Anticlines We Have Admired: A Review of Styles and Implications for Anticlines Observed in Seismic Reflection Data of the northern Appalachian Basin
Anticlines are ubiquitous in the Appalachian basin of Pennsylvania and West Virginia. We will examine some of the different styles including fault-bend folds that generally are westerly-directed, asymmetric salt/mud cored anticlines that verge east, box folds, salt sheets/tongues, and rollover anticlines along reactivated Rome trough normal faults. The forelimb of fault-bend folds and the associated thrust are recognized in 2D seismic data based on the acute angle between steeply-dipping reflectors above the inferred thrust and flat-lying reflectors below the inferred thrust. Examples are observed in 1) Fayette County (PA) in the Fayette anticline above the Tully and in the Chestnut Ridge anticline above the Onondaga Formation, 2) Westmoreland County (PA) in the Chestnut Ridge anticline above the Onondaga, 3) Somerset County (PA) in Negro Mountain anticline above the Tully and several in a west-verging duplex that affects the Trenton and an east-verging duplex with roof thrust above the Tully east of the Youghiogheny syncline, and 4) Upshur County (WV) above the Onondaga, where dates on tectonized veins in a core yield Neoacadian age dates, suggesting that these assumed Alleghanian folds hide older structural phases. Asymmetric evaporite/mud cored anticlines above the Silurian Salina Group are common and have been described many times before. In southwestern PA, some of these folds are associated with adjacent graben/synclines which clearly have variable times of Neoacadian infill. A broad (2400 m wide), low amplitude (34 m) anticline marked by the Onondaga strikes at least 32 km along the Hundred anticline in Wetzel County (WV) and may continue to a similar feature 14.5 km to the SW in Doddridge County (WV) east of the axis of the Arches Fork anticline. The broad anticline results from a sheet (or tongue) of Salina units that flowed west out of a syncline. Sediment infill indicates a probable Neoacadian age of the flow. Both the Arches Fork and the Wolf Summit anticlines (in Harrison and Doddridge counties, WV, respectively) appear to be partly the result of basin inversion, wherein west-dipping border faults of half-grabens of the Rome trough underwent later compression and horizontal shortening, resulting in the basement blocks backing up the formerly normal faults. We estimate about 1.75% shortening across the basement fault blocks would account for the amount of vertical structural relief and shortening observed at the Trenton horizon. Motion timing includes Alleghanian, Utica time, pre-Lockport time, and possibly Salina-Onondaga time. In Jefferson County (PA) west of the Hebron anticline EQT collected an image log in a vertical well that penetrated a structure observed on seismic as small fold associated with a small thrust. The image log reveals a complexly faulted Z-fold (box fold), consistent with the proposal of Gillespie et al. for some of the structures that appear to be faults in seismic.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019