Lower Eocene carbonate-cemented 'chimney' structures (Varna,Bulgaria)- Control of seepage rates on their formation and stable isotopic signature

摘要

The genesis of Lower Eocene calcite-cemented columns,"pisoid"-covered structures and horizontal interbeds,clustered in dispersed outcrops in the Pobiti Kamani area(Varna,Bulgaria)is related to fossil processes of hydrocarbon migration.Field observations,petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins,revealed that varying seepage rates of a single,warm hydrocarbon-bearing fluid,probably ascending along active faults,controlled the type of structure formed and its geochemical signature.Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions(delta~(18)O=-1+-0.5per thousand V-PDB),explaining columns' depleted delta~(13)C ratios of-43per thousand.Increasing seepage rates caused methane to emanate into the water column(delta~(13)C=-8per thousand)and raised precipitation temperatures(delta~(18)O=-8per thousand).Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation.Even higher-energy fluid flow and temperatures likely controlled the formation of "pisoids",whereby sediment was whirled up and cemented.