CONDENSED SECTIONS: THE KEY TO AGE DETERMINATION AND CORRELATION OF CONTINENTAL MARGIN SEQUENCES

Abstract

Abstract: Condensed sections play a fundamental role in stratigraphic correlation, both regionally and globally. Condensed sections are thin marine stratigraphic units consisting of pelagic to hemipelagic sediments characterized by very low-sedimentation rates. Areally, they are most extensive at the time of maximum regional transgression of the shoreline. Condensed sections are associated commonly with apparent marine hiatuses and often occur as thin, but continuous, zones of burrowed, slightly lithified beds (omission surfaces) or as marine hardgrounds. In addition, condensed sections may be characterized by abundant and diverse planktonic and benthic mi-crofossil assemblages, authigenic minerals (such as glauconite, phosphorite, and siderite), organic matter, and bentonites and may possess greater concentrations of platinum elements such as iridium. Condensed sections are important because they tie the temporal stratigraphic framework provided by open-ocean microfossil zonations to the physical stratigraphy provided by depositional sequences in shallower, more landward sections. Condensed sections represent a physical stratigraphic link between shallow- and deep-water sections and are recognized by the analysis of seismic, well-log, and outcrop data. Within each depositional sequence, condensed sections are best recognized and utilized within an area from the shelf/slope break landward to the distal edge of inner neritic-sand deposition. Where sedimentation rates are generally low, as in the deep ocean, a number of condensed sections may coalesce to form a composite condensed section. Data from detailed analyses of continental-margin condensed sections are presented to illustrate the nature and importance of condensed sections for dating and correlating continental-margin sequences and reconstructing ancient depositional environments.