U-PB CALCITE VEIN AGES AND THE CASE FOR ACADIAN STRUCTURAL DEVELOPMENT IN THE APPALACHIAN BASIN

New U-Pb isochrons on calcite veins in the Middle Devonian (~ 400 Ma) Marcellus Formation in an EQT core raised in West Virginia are 317 +/- 20 Ma and 332 +/-20 Ma. One of the dated samples on the 322 Ma isochron is from a low-angle, slickensided surface. These ages and their error bars straddle the tectonically quiet time between the Neoacadian and the Alleghanian orogenies, and the older isochron predates thrust ages in the fold and thrust belt by 40-75 Ma. Since the ages record the last time fluid migration occurred in the sampled vein-filled fractures, the veins record a Neoacadian or slightly younger age, not Alleghanian, although an earliest Alleghanian age for the younger isochron is possible. Preliminary data from cores in PA and NY suggest this fluid migration event was basin-wide.

The Neoacadian to slightly post-Neoacadian age of the veins supports an earlier contention that folds and faults in the Appalachian basin began developing in the Neoacadian. This proposed age was based on 2‑D seismic lines that appeared to display structural troughs infilled by Upper Devonian Elk and Bradford-time sediment (Jacobi et al., 2012). Because the seismic data were relatively low resolution, a tectonic infill (mushwad), rather than a sediment infill, could not be completely ruled out (and thus, an Alleghanian time of development remained possible). However, a high–resolution 3D seismic volume in western PA confirms that the infill there is depositional and of slightly pre-Elk time. Thus, these particular major Appalachian basin structures document Neoacadian initial development. We previously suggested that the initial structures were related to gravity “salt” tectonics (that involved silts, shales and limited salt of the Silurian Salina Group).

The ages of the veins correspond to the timing of early oil and gas generation based on a subsidence curve for the cored well in West Virginia. The proposed coeval early oil and gas generation and vein development is consistent with the observation of bitumen in the veins. Oil and gas migration up these early faults could have provided hydrocarbons for the Upper Devonian Elk and Bradford sands. The paleostress rotation near one of the thrusts (documented by J1 fractures rotating close to a thrust on an image log in a lateral) suggests that the faults were again open during J1 (Alleghanian) time.