Gulf of Mexico Basin Late Tertiary Deep-Water Biostratigraphic Zonation: Relationship to Standard Shelf Foraminiferal and Nannofossil Marker Terminology

Breard, S.Q., A.D. Callender, R.A. Denne, and M.J. Nault, Applied Biostratigraphix, 717 East 8th Street, Houston, Texas 77007

Extended Abstract

The science of biostratigraphy has seen its greatest development and usage in Gulf of Mexico Basin oil exploration and production due to certain characteristics of the basin: its monotonous quartz sand and shale sequences, growth faults, and salt tectonics. Although 3-D seismic has reduced the reliance on biostratigraphic data for correlations in routine settings, drilling in the deep waters of the Gulf of Mexico (especially the eastern fold belts) has returned biostratigraphy to a prominent position. Reasons for this include the structural/stratigraphic complexity of the region, proximity of salt sheets to many of the prospects, and the poor seismic quality at target depths (sometimes below 25,000' subsea).

Early zonation of the Gulf of Mexico Basin relied primarily on benthic foraminifera characteristic of updip neritic facies to differentiate sand-shale sequences and to aid in correlation across growth faults. Utilization of planktic foraminifera and nannoplankton (nannofossils) became increasingly important as more down-dip exploration occurred. The current focus (1990's) on deep-water exploration has provided biostratigraphy with the unique opportunity to develop new correlative events from which up-dip marker fossils are environmentally excluded. Many standard Gulf of Mexico Basin foraminiferal markers, such as Angulogerina B, Buliminella 1, Textularia X, and Cristellaria K, are virtually useless in deep-water biostratigraphic zonation. Advances in several areas, including increased understanding of bathyal paleoenvironments, more widespread utilization of local foraminiferal biofacies, improved taxonomic methodology in bathyal calcareous and agglutinated foraminifera, and better utilization of nannofossil abundance events, have contributed to finer subdivision of the stratigraphic framework of bathyal Neogene sediments in the Gulf of Mexico Basin.

The breakup of major oil company micropaleontology staffs, establishment of small industry consulting groups, and subsequent publication of improved down-dip zonations (Breard et al., 1993; Styzen, 1996; Lawless et al., 1997) have also improved understanding of down-dip Neogene stratigraphic sections targeted by current deep-water exploration. The recent publication of the long-awaited Gulf of Mexico Basin Biostratigraphic Index Microfossils volume (Rosen et al. , 1999) should contribute to a more unified understanding of the Gulf of Mexico Basin biostratigraphy.

Paleoenvironmental/paleobathymetric models have been proposed for the Gulf of Mexico Basin Miocene, Pliocene, and Pleistocene strata (Breard et al., 1993). These models have recognized a finer subdivision of bathyal paleoenvironments, and have identified which key paleoecological indicators species exhibit habitat change through time. Recognition of strictly abyssal paleoenvironments remain problematic, based on lack of zonal markers which are restricted to these depths.

Local biofacies have particular value in correlation of bathyal sections in the Gulf of Mexico Basin Late Tertiary (Breard et al., 1996). Highly useful are acme zones (increases in faunal/floral abundance), especially among agglutinated (arenaceous) foraminifera, and planktic foraminifera and nannofossil species (which typically indicate condensed zones). Due to the large distances between some deep-water settings and deltaic outflow, some planktic species (both foraminifera and nannofossil) are found to be rare in more proximal settings. Zones of reworking of older species, and recurring faunas/floras within repeating sedimentary facies have also been useful down-dip correlative tools.

Establishment of a bathyal benthic foraminiferal zonation has provided enhanced correlation points between established planktic extinctions (LAD's) and provides finer resolution in areas in which updip and mid-dip species do not range ( Breard et al., 1997). Increased understanding of bathyal foraminiferal taxonomy and accurate establishment of geologic ranges for bathyal foraminifera has provided biostratigraphers with the ability to include these taxa in down-dip stratigraphic correlation schemes (Van Morkhoven et al., 1986).

A detailed updated biostratigraphic chart (Figure 1) for the Miocene through Pleistocene of the Gulf of Mexico Basin is provided. This chart ties the standard shelf foraminiferal zonation (column 1) to down-dip benthic and planktic foraminifera (column 2) and nannoplankton (column 3) events and markers. Bathyal zonal markers provide greater correlation potential in areas of stratigraphic expansion and regions of rapid deposition where standard shelf benthic markers and planktic marke rs may be either rare or absent. Newly recognized bathyal extinctions are included for the down-dip Miocene. Recognition of many key Gulf of Mexico Basin bathyal index species in other areas (Caribbean, Central and South America) suggest widespread utility of this zonation in similar bathyal settings outside of the Gulf of Mexico Basin.

Successful deep-water biostratigraphic zonation of the Gulf of Mexico Basin and beyond, therefore, depends on the proper integration of planktic (nannofossils and foraminifera) markers and benthic bathyal foraminiferal (agglutinated and calcareous) markers, utilizing LAD's, local occurrences and acme events.

References Cited

Breard, S.Q., A.D. Callender, and M.J. Nault, 1993, Paleoecologic and biostratigraphic models for Pleistocene through Miocene foraminiferal assemblages of the Gulf Coast Basin: GCAGS Transactions, vol. 43, p. 493-502.

Breard, S.Q., A.D. Callender, and M.J. Nault, 1996, Local foraminiferal faunas: the key to enhanced stratigraphic resolution of expanded Cenozoic sections in the Gulf Coast Basin: GCAGS Transactions, vol. 46, p. 55-62.

Breard, S.Q., A.D. Callender, and M.J. Nault, 1997, Application of bathyal foraminiferal events to the Neogene through Pleistocene biostratigraphic framework of the Gulf of Mexico Basin: GCAGS Transactions, vol. 47, p. 65-73.

Lawless, P.N., R.H. Fillon, and R.G. Lytton, 1997, Gulf of Mexico Cenozoic biostratigraphic lithostratigraphic and sequence stratigraphic event chronology: GCAGS Transactions, vol. 47, p. 271-283, 3 charts.

Rosen, R. N., S.Q. Breard, A. D. Callender Jr, R. L. Fleisher, R. G. Lytton III, L. C. Menconi, M. J. Nault, Rhonda roederer, T. W. Shunick, and J. L. Tuttle, 1999, Gulf of Mexico Basin Biostratigraphic Index Microfossils; A Geoscientist's Guide, Foramini fera and Nannofossils, Oligocene through Pleistocene, Parts I and II: Gulf Coast Section SEPM Foundation, 215 data sheets, 3 charts.

Styzen, M.J., 1996, Late Cenozoic Chronostratigraphy of the Gulf of Mexico: Gulf Coast Section SEPM Foundation, chart in 2 sheets with text.

Van Morkhoven, F.PC.M., W.A. Berggren and A.S. Edwards, 1986, Cenozoic Cosmopolitan Deep-Water Benthic Foraminifera: Bull. Centres Rech Explor.-Prod. Elf Aquitane, Pau, France, Memoir 11, 421 p., 126 pls.