ABSTRACT: The Nature and Significance of Condensed Sections in Gulf Coast Late Neogene Sequence Stratigraphy
Bernard L. Shaffer
Sequence stratigraphic analysis is being applied in the Gulf Coast to facilitate genetic interpretations of stratigraphy and to improve predictions of reservoir facies. Among the components of sequences, the condensed section is of paramount importance in providing a chronostratigraphic framework and in the delineation of systems tracts.
Condensed sections are characterized by abundant and species diverse planktonic foraminiferal and calcareous nannoplankton assemblages, along with fine-grained pelagic to hemipelagic lithologies often rich in authigenic minerals such as glauconite and siderite. In deep-water environments, condensed sections correspond to sea-level highstands and are temporally equivalent to both the maximum flooding surface in the transgressive systems tracts and the highstand systems tracts on the shelf. This paper discusses various methods and techniques for recognizing and graphically displaying condensed sections, and defines and documents 16 condensed sections in the Gulf Coast pre-Wisconsin Plio-Pleistocene that have currently been identified. The Miocene-Pliocene transition is also re-evaluated in terms of planktonic biostratigraphic and global eustatic events.
All of these condensed sections can be dated by either nannofossil or planktonic foram apparent extinction datums contained within them; they have been related to the range of tops of classic benthic foram markers, and all are associated with a relative paleobathymetric deeping as indicated by benthic foram facies. Each are also contained within seismically resolvable and regionally mappable stratigraphic sequences. On seismic sections, they are characterized by continuous high-amplitude reflectors. On wireline logs, they are often recognized as low resistivity, high gamma ray shales. As indicated by the chronostratigraphic control provided by planktonic datums, they are not random events and can be shown to be regionally correlative.
A few minor abundance/diversity peaks may reflect local changes in sedimentation rates or may be associated with parasequence sets. Middle to late Pliocene condensed sections are more completely developed in the South Additions of the Louisiana offshore, especially the expanded sections in the East and West Cameron areas. In contrast, terrigenous clastic sediment starvation has resulted in several stacked condensed sections in the Green Canyon area.
Condensed sections vary in thickness from a few feet to several hundred feet, and two variations in their configuration occur. In deep-water settings, they may be relatively thin, single spikes with sharply defined boundaries, and represent both the transgressive and highstand systems track. Complex condensed sections are generally thicker, have serrated margins and less sharply defined boundaries, and reflect higher and more variable sedimentation rates in upper slope and shelf environs. They may develop between slope fans and lowstand prograding wedges or find expression as the maximum flooding surface in the transgressive systems tract.
Since most biostratigraphic datums are restricted to them, condensed sections provide the most reliable chronostratigraphic tie from slope to shelf paleoenvironments and play a preeminent role in sequence stratigraphic and systems tracts analysis. They are considered a fundamental stratigraphic unit, whether utilized in this context or for conventional stratigraphic correlations.
AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990