Jurassic lagoonal environments and quasi-abiotic platy limestone accumulation: microbial interventions

Within the regressive sequence (eustatic sea-level fall) that characterizes the French Southern Jura carbonate platform during the Kimmeridgian, a suc cession of three facies (called here units 1-3) may be observed in a palaeo lagoon near Orbagnoux. Unit 1 is a shallow-water, intensely burrowed, carbo nate deposit; unit 2 (consisting of laminated, organic matter-rich carbonat e) represents a shallow lagoonal environment, prone to stromatolite develop ment and organic matter accumulation; unit 3 consists of platy limestones a lmost devoid of identifiable biota. Trace element distributions indicate th at depositional and early diagenetic conditions must have been reducing dur ing deposition of unit 1 and, especially, unit 2, whereas they were much le ss reducing during deposition of unit 3. However, the latter does not conta in body or trace fossils, with the exception of structures interpreted as c alcified bacteria. The reasons why the apparently normal marine environment of unit 3 did not allow a normal biota to develop are explored and lead us to propose that: (i) unit 2 may be a Jurassic counterpart of modern kopara deposits (stromatolites of French Polynesian atolls); and (ii) unit 3 may be the result of palaeokopara reworking. This model invokes the extensive i ntervention of microbes in the formation of these carbonate deposits.