Correlation of a Hierarchy of Cycles forced by Eccentricity and Precession: Middle Wills Creek Formation, Central Appalachian Basin

Abstract

Applying a hierarchic 'Milankovitch' orbital forcing model, a fourth-order (400 ka, long eccentricity) sequence and its internal cyclic structure, in the middle part of the Wills Creek Formation, can be correlated between Pinto, Maryland and Mt. Union, Pennsylvania, a distance of 150 km. This fourth-order sequence comprises four fifth­-order (100 ka, short eccentricity) cycles arranged in an asymmetric shallowing-upward pattern, in which the second fifth-order cycle contains a markedly deeper facies or is more carbonate-rich. Fifth-order cycles in the Wills Creek Formation consist of three to five sixth-order (20 ka, precessional) cycles generally arranged in an asymmetric pattern in which the second sixth-order cycle contains the deepest facies. Within sixth-order cycles, the basal deposits (occurring just above surfaces produced by precessionally forced sea-level rises) are carbonate-rich while the upper, shallower parts of cycles are characterized by an abundance of green (or sometimes red) shale. The deepest (most open marine) carbonate facies are oosparites and biosparites. The shallowest facies may be either mudcracked or simply massive (bioturbated?) shale. Within the studied fourth-order sequence at both localities, symmetry, thickness and fifth-order cyclic structure are similar. The most significant difference between the two localities is that the sequence at Mt. Union contains more onshore and/or nonmarine facies than the correlative sequence at Pinto. The consistency in the pattern of hierarchic cyclic structure between such distant and facies-distinctive localities reinforces an interpretation of Milankovitch orbital forcing as an explanation for Wills Creek cyclic stratigraphy.