Correlation of a Hierarchy of Orbitally Forced Allocycles in the Lower Silurian McKenzie Limestone Member of the Mifflintown Formation, Pennsylvania and Maryland

Abstract

The lower part of the McKenzie Member of the Mifflintown Formation (Latest Wenlockian) in Pennsylvania and Maryland occurs as a hierarchically stacked set of allocycles whose causal mechanism is orbital forcing. At five localities in Maryland and Pennsylvania the lower McKenzie Limestone is divisible into two fourth-order (400 ky duration) sequences, which in tum may be divided into four fifth-order (100 ky duration) cycles. The predominant cyclic pattern, averaging 8.2 feet in thickness, is a highly asymmetric (shallowing-upward), 100 ky, fifth-order cycle. The lower parts of 100 ky cycles consist of thin (2 - 6 inches) beds of micrite, fine to coarse calcarenite and intraclastic conglomerate separated by variably thick (0.5 < 2.5 feet) intervals of gray to black shale. The upper portions of fifth-order cycles are dominated by shale, containing fewer and thinner limestone beds. Within these sequences, smaller-scale, sixth-order cycles are typically initiated by coarse calcarenite or micrite beds. Fifth-order cycles can be grouped into two fourth-order sequences averaging 33 feet thick. These larger fourth-order sequences are also asymmetric, consisting of limestone-dominated fifth-order cycles in their lower portions and shale-dominated fifth-order cycles in their upper portions. Correlation of these cyclic patterns over a distance of 160 miles indicates a eustatic causal mechanism. Specifically the predominant 6.5 - 10 feet thick fifth-order cycles are interpreted as a response to the short-term (100 ky) eccentricity mechanism and the larger composite sequences are thought to represent the long term (400 ky) eccentricity cycle. The fundamental building block of these sequences is the sixth-order cycle produced by the 20 ky precessional cycle. The hierarchic structure of these correlative patterns suggests a hierarchic mechanism, specifically Milankovitch orbital forcing.