21-EPN-FT1-005: Reading the sedimentary archive of discontinuity surfaces
Visit by Simon Andrieu, Aarhus University (Denmark) to TA2.9 Ion Probe Facility (IPF), CRPG (France).
Dates of visit: 18-22 October 2021
Report Summary: Discontinuity surfaces, associated with seafloor cementation, are hence of primordial importance for fully apprehending the geological record, yet they have received far less attention than the sedimentary rocks surrounding them. Fundamental problems that are still not sufficiently understood concern the lateral change of discontinuities. In this project, we tackle this issue by studying the lateral variation of five distinct discontinuity horizons present in the Middle Jurassic of the High Atlas (Morocco), where outstanding exposures permit to track these surfaces over tens of kilometres. Hence, the purpose of this work is to characterise at a high-resolution the large-scale variation of petrographic and geochemical (C, O and Sr-isotopes) properties of discontinuities (matrix and cement phases) along dip and strike of a Jurassic moderately deepening ramp. δ18O (176 values) and δ13C (105 values) signatures were obtained on twenty-one cements and grain types, including 14 different early calcite cements and fabrics corresponding to dogtooth cements (7), turbid synaxial cements (1) and micritic/microsparitic fabrics (6).
Data confirm that dogtooth cements can precipitate in marine phreatic, meteoric phreatic and shallow burial environments. The highly negative δ18O values of micritic fabrics and turbid synaxial cements, which form in seawater, indicate that they transformed during subsequent diagenesis (i.e during meteoric water circulation or shallow burial). It indicates that they precipitate initially with an unstable mineralogy (aragonite or high-magnesium calcite). δ18O and δ13C data on early cements suggest that a same discontinuity can change laterally from a subaerial exposure surface to a marine surface.
Read the full scientific report, with kind permision from Simon Andrieu.