Milankovitch cyclicity and its record in borehole logs of carbonates

Three of the earth's orbital variations - the approximately 21,000-year precession, the approximately 100,000-year short cycle in eccentricity, and the 413,000-year long eccentricity cycle - combine to form a coherent cyclic pattern of climatic variation in which periodicities occur in ratios of about 1:5:20. In continental settings such periodicities are recorded as cyclic alternations of carbonate-rich lacustrine and playa deposits (Triassic Newark Group, Eocene Green River Formation). In pelagic settings they drove variations in carbonate productivity (Albian Fucoid Marls, Italy) and possibly in detrital supply. These patterns may be obscured by other cyclic (obliquity cycle) or noncyclic variations. Cyclicity is best revealed by instrumental scans, such as high-resolution borehole logs of various sorts from Late Cretaceous chalk sequences of Europe and North America. Such logs may be the key to the development of a cyclostratigraphy with the potential for long-distance correlation and a refined geochronology.