A SYNSEDIMENTARY EXTENSIONAL-COMPRESSIONAL FAULT SYSTEM IN AN INCISED PALEOVALLEY IN THE UPPER MISSISSIPPIAN BLUESTONE FORMATION, SOUTHERN WEST VIRGINIA: A RE-EXAMINATION OF THOMAS' EARLY WORK ON THE HURRICANE RIDGE SYNCLINE

Bill Thomas' Ph.D. dissertation, completed in 1959 at VPI just as he turned 23, was a pre-plate tectonic study on the interaction between tectonics and stratigraphy, and included an early nod towards source to sink depositional processes. These research themes would dominate the rest of his career. His conclusions suggested synsedimentary synclinal folding along the Appalachian structural front was recorded by slumps and stratigraphic variations in Upper Mississippian strata. Since publication of that work (submitted 1963, published GSA Bull. 1966), there have been significant advances in understanding of sedimentary environments and thrust-belt structure, in addition to rejection of geosynclinal theory in favor of the plate tectonic paradigm. Here, I update his work considering 64 years of advances and fresh exposures that elucidate structures he was only able to partially observe.

New construction has exposed a >150 m-long by 12 m-deep, NE-SW-trending incised paleovalley in coastal plain facies of the upper Bluestone Formation ~50 m below the pre-Absaroka unconformity and ~1 km NW of the NW limb of the Hurricane Ridge syncline. The paleovalley is filled with a trough crossbedded litharenite overlain by red mudstone interpreted as a fluvial or estuarine valley fill and associated floodplain. The cross-strike exposure is cut by 9 listric normal faults that strike SW and dip NW. The faults root in a basal extensional detachment at the top of a 30 cm lacustrine limestone overlying 1-2 m of gray mudstone. Strata in down-to-NW hanging walls thicken relative to each footwall documenting synsedimentary growth. Basal strata in each hanging wall block are rotated and dip as much as 40˚ SE; dip decreases upsection to subhorizontal. The NW toe of the listric system transitions into a 3 m, top-to-NW thrust splay dipping 60˚ SE; ironically, the underlying gray mudstone is deformed into a miniature mushwad.

The fault system resembles structures Thomas described as slumps and angular unconformities, although folding is localized. Considering previous work documenting paleoseismicity in the underlying Hinton Formation, these new observations confirm synsedimentary deformation in the Appalachian foreland at ~320-317 Ma between Neoacadian and Alleghenian deformation in the Appalachian internides.