DIAGENESIS OF BARREMIAN-APTIAN PLATFORM CARBONATES (THE URGONIAN LIMESTONE FORMATION OF SE FRANCE) - NEAR-SURFACE AND SHALLOW-BURIAL DIAGENESIS

The diagenesis of carbonate platform sediments is controlled by the or iginal facies and mineralogy, climate, sea-level changes and burial hi story; these controls are clearly seen in the diagenesis of the Urgoni an platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced d uring this phase were controlled by several falls in relative sea-leve l during the Barremian to mid-Aptian punctuating platform sedimentatio n, the original mineralogy of the sediment and the prevailing semi-ari d/arid climate in the region at this time. Following a relative sea-le vel rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non-luminescent, calcite cement. This cement was cut by late r stylolites, suggesting a relatively shallow-burial origin. Stable is otope (mean values - 7.94 parts per thousand delta(18)O and 0.36 parts per thousand delta(13)C) and trace element (mean values of Fe 334 ppm , Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitat ed from meteoric fluids at temperatures slightly elevated relative to depositional temperatures. A variable thickness of replacive dolomite which occurs preferentially within the shelf-margin facies of the lowe r part of the Urgonian post-dates mechanical fracturing and chemical c ompaction, but pre-dates the main phase of stylolitization. It is prob able that the dolomitizing fluid was sourced by the early compaction-d riven release of connate fluids held within the underlying muddy units . The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1-1 .5 km). The overall diagenetic history of the Urgonian Limestone Forma tion is a reflection of the pre-conditioning of the platform limestone s by climate, sea level, tectonics and the shallow burial depths exper ienced by the platform during the later Mesozoic.