Biostratigraphic Techniques for Locating the Position of Ancient Shelf Margins: Examples from the Neogene of the Northern Gulf of Mexico
G.D. Jones, A.C. Gary, B. Kohl, R.H. Fillon, H.H. Roberts, B.J. O’Neill, R.J. Witmer, 2013. "Biostratigraphic Techniques for Locating the Position of Ancient Shelf Margins: Examples from the Neogene of the Northern Gulf of Mexico", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Cenozoic infilling of the northern Gulf of Mexico Basin with siliciclastic sediments occurred in a broadly offlapping pattern as successively younger shelf margins prograded basinward. Ancient shelf margins divide the basin profile into shelf and slope realms, with each realm containing economically important sandy reservoir units of fundamentally different geometries and stratal architecture. In the northern Gulf of Mexico, salt tectonics, related growth faulting, and shale compaction severely hinder recognition of ancient shelf margins on dip-oriented seismic sections, often making prediction of potential reservoir sand distributions problematical.
We demonstrate that biostratigraphic signals derived from the quantitative analysis of benthic foraminifera preserved in well bore ditch cuttings are effective for constraining the position of ancient shelf margins. The signals are computed using the Integrated Paleontologic System (IPS) and include an objectively calculated paleobathymetry curve, similarity matrix, and curves showing the percentages of shelf, slope, and carbonate-bank assemblages (CBA). IPS is a data analysis software tool originally developed by UNOCAL and subsequently enhanced by an industry/academic consortium at the University of Utah.
Four coreholes taken along a shelf-to-upper slope dip profile across the latest Pleistocene Lagniappe Delta (located southeast of the modern Mississippi “Birdsfoot”) have provided a wealth of geologic information on the dynamics of shelf-margin siliciclastic sedimentation in the northern Gulf of Mexico. To better understand the biostratigraphic response of prograding siliciclastic shelf margins in this region, we vertically stack age-equivalent paleontologic data from shelf margin and shelf coreholes on top of similar data from the upper bathyal site. This concatenated data set produces an upward stratigraphic succession that would be expected if the current shelf margin was to prograde over the bathyal corehole location.
Using IPS, we calculate key biostratigraphic signals for the concatenated Lagniappe section, and use them as a model for interpreting similar data sets from exploration wells with upward successions of bathyal, shelf-margin, and shelf facies. The following biostratigraphic signals characterize Neogene shelf margins in the northern Gulf of Mexico: (1) first upward appearance of in situ CBA (2) progressive upward shift in IPS-derived paleobathymetry curve from upper bathyal to outer neritic/upper bathyal (3) upward shift from predominantly slope-restricted to shelf-restricted taxa.