized trilobite, "Cedaria" woosteri, from the Eau Claire Formation, suggests an age of Epoch 3 (Guzhangian Age). Sclerites of pterocephalid trilobites recovered from the upper Eau Claire Formation suggest a Furongian Epoch (Paibian to Jiangshanian? Age) assignment. Undetermined trilobites from the lower Knox Dolomite are consistent with an age of Furongian (Jiangshanian?).
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013