Abstract

Lower Cretaceous sandstone turbidites in the Scapa Field (North Sea) contain a complex variety of authigenic quartz microfabrics. Early diagenetic microfabrics predate burial calcite cements and compaction. They comprise abundant grain-rimming microquartz, intergranular aggregates of microquartz with authigenic illite-smectite, and localized isopachous cryptocrystalline quartz and chalcedonite fringes. These precipitates contain evidence of precursor opal-CT, in the form of dissolved, recrystallized, or pseudomorphed lepispheres and rim cements. Late diagenetic quartz microfabrics are postcompactional and either replace or overgrow burial calcite cements. They display a paragenetic trend from microporous, fibrous silica (quartzine or chalcedonite) to equant, syntaxial, and mosaic mesoquartz cements. None of these contain evidence for opal-CT precursors, and they are likely to have precipitated directly from silica-rich pore fluids. Cathodoluminescence examination of calcite-cemented sandstone intervals reveals that siliceous sponge spicules were a major depositional component, and were the most likely silica source for opal-CT and subsequent quartz cementation. Dissolution of biogenic silica and formation of opal-CT began shortly after deposition, with replacement of opal-CT by quartz during burial of the Scapa Member to 1-2 km. The early diagenetic silica transformations are identical to those recorded from Cenozoic porcelanites and cherts. Thermodynamic models developed for the diagenesis of biogenic cherts can therefore be applied to the Scapa sandstones, and qualitatively explain the development of contrasting quartz microfabrics on a bed-to-bed (or smaller) scale. In contrast, direct precipitation of chalcedonic quartz during late diagenesis required a resurgence of high supersaturations. This is difficult to account for without some import of silica to the sandstones and/or cooling of pore fluids. Precipitation most probably occurred from allochthonous, deep basinal brines associated with early stages of hydrocarbon migration.

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