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Abstract: Cosmopolitan Tertiary Bathyal Benthic Foraminifera
F. P. C. M. van Morkhoven (2)
A three-year study of Tertiary bathyal benthic foraminiferal assemblages revealed that many species have extensive, often near-global geographical distributions combined with restricted vertical stratigraphic ranges. The study was based on an examination of closely sampled surface profiles and cored subsurface sections from widely separated areas throughout the world, including an almost completely cored composite section of Tertiary strata from the DeSoto Canyon in the northern Gulf of Mexico.
The preliminary range chart of Tertiary bathyal benthic Foraminifera, issued with this volume, gives the total stratigraphic ranges of 120 selected species, as they are known to us at present. Ranges are shown against the standard pelagic foram zones of Blow (1979) and the calcareous nannoplankton zonation of Martini (1971). Also shown for each species are the areas in which they have so far been observed to occur, and the extinction tops of some of the more conspicuous ones are indicated.
The correct nomenclature of such cosmopolitan species poses serious problems. Undoubtedly, many geographical variants were described and named differently by authors from widely scattered regions. Some species names used on our chart will need to be revised and the correct identifications of those shown in open nomenclature are yet to be determined. This task, together with additional checking of their vertical ranges, etc., is being carried out at present at the Woods Hole Oceanographic Institute.
In our opinion, bathyal benthic Foraminifera show great promise of becoming a potentially important global stratigraphic tool. They can be used for isochronous correlations and age dating in areas where planktic organisms are mostly absent (such as parts of the California Tertiary) or between areas with very different planktic faunas (i. e. high vs. low latitude, or tethys vs. boreal areas). Many species exhibit gradual morphological changes through geologic time. The detailed study of such 'chronoclines' will add significantly to the resolution of an envisaged global zonal scheme based on this group of fossils.
Another important attribute of bathyal benthic Foraminifera lies in their usefulness as indicators of paleo-depth of deposition. Many species were observed to show clear ecophenotypic variations with depth, while others were found to be restricted to certain depth limits within the bathyal realm. This seems to make them particularly useful for detecting major sea-level changes which often are best reflected in slope deposits.
Lastly, the usefulness of these deep water benthic organisms for paleo-oceanographic studies should be emphasized. The apparent sudden extinction of a good number of species around N.9-N.11 time, as apparent on our range chart, would seem to reflect the Mid-Miocene oxygen-isotopic event (Woodruff, 1979) when colder antarctic bottom water started invading the oceans.
Blow, W. H., 1979, The Cainozoic Globigerinida: E. J. Brill, Leiden.
Martini, E., 1971, Standard Tertiary and Quaternary calcareous nannoplankton zonation: Proc. 2nd Planktonic Conference, Roma, 1970.
Woodruff, F., 1979, Deep sea benthic foraminiferal changes associated with the Middle Miocene oxygen isotopic event, DSDP Site 289, Equatorial Pacific: MS thesis. University of Southern California. (unpublished).
1. Published with the permission of Shell Oil Company
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(2) Stratigraphic Services, Shell Oil Company, Houston, Texas
Copyright © 1999 by The Gulf Coast Association of Geological Societies