Western Tethys Early and Middle Jurassic calcareous nannofossil biostratigraphy

Abstract

An exhaustive biostratigraphic study based on the calcareous nannofossil content from Lower and Middle Jurassic Lusitanian Basin sections in Portugal is here presented. Located at the strategic intersection of important Jurassic water masses during a period of drastic environmental changes, the Lusitanian Basin conveyed mixed phytoplanktonic communities in its surface waters. Approximately 800 m of exceptionally continuous Lower and Middle Jurassic series bearing detailed ammonite biostratigraphy and two Global Stratotype Section and Point are studied. Vertical and lateral ubiquity of calcareous nannofossils in marine sediments and rapid evolutionary rate and radiation make this fossil group a remarkable biostratigraphic tool. Nearly 100 bioevents are acknowledged in ~24 Myr time span this work refers to, bridging 11 nannofossil zones and 29 subzones. A thorough revision of the existing literature referring to Early and Middle Jurassic calcareous nannofossil biostratigraphy is presented, and causes and implications for zonal marker discrepancies discussed under a palaeoceanographic and Jurassic nannofossil evolution angle. In order to create a solid biostratigraphic frame and to compare it with C and O stable isotopic trends, brachiopod calcite shells were analysed for geochemistry. This work yields for the first time a very detailed and inclusive biostratigraphic scheme for the Early and Middle Jurassic epochs.