Eocene-Oligocene calcareous nannofossil assemblages from ...
Eocene-Oligocene calcareous nannofossil assemblages from piston cores taken in the vicinity of deep sea drilling sites 511 and 512, Falkland Plateau
It was originally hoped that Deep Sea Drilling Project (DSDP) site 512 could be located at a shallow depth (about 1,500 meters) somewhere along the steeply sloping northern margin of the Maurice Ewing Bank so that the exposed Oligocene-Paleocene section could be sampled by hydraulic piston core. Unfortunately, lack of adequate seismic profiles did not allow sufficient evaluation of potential hydrocarbon hazards, and the site was eventually relocated some distance to the southeast in about 1,850 meters of water where the older sediment section is more abbreviated (see figure 1 of Parker et al., Antarctic Journal, this issue). Nevertheless, the Islas Orcadas piston cores in the vicinity of the initially proposed drill site do provide a valuable insight into the nature of the sediments higher up on the Maurice Ewing Bank. Among those cores are four which can be correlated closely with the middle Eocene and lower Oligocene sections cored at DSDP sites 512 and 511 respectively. The calcareous nannofossil stratigraphies of these cores are given in figures 1 and 2. These figures and the following synopsis are taken from a more extended article by Wise and Mostajo (in press). Correlation of middle Eocene piston and drill cores from the Maurice Ewing Bank suggests the following sequence of calcareous nannofossil datums within or bounding the Discoaster bifax Subzone of the Reticulofenestra umbilica Zone in this area (listed in descending stratigraphic order): Last appearance datum: Chiasmolithus solitus First appearance datum: Reticulofenestra hisecta bisecta Last appearance datum: Sphenolithus furcatolithoides First appearance datum: Reticulofenestra umbilica Since the entire Discoaster bifax Subzone was not drilled or cored at any one locality, we cannot ascertain the reliability of these datums in this region. Further drilling would be required. Nor
SHERWOOD W. WISE, JR. Antarctic Marine Geology Facility Department of Geology Florida State University Tallahassee, Florida 32306 ELENA L. MOSTAJO Argentine Museum of Natural Sciences 1405 Buenos Aires, Argentina
During April of 1978, a series of ARA Islas Orcadas cruise 16 piston cores were taken along the heavily eroded northern margin of the Maurice Ewing Bank on the eastern Falkland Plateau (Wise et al. 1978) in order to trace the outcrop belt of older (pre-Pliocene) sediments thought to be exposed there (Ciesielski and Wise 1977). Basal sediments of the thirteen piston cores taken along the northern margin range from Paleocene to Miocene in age (Jones et al. 1979) and suggest the presence of a high latitude Paleocene to Oligocene section of mixed calcareous-siliceous oozes in excess of 200 meters thick.
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middle Reticulo- DiscoEocene fenestra aster umbilica bifax
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A M, E, 0 C AV F A A M, E, 0 F F ? A AV F CF A C, E,0 C F A CV C A C, E, 0 C F F A CV F A C, E, 0 C F A CV F A C, E, 0 F F F A F ? A C, E, 0 F C A F R R R F M, E CFA F R F F M, E C C F R R F M, E R C C FA C M, E C C CV F A M, E A A A AV F
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Note: V = very abundant; H = highly abundant; A = abundant; C = common; F = few; R = rare; fr = fragment; P = poor; M = moderate; G = good; E = etched; 0 =
overgrown; Fl = flow-in. Abundances in lowercase denote reworked or contaminant specimens. a Cyclicargolithus sp.
Figure 1. Distribution of middle Eocene calcareous nannofossils in
1983 REVIEW
ARA
Is/as Orcadas cores 16-43, 16-44, and 16-46.
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10 16-45, 5 10 16-45, 15 10 16-45, 25 10 16-45, 35 10 16-45, 45 10 16-45, 55 10 16-45, 65 10 16-45, 73
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Note: V = very abundant; A = abundant; C = common; F = few; R = rare; P poor; M = moderate; 0 = good; E = etched; 0 = overgrown. Figure 2. Distribution of early Oligocene calcareous nannofossils In
can we ascertain the duration of time encompassed by the Discoaster bifax Subzone in this region. Based on Bukry's (1973a) calculations, the subzone spans only about 1 million years. If that is correct, then the sedimentation rate at site 511 during that time would have been a phenomenal 70 meters per million years (Wise in press). An alternative explanation is that the interval actually spans a greater amount of time. Perhaps Chiasmolithus solitus persisted longer in the higher latitudes than in the tropics where Bukry's (1973b) zonation is grounded. If that were so, then the subzone would encompass a greater amount of section. Many of these questions could best be answered by further drilling in the vicinity of the Is/as Orcadas piston cores discussed here. The section there is shallower and is far more calcareous in the Oligocene than the equivalent section at DSDP site 511. In addition, exposure of progressively older sediments down the long northern slope of the Maurice Ewing Bank places the entire Tertiary section within sampling range of the hydraulic piston corer. The acquisition of such a section would be of great value to high latitude biostratigraphy. This research was supported by National Science Foundation grant DPP 80-20382. References Bukry, D. 1973a. Coccolith stratigraphy, Eastern Equatorial Pacific, leg 16, Deep Sea Drilling Project. In T. H. van Andel, C. R. Heath, etal.,
156
ARA
Islas Orcadas core 16-45.
(Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 16. Washington D.C.: U.S. Government Printing Office. Bukry, D. 1973b. Low-latitude coccolith biostratigraphic zonation. In N. T. Edgar, J . B. Saunders, etal., (Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 15. Washington, D.C.: U.S. Government Printing Office. Ciesielski, P. F., and S. W. Wise, Jr. 1977. Geologic history of the Maurice Ewing Bank of the Falkland Plateau (Southwest Atlantic sector of the Southern Ocean) based on piston and drill cores. Marine Geology, 25, 175-207. Jones, S. C., S. W. Wise, Jr., D. R. DeFelice, et al. 1979. Basal sediment ages of ARA Islas Orcadas cruise 16 piston cores. Antarctic Journal of the U. S., 14(5), 150-153. Parker, M. E., M. A. Arthur, and S. W. Wise, Jr. 1983. Carbonate cycles in Aptian-Albian "black shales" of the Falkland Plateau. Antarctic Journal of the U.S., 18(5). Wise, S. W., Jr. In press. Mesozoic and Cenozoic calcareous nannofossils recovered by [)SDP leg 71 drilling in the region of the Falkland Plateau, southwest Atlantic Ocean. In W. J . Ludwig, V. A.
Krasheninnikov, et al., (Eds.),
Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office.
Wise, S. W., Jr., S. C. Jones, P. R. Ciesielski, D. T. Georg, D. S. Woodroffe, and S. S. Jacobs. 1978. Islas Orcadas cruise 16. Antarctic Journal of the U.S., 13(4), 92-94. Wise, S. W., Jr., and E. L. Mostajo. In press. Correlation of EoceneOligocene calcareous nannofossil assemblages from piston cores taken in the vicinity of Deep Sea Drilling Project sites 511 and 512, southwest Atlantic Ocean. In W. J . Ludwig, V. A. Krasheninnikov, et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office.
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