Dr. Andy Gale
andy.gale@port.ac.uk
University of Portsmouth
Saturday, 30th of November 2019 (16:00-16:45) at the international symposium on Chalk & Flint
The epicontinental Chalk Sea occupied a vast region, extending from western Europe to central Asia, and lasted for approximately 40 million years, from the Cenomanian (Cretaceous, 100 Ma) to the end of the Danian (Paleocene, 60Ma). Its characteristic deposit, nannofossil chalk, is a typical sediment of deep-water ocean basins, not relatively shallow settings. Why did such a deposit develop and dominate successions for so long? The high sea-level stand and Greenhouse of the Late Cretaceous are partly, but not entirely responsible, as are oceanic conditions and locally arid climates. Chalk sedimentation, and subsequent diagenesis, were partly controlled by orbital cyclicity which created the regular rhythmicity so commonly seen in chalks, and for this reason individual beds can extend over great distances. Bottom currents also played an important role, creating channels and erosion surfaces, sometimes witnessed by hardground formation. Diagenesis beneath the Chalk Sea floor was dominated by the dissolution and reprecipitation of silica and calcium carbonate phases (silica and aragonite), which picked out and augmented the primary climatic cycles. However, the absence of pervasive cementation in chalks is both unusual and difficult to explain – most limestones are hardened. The demise of the Chalk Sea, and its replacement by clastic sediments in the Selandian, can be related to major palaeoclimatic reorganisation