Biostratigraphy, sedimentology and sequence stratigraphy of the latest Hauterivian - Early Barremian drowning episode of the Northern Tethyan margin (Altmann Member, Helvetic nappes, Switzerland)

Abstract.: During the Early Cretaceous, major palaeoceanographic changes are mirrored on the northern Tethyan carbonate platform by changes in the carbonate factory and by platform drowning. The Altmann Member of the central European, northern Alpine Helvetic thrust and fold belt, contains the sedimentary record of one of these drowning events which occurred during the Late Hauterivian - Early Barremian. It consists mainly of highly condensed beds, which are rich in glaucony and phosphates. The Altmann Member was hitherto only poorly dated. New ammonite findings and a re-evaluation of existing ammonite fauna allow to precisely date this drowning episode, starting in the Pseudothurmannia seitzi biozone (latest Hauterivian) and lasting until the Coronites darsi biozone (latest Early Barremian). These new age dates, coupled with sequence stratigraphic interpretations allow to better understand the unfolding of the drowning episode, which proceeded in two stages: The first stage consisted in an important phase of marine transgression during the latest Hauterivian, during which carbonate production was highly reduced; the second stage is recorded during the latest Early Barremian by an important sequence boundary, which is associated with a phosphatized hardground, followed by rapid sea-level rise and the deposition of outer ramp sediment associated with the backstepping of the platform. Almost the whole early Barremian is likely to be condensed in this phosphatized hardground, which is associated to a second order sea-level lowstand. The onset of the drowning event is linked to the Faraoni oceanic anoxic event, whereas during the Early Barremian, phosphatization might be the result of important winnowing during a period of highly eutrophic condition