Correlation Of Milankovitch-Band Cyclicity In The Peritidal Carbonates Of The Tonoloway Formation, Central Pennsylvania And Western Maryland

Abstract

A hierarchic cyclic pattern consistent with the Milankovitch model of orbital forcing characterizes the Upper Silurian Tonoloway Formation. At three localities in Pennsylvania and Maryland, the Tonoloway Formation is divisible into three Third Order sequences (2 million years duration). The first of these Third Order sequences (44 to 53 meters thick) is an incomplete, shallowing-upward cycle divisible into four shallowing-­upward Fourth Order sequences (400 ky eccentricity), averaging 13.2 meters thick defined by the most open facies in the first Fifth Order cycle and the most restricted in the last. Each Fourth Order cycle is divisible into four shallowing-upward Fifth Order sequences (100 ky eccentricity), averaging 3.3 meters thick, defined by the most open facies in the second Sixth Order cycle and the most restricted in the last. Each Fifth Order sequence is divisible into three to five sharply bounded, shallowing-upward, Sixth Order cycles (20 ky precessional) containing a sharp, sea-level-fall surface separating its upper shallower facies from its lower deeper facies. These asymmetric cyclic patterns can be correlated between Mount Union, Pennsylvania and Pinto, Maryland, a distance of 150 kilometers. The similarity of symmetry and thickness of all cycles over this distance indicates that the principal cause of the stratigraphic fabric of the Tonoloway Formation was eustacy driven by multiple cycles of orbital perturbation. The greater overall thickness of the interval in Pennsylvania indicates greater rates of subsidence in this area.