Microstratigraphy of the Lower Mississippian Sunbury Shale: A record of solar-modulated climatic cyclicity

Abstract

Microstratigraphic analysis of the Sunbury Shale has yielded a high-resolution record of probable short-term climatic changes in the Early Mississippian central Appalachian basin. The formation is a laminated black shale that contains pervasive millimetre-thick couplets composed of alternating thin black and thick dark gray laminae, and decimetre-thick bands that are alternately dark and light. Total organic carbon content varies at length scales corresponding to both orders of cyclicity, and correlation of total organic carbon values to X-radiograph gray-scale densities permitted rapid stratigraphic analysis of millimetre-scale lithologic variation in the 5.0-m-thick formation. Spectral analysis of gray-scale density time series revealed strong power concentrations at intervals of 23 ±2 and 70 ±5 couplets. These results are interpreted to represent varved deposition of the Sunbury Shale modulated by the ∼22 yr Hale and ∼70-90 yr Gleissberg solar activity cycles. These cycles were probably recorded because of deposition in a stratified anoxic environment that was sensitive to short-term climatic fluctuations and subject to high sedimentation rates (4.5 mm/yr).