CEMENT GEOCHEMISTRY OF PHOTOZOAN CARBONATE STRATA (UPPER CARBONIFEROUS - LOWER PERMIAN), FINNMARK CARBONATE PLATFORM, BARENTS SEA

Abstract

Stable-isotope, petrographic, microprobe, and fluid-inclusion analyses were carried out on calcite and dolomite cements from two cored wells covering the lower, high-porosity stratigraphic interval (Upper Carboniferous-Lower Permian) of the Finnmark carbonate platform. Cathodoluminescence petrography and microprobe profiling of cement zones reveal evidence for two main transitions in the cement growth history: (1) a possibly gradual and continuous transition from eogenetic (finely crystalline; < 0.1 mm) calcite and dolomite cements to mesogenetic (more coarsely crystalline) calcite and dolomite and (2) a discontinuous transition to a latest stage of saddle dolomite and anhydrite cementation. Fluid-inclusion data show that high-salinity brines replaced earlier marine to meteoric pore waters throughout the entire section prior to precipitation of the mesogenetic cements, possibly resulting from a series of brine-reflux episodes during successive stages of platform deposition. Carbonate delta(13)C variations in both whole rocks and cements are related to proximity of shaly beds, from which isotopically negative organic carbon was released, probably during both eogenetic and mesogenetic diagenesis, to mix with isotopically positive marine-carbonate carbon. Carbonate cements show significant enrichment in Fe and Mn near shale, suggesting release during burial diagenesis of clay minerals. On the basis of the shallow-water setting of these strata during a time of major, high-frequency sea-level fluctuations, the absence of negative delta(13)C excursions at cycle tops is suggested to reflect limited development of vegetation, possibly because of and climate. The above results support the scenario of a photozoan platform succession undergoing extensive eogenetic stabilization with variable marine, evaporitic, and meteoric waters, followed by gradual mesogenetic cementation during progressive burial in a refluxed, hypersaline brine. This model contrasts with scenarios of extensive meteoric-phreatic calcite cementation indicated by cement-geochemistry studies of many other carbonate platforms, but it may be widely applicable to high-latitude Carboniferous-Permian sequences of similar depositional affinity.