U-Pb Calcite Ages and the Case for Acadian Structural Development in the Appalachian Basin: Implications for Oil and Gas Migration
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 samples is from a low-angle, slickensided (“worked”) Marcellus surface. These ages and their error bars primarily straddle the tectonically quiet time between the Neoacadian and the Alleghanian orogenies. The ages record the last time fluid migration occurred in the sampled vein-filled fractures. Thus, the veins record a Neoacadian or slightly later age, and are clearly not Alleghanian. The Neoacadian to slightly post-Neoacadian age of the veins supports an earlier contention that the major folds and faults in the Appalachian basin began developing in the Neoacadian. This proposed age was originally based on 2-D seismic lines that appeared to display structural troughs that had Upper Devonian Elk and Bradford-time sediment infilling the structural trough (Jacobi et al., 2012). Because the 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 there the infill is almost certainly depositional and of slightly pre-Elk time. Thus, these particular major Appalachian basin structures document Neoacadain 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 and their error bars exactly correspond to the timing of early oil and gas generation that was determined from a subsidence curve constructed for the cored well in West Virginia. That the timing of early oil and gas mirrors the vein age is consistent with the observation of bitumen in the veins. Oil and gas migration up these early faults could have provided hydrocarbons for the Elk and Bradford sands. It is probable that the veins did block significant hydrocarbon migration in immediately post-Neoacadian time. The possible stress rotation near one of the thrusts (documented by the rotating orientation of J1 fractures observed on an image log in a lateral) suggests that the faults were again open during J1 (Alleghanian) time. This later reactivation of the thrust faults during the Alleghanian would have provided a second time of potential hydrocarbon migration.
AAPG Datapages/Search and Discovery Article #90335 © 2018 AAPG 47th Annual AAPG-SPE Eastern Section Joint Meeting, Pittsburgh, Pennsylvania, October 7-11, 2018