Brian Coffey¹, Fred Read¹
(1) Virginia Tech, Blacksburg, VA

Abstract: Sequence stratigraphy of a Paleogene, mixed carbonate-siliciclastic shelf, North Carolina, U. S. A

Thin section analysis of plastic impregnated well cuttings, along with wireline logs, biostratigraphic control, and seismic data from the subsurface Paleogene interval of the North Carolina coastal plain were used to generate a basin-scale, lithology-based sequence stratigraphy. Subsurface facies include: silty and shelly sands and shell beds (estuarine/lagoon/protected inner shelf facies); clean quartz sands and sandy mollusk-fragment grainstones (shoreface/shallow shelf); phosphatic hardgrounds (current and wave-swept shoreface and shallow shelf); bryozoan and echinoderm grainstones/packstones (storm reworked middle shelf); and fine skeletal wackestones and planktonic marls (slightly storm-winnowed to sub-wave base, deeper shelf).

Four supersequences, (Paleocene, Lower-Middle Eocene, Late Eocene-Lower Oligocene, and Late Oligocene-Early Miocene) bounded by Type I sequence boundaries occur. LSTs are evident on seismic lines just seaward of the terminal shelf break of each supersequence. Seismic data show horizontal, aggradational layering occurs over much of the landward portion of the shelf and low angle, clinoform reflectors near the depositional shelf break. Clinoform geometries define a broad spur along the Eocene shelf break, underneath the present-day Outer Banks. Maximum flooding surfaces are characterized by low-angle downlap onto supersequence boundaries/or tops of lowstand wedges downdip, whereas updip, they downlap onto the top of the transgressive system tract. Each supersequence contains several 3rd order sequences bounded by Type II sequence boundaries. Marine glauconitic and phosphatic condensed surfaces are common at sequence boundaries, but also occur in other systems tracts subjected to marine erosion/nondeposition.

The use of thin-sectioned well cuttings, coupled with seismic data has provided the first sub-seismic, detailed facies-based sequence stratigraphic framework for the thick subsurface section of the North Carolina coastal plain. The technique offers a low-cost alternative to coring programs, of potential use for detailed sequence analysis of other carbonate basins.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana