Ecology and Depositional Environments of Chalk-Marl and Limestone-Shale Rhythms in the Cretaceous of North America (Abstract)

Abstract

Chalk-marl and limestone-shale couplets less than a meter thick widely characterize pelagic Cretaceous sediments of the Western Interior and Gulf Coastal Plain, United States and northern Mexico. These rhythmic deposits are best developed during transgressive (eustatic) peaks between Late Aptian and earliest Campanian time. Individual couplets can be traced for hundreds of kilometers, and from offshore epicontinental and marginal continental platforms into basinal facies, where they broadly grade into clay shales. Shallow and deep water couplets are similar in many ways. Their cyclicity ranges between 30 and 50,000 years. measured radiometrically, suggesting climatic cycles. Most have graded lower contacts between shale and carbonate, but sharply defined boundaries on top of carbonate units; in some cases these are eroded and/or partially cemented surfaces (firmgrounds). In each couplet, clay content diminishes upward as mainly biogenic pelagic carbonate increases. Many lines of evidence suggest diminishing sedimentation rate, and decreasing water content of the sediment, from the bottom to the top of each cycle. Marine varves consisting mainly of alternating clay enriched and coccolith — enriched bands a mm or less thick are variably preserved in the rhythms, and best studied in offshore deep basinal facies.