Samuel N. Bishop¹

(1) University of Cambridge, Cambridge, England

ABSTRACT: Diagenesis beneath sequence boundaries: the Corallian Group (Upper Jurassic), southern Dorset, UK

The Corallian Group, a mixed succession of shallow marine carbonate and siliciclastic sedimentary rocks, contains candidate sequence boundaries. It has been suggested that the early diagenesis was related to cycles of relative sea level change, with relative sea level fall leading to the influx of meteoric water. Stable isotope data from early calcite cements, however, suggest that the early diagenesis was dominated by bacterially mediated reactions taking place in modified marine pore fluids. Corallian limestones contain early calcite cements which post-date authigenic pyrite. These have negative d13C values and 'marine' d18O values (e.g. d13C=-5.3o/oo, d18O=-0.5o/oo). The carbon was probably sourced from organic matter (via bacterial sulphate reduction, and perhaps also methanogenesis), seawater and marine shell material. Calcite cement generations can be correlated across candidate sequence boundaries. Rocks above and below the boundaries experienced the same succession of diagenetic changes, although not necessarily simultaneously. Rare pendant and meniscate cements (occurring mostly in oolitic intraclasts) probably reflect local exposure of ooid shoal tops. Early cements in Corallian sandstones also precipitated from modified seawater. Pyritiferous calcite concretions occur in the Nothe Grit Formation, a sandstone-dominated unit occurring at the base of the Corallian Group. d13C values (-28.6 to -38.6o/oo) suggest that some of the carbon in the calcite was supplied by the oxidation of biogenic methane, probably at the base of the sulphate reduction zone. Although the stratigraphic succession may have been controlled by changes in relative sea level, there is no evidence for subaerial exposure at sequence boundaries.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado