SEQUENCE STRATIGRAPHY AND SPECTRAL GAMMA RAY ANALYSIS OF UPPER ORDOVICIAN CARBONATES OF THE NORTHERN APPALACHIAN BASIN: LINKING SURFACE AND SUBSURFACE STRATIGRAPHY

Throughout the last half-century, gamma ray logs have been utilized extensively for establishing subsurface correlation. In particular, substantial data are available from the subsurface of southern NY, OH and PA and have been used to establish general subsurface patterns in the Black River and Trenton groups. However, new technology has enabled the resolution of gamma radiation into a spectrum of source elements by establishing the proportion of K, Ur, and Th radiation emitted from the host rock. Moreover, newer handheld devices have similarly, enabled the evaluation of surface stratigraphic successions with respect to K, Ur, and Th radiation values.

Through the use of these tools, and an established detailed framework of bio-, event, and sequence stratigraphy it has become possible to extend well-constrained stratigraphic intervals recognized at the surface into the subsurface. In this work, the Black River and Trenton Group carbonates (Mohawkian) are being investigated throughout the subsurface of the northern Appalachian Basin and are being equated to surface exposures in the type Mohawkian area, central PA, and central KY. Through the use of resolved spectral gamma ray analysis, it has become possible to attribute sharp excursions in subsurface gamma ray profiles to K-rich horizons (probable K-bentonites), to Ur-rich intervals (condensed intervals, sequence boundaries, and organic rich shales), and to Th-rich intervals (strata with higher siliciclastic content).

Although this work is still in its infancy, initial work on surface exposures using the Exploranium MiniSpec, has helped to refine our existing sequence stratigraphic framework. Interestingly, the gamma ray values obtained show excellent resolution (down to half meter scales) and help to identify a series of parasequences, flooding surfaces, K-bentonites, and sequence boundaries already recognized in outcrop. These techniques underscore the continuity of small-scale cycles over a large region, and thus suggest allocyclic controls on deposition rather than autocyclic controls. Nonetheless, studies of this magnitude and resolution have not been performed in this region, and such a study will provide new insights on the synchroneity/diachroneity of climatic, tectonic, and sea-level modifications during this time.