Print this pageSubmarine Cementation in Chalks of the Vaihall and Hod Fields, Norwegian North Sea: Origin of "Dense Zones" as Submarine Hardgrounds
MATTHEWS, ANNA L., BRUCE W. SELLWOOD, and CATHY FARMER
"Dense Zones" are a characteristic feature of the Hod/Tor boundary and Tor
Formation of the Valhall and Hod structures. Petrographic and geochemical
investigations of "Dense Zone" sediments shows that they were initiated through
submarine cementation, forming as deep-water hardground systems. Fracture- and
burrow-like "fissures" are lined with multi-phase cements and/or successive
chalk fills. Initial cements of the "fissure" systems comprise laminated calcite
microspar, inclusion-rich radial fibrous calcite and blocky calcite, and are
generally isopachous. Late-stage cements consist of coarse calcite druses.
Under cathodoluminescence planktic forams appear to be unaltered. Chalk matrix is dull orange, suggesting some alteration. Spar within "fissures" shows complex, bright zonation, suggesting formation under sub-oxic conditions. Coarse druses are dull, representing burial cements. Isopachous cements have the morphology of marine calcites. Oxygen and carbon isotopic evidence indicates that isopachous cements formed close to the Cretaceous sea-floor at water temperatures of ~10 degrees C, under conditions of sulphate reduction.
These "Dense Zone" hardgrounds reflect times in the Late Cretaceous and Early Tertiary when the area now represented by the Valhall and Hod fields, was above the general level of the Cretaceous sea-floor and by-passed by accumulating sediment. The recognition of these hardgrounds and their environment of formation has implications for the heterogeneity of Valhall and Hod chalk facies and the timing of growth of the Valhall and Hod structures.