Sequence Stratigraphy as a Tool for Interpretation of Cockfield and Yegua Formations in Southwestern Louisiana

Brian E. Lock, Sheila L. Voorhies

Sequence stratigraphy, which consists largely of old well-established concepts recently given new validity by the advances made in seismic stratigraphic interpretation, involves grouping strata into genetically and chronostratigraphically constrained units. Peter R. Vail and his co-workers have demonstrated that these units (depositional sequences and systems tracts) comprise predictable depositional and lithologic facies.

The Cockfield and Yegua Formations of southwestern Louisiana provide some very clear examples of features predicted by the Vail model. Thick turbidite sands from the downdip facies (the Arco Hoffpauir well, south central Calcasieu Parish, for example) can be assigned to the lowstand slope fan subdivision of Vail's lowstand depositional systems tract. Further updip, the Cockfield of northernmost Calcasieu Parish and of Beauregard Parish represents the transgressive systems tract. In southern Beauregard Parish, the sands are of distinctive open-shelf facies with geometries indicative of transport by retreating storm surge waters. In the central part of the parish, distributary mouth-bar sands occur, and in the north, log patterns indicate fluvial point-bar sands.

Above the Cockfield is a distinctive calcareous unit, the Moodys Branch Marl, which has already been identified (by Vail and Baum working with outcrops in Alabama) as a condensed section indicative of maximum sediment starvation corresponding to the approximate time of maximum transgression. Significantly, a feature that was originally interpreted as a simple unconformity, but that increases in magnitude toward the basin, has been described at this horizon by the present authors. This feature is here reinterpreted to be a consequence of basinward sediment starvation affecting the uppermost zones of the Cockfield beneath the Moodys Branch.

The overlying Jackson (Yazoo) shales, above the Moodys Branch, probably represent the downdip portions of the highstand systems tract, although no distinctive downlap patterns have yet been recognized.

Sequence stratigraphy is shown to be applicable to the Gulf Coast subsurface and to have important predictive capabilities that represent a significant advance in our understanding of sedimentary basins.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.