Limestone-Shale Bedding and Perturbations of the Earth’s Orbit

Abstract

Stratigraphic sequences in Carboniferous and Cretaceous limestones were measured with particular attention to strength and spacing of bedding planes. Spot sampling at regular intervals transforms these data into numerically manageable form, replotted in smoothed curves of sequential bed thickness. Beds are bundled into sets about 1 m thick. Power spectra of the Cretaceous data yield three frequency peaks, in the vicinity of 50 cm, 1 m, and in the less-well defined low-frequency end. Depositional rates indicate that the bundles represent about 100,000 years, implying correlation with the Earth’s 100,000 year short cycle in eccentricity. That would match the 50 cm peak with the orbital cycle in obliquity and the low-frequency peak possibly with the long cycle in eccentricity. Sub-structure of the bundles, revealed by relative position of bedding planes, yields curves showing three to four groupings of bedding planes per bundle. These are remarkably similar to insolation minima per eccentricity cycle, as calculated from BERGER’s algorithm, and suggest that the main beds reflect the precessional cycle. It appears that limestone deposition, as well as Pleistocene ice volume, is a sensitive index to climate. It reflects the Earth’s orbital perturbations at least through the latter half of Phanerozoic time.