Facies and climate/environmental changes recorded on a carbonate ramp: A sedimentological and geochemical approach on Middle Jurassic carbonates (Paris Basin, France)

A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Objectives are to document and explain the facies changes in the context of the climate/environmental evolution at the NW European scale. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. At a greater time scale, 4 biosedimentary packages are distinguished by their allochems associations. (1) An intracratonic carbonate environment with coral reefs and crinoid-rich facies is typical of the Early Bajocian. (2) A major facies change occurred at the Early/Late Bajocian transition with a shift from crinoid- and coral-rich facies to ooid-rich facies. During the Late Bajocian, a southward-dipping ooid ramp with successive progradational trends was emplaced. (3) A large lagoon protected by ooid shoals developed during the Bathonian in a muddy ramp setting. (4) At the Bathonian/Callovian boundary, a second major change occurred with the lagoonal facies being superseded by an ooid-bioclastic (crinoids, corals) ramp associated with a waning of carbonate productivity (retrogradational trend). Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ13C and mineralogical data, our new δ18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment.