Structural Analysis Of The Wissahickon Formation Along The Northern Section Of The Wissahickon Creek Valley, Philadelphia, PA

Abstract

The structural history of the Wissahickon Formation has been for considerable time questionable and under continuous debate. This current investigation involving the mesoscopic analysis of pelitic schists and psammitic gneisses supported by microscopic study of rock fabrics and thin section analysis discloses a complex structural history consisting of 3 episodes of fold generation. The Primary Structures consist of F1 isoclinal similar folds, L1 lineations (mineral alignments) and S1 axial plane schistosity seen predominantly near the Rosemont Fault Zone. The Second Structures and most predominant are characterized by subisoclinal F2 folds predominantly Class 1C in type, L2 lineations (mineral alignments, S1/S2 cleavage intersections and minor F2 fold axes) and axial plane "Cleavage Rucking" (S1-S2 cleavage intersection) seen throughout most of the area grading into a transposition S2 schistosity near Rosemont Fault Zone. These F2 structures are quite large, structurally influencing the flow of the river in various areas. The last episode is very minor, consisting of F3 open parallel, accordian, and chevron folds seen primarily on the limbs of F2 structures, L3 lineations (axes of F3 folds and crenulations) and S3 axial plane crenulation cleavage. This polyphase deformational history of the Wissahickon Formation in this region appears not unlick and in face quite similar to that seen in most thrust belts throughout the world.