ABSTRACT: Foundered Shelf Edges - Examples from the Yegua and Frio

EWING, THOMAS E. and FRANK VINCENT

Foundered shelf edge (FSE) sequences are a distinctive reservoir-bearing component of Paleogene stratigraphic units in the Gulf Coast Basin. FSEs are defined by a sudden movement of the shelf edge to a more landward point, due to a combination of large-scale slumping, sliding and erosion--the foundering phase. Following is a filling phase, including deep-water slope fans and shallow-water prograding complexes, and finally a healing phase of transgressive and highstand deposits. In the present examples, foundering is associated with valley incision on the adjacent shelf, and filling-phase deposits correlate with the valley-fill sandstones, implying that foundering occurred during a fall or lowstand in relative sea level.

In central Wharton County, Texas, the Yegua 50 displays a well-defined FSE. To the north, complex incised valley deposits occur in McFarlan field. Immediately downdip, sandy grainflow deposits overlie scoured Yegua 60 and 70 sediments. Above is a thick claystone, followed by a thin prograding sandstone and the open-marine Yegua 50 marker. Downdip, the claystone thickens and condensed sections develop; the prograding package becomes indistinct. The basal sandstones become organized into channeled slope-fan deposits (Phase Four and Shanghai fields). The thickness of the Y-50 fill is 0-150 ft on the shelf, but is up to 1800 ft downdip.

In St. Landry Parish, Louisiana, FSEs are well developed both in the Lower Cockfield and in the "Basal Frio" (Frio/Vicksburg; Textularia seligi - Anomalina bilateralis). Detailed cross-sections show the incision of the FSE into older shelf strata, and the correlation of the FSE with updip valley incision. There is a continuous sandstone sequence from updip blocky incised-valley sandstones into a downdip deep-water sandstone overlain by a thin shallow-water sandstone.

Hydrocarbons have been found in pre-foundering sandstones, in the grainflow deposits, in downdip slope fans, and in updip incised valleys. Prediction of reservoir rock is the primary challenge to continued hydrocarbon exploration in all parts of the FSE system.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana