Geologia Croatica, Vol. 61 No. 2-3, 2008.
Original scientific paper
A Lithocodium and Bacinella signature of a late Hauterivian, local microbial event: the Urgonian limestone in South-East France
Marc A. Conrad
Bernard Clavel
Abstract
Sediment binding crusts of Lithocodium aggregatum and associated Bacinella irregularis abound in a shallow water limestone layer, in South–East France. The sudden appearance of these crusts denotes a local, conspicuous change in an otherwise balanced, open marine depositional environment. Based on regional correlation, the layer belongs to the Lower Hauterivian, Ligatus Zone, directly dated by an assemblage of orbitolinid foraminifera, and indirectly by a specimen of ammonite. It is older than the Faraoni oceanic anoxic event (OAE), (latest Hauterivian, Angulicostata auct. Zone), and presumably not linked with a global climate change. Two facies are recognized: (1) Lithocodium bindstone, with Bacinella, other microencrusters, and fluorescent automicrite of microbial origin; (2) fl oatstones containing numerous fragments of Lithocodium and accompanying biota. Coarse rhombohedral dolomite is locally present. Two modern analogues of microbial mediation are put forward to explain the Lithocodium event: (a) Lagoa Vermelha, in Brazil, along with anoxic, marginal marine conditions; (b) Highborne Cay, in the Bahamas, along with oxic, open marine conditions. A stack of four Upper Hauterivian parasequences is described: Parasequence 1 ends with oolitic deposits; the somewhat deeper water Parasequence 2 occurs only in the south-eastern part of the study area, towards the Subalpine depocenter; Parasequence 3 matches the layer with Lithocodium and Bacinella, of variable thickness (0.8 m – 11 m), resulting from syn-sedimentary tectonic activity in the area of the Vuache fault system which is a major feature; Parasequence 4 corresponds with the resumption of the normal, Urgonian carbonate factory.
Keywords
Lithocodium; Bacinella; Urgonian; automicrite; dolomite; Hauterivian; Jura; Pre-Subalpine Domain
Hrčak ID:
30653
URI
Publication date:
22.12.2008.
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