Print this pageDiagenetic Reactions in Upper Jurassic Interbedded Sandstones and Mudstones over the Depth Range of 2-5 km from the Moray Firth and South Viking Graben, UK North Sea
EI-FEGHI, ZAID, STUART D. BURLEY, JOE H. S. MACQUAKER, and JAMES D. MARSHALL
Upper Jurassic mudstones and interbedded sandstones exposed on the East Sutherland coast of Scotland have time and depositional equivalents at depths of between 2-5 km in the Moray Firth Basin and South Viking Graben. With increasing burial the clay mineralogy of the mudstones changes from a kaolinite-dominated assemblage with subordinate I/S to an illite-dominated assemblage with only minor I/S and there is a dramatic decrease in the feldspar and calcite contents of these rocks. The sandstones contain abundant pre-burial calcite cement and a later post-quartz overgrowth calcite. Texturally, the calcites exhibit aggrading neomorphic fabrics and CL indicates that they have uniform, unzoned growth fabrics. delta{13}C of the early calcites ranges from +2 to -13o/ooPDB whilst delta{18}O values range from -2 to -15o/ooPDB. These data suggest that the early carbonate cements have a simple mixed marine and sulphate reduction source of carbon and that they formed during breaks in sedimentation close to the sediment-water interface. In the mudstones, as burial progressed, the conversion of I/S and kaolinite to illite and quartz provided a source of acidity for the dissolution of feldspar and carbonate grains supplying the solutes for late quartz and calcite cements in the sandstones. Moreover, the increased temperatures (>60 degrees C) associated with these reactions resulted in the recrystallisation of the early calcite cements. The diagenesis of the sandstones is thus dominated by the products of reactions taking place in the interbedded mudstones.