STRUCTURE AND DISTRIBUTION OF THE HAMILTON GROUP IN THE UPPER AND LOWER AUGUSTA REGIONS, SUNBURY, PA 7.5 MINUTE QUADRANGLE

Field mapping in the Upper and Lower Augusta regions of the Sunbury, PA (7.5' quadrangle) demonstrates that the Hamilton Group (Middle Devonian) can be subdivided into formations and members at the 1:24,000 scale in central Pennsylvania. This field area consists of the northeasterly oriented, doubly plunging first-order Selinsgrove Anticlinorium. The breached core of the anticlinorium, exposed along the Susquehanna River, consists of Upper Silurian and Lower Devonian carbonate units (Tonolloway, Keyser, Old Port, and Onondaga Formations). The northern and southern limbs of this structure are composed of the Middle Devonian Hamilton Group (Eifelian to Giventian), a siliciclastic succession which is subdivided into the Marcellus Formation and the Mahantango Formation. The Mahantango Fm. is further subdivided into the Turkey Ridge Mb., Dalmatia Mb., Fisher Ridge Mb., Montebello Sandstone, Sherman Ridge Mb., and Tully Limestone. The heterogeneity of the Hamilton Group is responsible for the variability in structures both observed and inferred in the field. Second-order folds in the area of study consist of asymmetrical and gently easterly and northeasterly plunging folds that range from tight to open, third-order kink folds (observed in Tonolloway Fm. and Fisher Ridge Mb.), and fourth-order chevron and kink folds (observed only in the Tonolloway Fm.). Some of the second-order folds were observed to have undergone flexural slip and may be cross cut by reverse faults. Axial plane cleavage and pencil cleavage in the lower part of the Mahantango Fm. are very common throughout the map area. This mapping project will help locate the Hamilton Group where it is not well exposed on the westward side of the Susquehanna River in the Sunbury, PA 7.5 quadrangle. Possible organic maturation of units in the Onondaga and Marcellus Formations in the southern part of the field area may also help reveal the thermal conditions of this region during Alleghanian tectonics.