--> ABSTRACT: Timing of BSR and Carbonate Replacement after Evaporites in Organic Rich Deposits, Lorca Basin, Messinian, SE Spain, by C. Taberner, J. M. Rouchy, M. Russell, and J. Grimalt; #91021 (2010)

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Timing of BSR and Carbonate Replacement after Evaporites in Organic Rich Deposits, Lorca Basin, Messinian, SE Spain

TABERNER, C., J. M. ROUCHY, M. RUSSELL, and J. GRIMALT

Diagenetic carbonates formed after partial replacement of gypsum, at least in five sequences preceding the main Messinian evaporite unit in Lorca Basin. These carbonates are found interbedded in diatomites and organic rich shales (TOC up to 23 %). The results from the integrated inorganic/organic study of one of these sequences supports bacterial sulphate reduction (BSR and degradation of organic matter (OM) as the local source of CO[2] consume in the precipitation of calcite replacing former gypsum crystals. Delta{34}S and delta{18}O of native sulphur and barite found infilling cavities in the carbonates prove these to be by-products of this process. Sulphurization of OM in the shales, the good preservation of fatty acids, as well as the existence of pristine aragonite levels related with this sequence, all suggest that BSR occurred during the initial stages of diagenesis and thus discounts any thermal effect in these samples These deposits have not been affected by later diagenetic processes and therefore record either the original environmental features or those originating at the very early diagenetic stages during the Messinian. The markers for BSR present in the studied rocks highlight the efficiency of this process where significant amounts of organic matter fuelled the process in a hypersaline environment. Due to the good preservation of the studied sediments and organic markers, these sequences in the Lorca basin constitute a case model for the study of BSR during early diagenesis, and in particular for the main controls on sulphurization of organic matter.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.