Disseminated 'jigsaw piece' dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

摘要: Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments the North Sea have provided basis for a proposed new interpretation early diagenetic authigenesis highly bioturbated marine sandstones. The is present throughout Franklin Sandstone Formation Elgin Fields as discrete, non-ferroan, generally unzoned, subhedral to anhedral 'jigsaw piece' crystals. These are similar size detrital silicate grains typically account =5% rock volume. crystals never seen form polycrystalline aggregates or concretions, ever envelop adjacent grains. They uniformly dispersed sandstones, irrespective grain clay content. Dolomite predated all other significant events visible thin section. overgrown by late ankerite, bulk samples display stable isotope compositions that lie on mixing trend between these components. Extrapolation this suggests has near-marine delta18O values low, positive delta13C values. unusual can be reconciled if it formed near-surface zone active bioturbation. water plentiful reservoir Mg pore fluid regionally consistent delta18O. Labile bioclastic debris (e.g. aragonite, Mg-calcite) supplied isotopically carbon fluids during shallow-burial dissolution. Such dissolution took place response ambient 'calcite sea' conditions, but may been catalysed organic matter oxidation reactions. Bioturbation not only ensured dissolving carbonate was also prohibited coalescence consequent cementation framework. Continued exchange Mg2+ Ca2+ with sea-water maintained sufficient Mg/Ca ratio (rather than calcite) form. Irregular crystal shapes resulted from dissolution, both enclosed fine calcitic shell debris, before ankerite precipitation deep-burial diagenesis.