A Jurassic counterpart for modern kopara of the Pacific atolls: lagoonal, organic matter-rich, laminated carbonate of Orbagnoux (Jura Mountains, France)

Abstract : Two types of laminated sediments are compared in this paper: the Kimmeridgian bituminous laminites of Orbagnoux (French Southern Jura Mountains) and the present-day ‘kopara' of the French Polynesia atolls or Kiritimati (Christmas) Island from the Pacific Ocean. The kopara is made of laminated sediments, several tens of centimetres thick, that cover the floors of most shallow (<2 m deep) lakes and ponds on the rims of atolls in the Tuamotu Archipelago and Society Islands. The millimetre-scale laminations come from the alternation of organic-rich and CaCO3-rich laminae. The top few centimetres host a succession of bacterial populations, from aerobic to strictly anoxic. Most carbonate grains are precipitated in situ, due to bacterial activity (s.l.). The Kimmeridgian bituminous laminae of Orbagnoux are partly made of flat stromatolites that show many similarities with the kopara. This analogy allows us to refine the interpretation of the Southern Jura platform where laminated, organic matter-rich, carbonates were deposited in many places.