KRONEN, JOHN D., Jr., Department of Geology and Geophysics, University of Hawaii, Honolulu, HI

ABSTRACT: Recognition of Condensed Sections and Flooding Events as Deduced From High-Resolution Sequence Stratigraphy of the Great Barrier Reef, Australia: 0-1.5 Ma

Sequence stratigraphic interpretations of seismic reflection and core data collected from the outer shelf and upper slope of the ODP Leg 133 Great Barrier Reef transect reveal nine mixed carbonate-siliciclastic depositional sequences each bound by sequence boundaries, and each containing lithologically distinct and relatively thin condensed sections. Because of high sedimentation rates (140 to >1000 m/Ma) in this region, resolution of third- (0.4 to 1.2 my) through fifth-order (0.01 to 0.1 my) changes of sea level is possible. A condensed section is identified within each sequence, and in most cases is associated with the high-stand systems tract (HST) downlapped maximum flooding surface as seen on seismic reflection profiles. They are all are marked by the presence of in-situ (i.e nonreworked) "pristine" glauconites. These sections are also unique because they are marked by increases in abundance of quartz, clay, and shallow-water bioclasts, increases in magnetic susceptibility, and decreases in total carbonate abundance. Increases in glauconite are interpreted to reflect relative decreases in sedimentation rates whereby sediment residence time is prolonged within pore-water suboxic bacterial degradation zones. Increases in quartz and clay contents during maximum flooding are interpreted to be associated with sediment starvation whereby these background contents become concentrated. Increases in bioclasts are attributed to relatively rapid build-ups of shallow-water reefal communities that accompanied recurrent flooding events; during these times shallow-water ca bonate growth accelerated and built upward to keep up with rising sea level. Decreased accommodation potential during relative highstand phases induced HST progradation and sedimentation of reworked glauconites, carbonates, and terrigenous sands, silts, and clays.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.