Exploration significance of erosional unconformities downdip Yegua

Edwards and Tuttle J.L., 1989: Exploration significance of erosional unconformities in the Middle to Late Eocene Downdip Yegua Northern Gulf Coast Basin: The Downdip Yegua - State of the Trend, Houston Geol. Soc.

TEXT

Detailed log correlation, micropaleontological surveys, and regional seismic data have documented the presence of regionally extensive erosional unconformities in downdip areas of the Middle to Late Eocene Yegua - Cook Mountain Formations in the Texas and Louisiana Gulf Coast Basin. The erosion surfaces extend from Wharton County, middle Texas Gulf Coast, to Beauregard Parish in Louisiana, a distance of approximately 200 miles.

Exploded seismic window in Texas Yegua - 100k - click to enlarge

The stratigraphic phenomena that are associated with these unconformities can readily be confused with other causes, such as post-depositional faults, growth faults, facies changes, pinchouts of various types, and progradational wedges. But the proper recognition of the erosion surfaces greatly increases the power of sand prediction in the middle and downdip parts of the trend.

The erosion surfaces have highly variable geometries, ranging from strike-continuous surfaces that can be traced for many tens of miles along strike and which erode through as much as several thousand feet of older section, to narrow, arcuate surfaces that have a dip orientation and widths of 10 miles or less. In some cases, the surfaces truncate older erosional events within the Yegua - Cook Mountain section.

Detailed log correlation has provided a high resolution framework for demonstrating that the erosion surfaces formed at many different times during Yegua - Cook Mountain deposition. The same degree of resolution does not appear to be available with existing micropaleontological or seismic data.

An origin by catastrophic slump failure of the shelf edge seems necessitated by the strike-continuous nature of the most prominent erosion surfaces. It is possible that some of the linear dip-oriented examples were also scoured, at least in part, by bottom-hugging gravity flows. Timing of erosional episodes in relation to eustatic sea-level changes has not been demonstrable due to the higher frequency of erosional events compared to the frequencies of sea-level fluctuation events as identified in the literature (i.e. Hag et. al., 1988).

The exploration significance of the these erosion surfaces is as follows:

1. Older strata are eroded away in downdip areas, so that downdip projections of sand within depositional Systems subsequently removed by erosion are invalid.

Dip stratigraphic section in Yegua - Cook Mountain showing unconformity - 17k - click to enlarge

2. Scour of the erosion surfaces creates deep water (sub-wave base) conditions favorable for the action of gravity flow mechanisms. The resulting bathymetric basins generally form on the updip portion of the slope. The geometry of the bathymetric basins and the provision of sediment to the basin that was created by erosion will determine the potential for preservation of gravity flow deposits upon the erosion surface, or at other levels within the deep water basin fill. Some sandy gravity flow deposits may equivalent to the "basin floor fan" systems tract recognized in seismic facies analysis (Posamentier and Vail (1988). Such deposits may occur as a stratigraphic trap that pinches out against the unconformity and is sealed by shales both below and above.

3. The abrupt change from shallow to deep water conditions caused by the erosional events, results in a change in depositional geometries from horizontal aggradational strata updip (below the erosion surface) to clinoform progradational strata downdip (above the erosion surface). subsequent erosional events will cut into inclined strata, and therefore be more difficult to recognize. The resulting complex depositional-erosional slope deposits may harbor many types of sand bodies with a good potential for stratigraphic trapping but poor potential for sand body prediction.

4. The unconformity may act as a structural decollment surface for the overlying strata. Shallow structural events may hence not project into deeper strata.

The accompanying dip-oriented cross section front the Texas downdip Yegua trend in Liberty county illustrates stacked regressive sequences updip truncated by a basinward inclined erosion surface of approximately lower Yegua age. Above the erosion surface are blocky sands that formed as gravity flow deposits. Note the production from the updip, shallow well. The sand is overlain by a thick shaly section that probably shows a clinoform geometry on seismic data.

Block diagram showing geometry of unconformity - 61k - click to enlarge

The accompanying window from a regional seismic dip line from the middle Texas downdip Yegua has been segmented and separated to illustrate some of the geologic events that occurred in this area. Dashed lines show the presence of inferred major unconformities. The upper of these surfaces formed during Yegua - Cook Mountain sedimentation. The truncation of horizontal strata can be seen updip to the left, while the filling of the bathymetric basin created by erosion can be seen above the erosion surface to the right.

References

Edwards, M.B. and Tuttle, J.L., 1985, Sandstones of the Downdip Yegua, Middle Texas Gulf Coast Basin. Proprietary Study (unpublished).

Edwards, M.B. and Tuttle, J.L., 1987, Downdip Yegua Trend, Southeast Texas Southwest Louisiana. Proprietary Study (unpublished).

Haq, B., Hardenbol, J. and Vail, P.R., 1988, Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change. In: Sea-level Changes: an Integrated Approach, SEPM Spec Pub; No. 42, p. 77-108.

Posamentier, H.W. and Vail, P.R., 1988, Eustatic controls on clastic deposition II - sequence and systems tract models. In: Sea-level Changes: an Integrated Approach, SEPM Spec Pub; No. 42, p. 77-108.

Acknowledgments

I appreciate the support of the oil and gas companies that funded the proprietary studies from which the above information is extracted. J. Loyd Tuttle and Paleo Control Inc. carried the micropaleontological studies that resulted in the recognition and/or corroboration of some or the erosion surfaces. A large number of Regional seismic lines was made available by TGS Onshore Geophysical Company, which also provided the seismic data shown in the accompanying figure.

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