(Malvinas) Plateau based on piston and drill cores
Figure 3. Postulated paleocirculation in the region of the Falkland Plateau during the Camp a n ian. Maestrjchtian (from Ciesielski et al., 1977). The paleogeographic distribution of the land area (featuring a closed isthmus between South American and Antarctica) is based on the reconstruction proposed by Barker et al. (1977, figure 3a). Precise land and bathymetry configuration largely conjectural.
Constans, RE., and S.W. Wise. 1974. Fluctuations in the carbonate compensation depth recorded in deep sea cores. Geological Society ofAmerica. Abstracts with Programs, 7: 1036. Combos, Jr., A.M. 1977. Paleogene and Neogene diatoms from the Falkland Plateau and Malvinas Outer basin: Leg 36, Deep Sea Drilling Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. p.575-688. Kaharoeddin F,A., F.M. Weaver, and S.W. Wise. 1973. Cretaceous and Paleogene cores from the Kerguelen Plateau, southern ocean. AntarctzcJournal of the U.S., VIII(5): 297-298. Qsilty, P.G. 1973. Cenomanian . Turonian and Neogene sediments from Northwest of Kerguelen Ridge, Indian Ocean, Journal of the Geological Society ofAustralia, 20: 361-371.
Late Mesozoic-Cenozoic history of the eastern Falkland (Malvinas) Plateau based on piston and drill cores SHERWOOD W. WISEJR. and PAUL F. CIESIELSKI Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
An objective of Florida State University's participation in
ARA Islas Orcadas coring has been the acquisition of piston
cores near Deep Sea Drilling Project (DSDP) leg 36 drill sites (Barker et al., 1977) on the eastern Falkland Plateau (figure 1). Older (pre-Pliocene) geologic strata crop out near the surface over much of this portion of the Plateau. Piston core samples from these older units can be correlated with reference sections established at the drill sites. Study of the stratigraphic sequence, which dates back to the Jurassic at the drill sites, provides most of the evidence on which present reconstructions of the sedimentologic and paleoclimatologic history of this sector of the southern ocean are based. A piston core survey of the older units has been done during the past two field seasons during Islas Orcadas cruises 7 and 11 (Warnke et al., 1976; Sclater et al., 1977). We selected piston core stations in partial collaboration with Peter F. Barker (geophysicist, University of Birmingham, England) who was with Wise on DSDP leg 36. These stations were located along Glomar Challenger and Robert Conrad October 1977
seismic profiles across the plateau at points where the probability of encountering older sediment appeared most likely. Waters of the eastern portion of the plateau are shallow (800 to 1500 fathoms), and cores can be raised in less than an hour by the highly efficient Argentine deck crew of the Islas Orcadas. Most of the cores for our study were taken during cruise 7 under the shipboard supervision of Ciesielskj. On cruise 11, Ciesielski and Wise were chief geologists. Core catcher samples were age-dated aboard ship by microscopy. Thus, age determinations made by any given station could be used in final site selection for subsequent stations. The scientific results of the study to date are in press (Ciesielski and Wise, in press; Ciesielski et al., in press, 1977); a summary follows. Paleontologic and sedimentologic analyses of 55 piston cores (including 26 from the Lamont-Doherty Geological Observatory and Eltanin collections; see figure 2) and three DSDP leg 36 drill core sequences combined with seismic reflection profiler data provide the basis for the study. Micropaleontologic dating of the cores has been done using the high latitude biostratigraphic zonations summarized by these workers: Calcareous nannofossils: Wise and Wind (1977). Silicoflagellates: Ciesielskj (1975), Busen and Wise (1977). Diatoms: McCollum (1975), Gombos (1977), Weaver (1976). Our primary data is presented in a geologic map (figure 3) and cross-section (figure 4) of older (pre-Pliocene) units sampled beneath a thin veneer (1 to 2 meters) of Plio/Pleistocene siliceous ooze and glacial marine clastics that mantle the Plateau. Thirty-one of the piston cores penetrated pre-Pliocene sediments, the oldest being Campanian in age. Comparison of faunal and floral assemblages of Cretaceous sediment from opposite sides of the Maurice Ewing 67
701w
61W
60'W
55W
OW
45'N
40'W
35'W
mo
45S
50S
A
55'S 0
PASSAGE
,
XrW
Figure 1. Location of the Maurice Ewing Bank study area (black retangle) at the eastern end of the Falkland Plateau, Southwest Atlantic sector of the southern ocean. The polar front position is from Gordon of al. (in press). Bathymetry (in fathoms) for this and all subsequent map figures is from Leonardi and Ewing (1 971). 48
42
40 38' - -.--- -. 49'
50'
51'
52'
53.
Figure 2. Core location map (from Ciesielski and Wise, 1977). 54
Bank shows significant paleoecological differences which compensation depth, coupled with a significant variation in suggest that the Falkland Plateau served as an important topography across the bank, contributed to marked changes barrier between water masses in the South Atlantic during in lithologic facies ranging from coccolith ooze near the this time. During the Tertiary, a relatively high carbonate apex of the bank to diatom ooze and zeolitic clay on its 68
ANTARCTIC JOURNAL
48
46
44
42
38
40
49,
5O
GIL
Figure 3. Geologic map of older (pre-Pliocene) strata sampled in the area directly beneath a thin (1 to 2 meter) areawide cover of P1 jo/Pleistocene siliceous ooze and glacial marine clastics (from Ciesielski and Wise, in press). Superimposed are Glomar Challenger leg 36 and Robert Conrad 16-06 seismic reflection profile tracks used in constructing the geologic crosssection of figure 4.
ILL
54. R/V ROBERT COGAD CRUISE 16
LARGER RIDGE
MIOCENE
PALEOCENE
DIV GLOMAR CHALLENGER LEG 36
FAULT
OLIGOCENE/ EOCENE
CRETACEOUS
Figure 4. Geologic cross-section across the Maurice Ewing Bank as interpreted from piston and drill core data and from
Glomar Challenger leg 36 and Robert Conrad 16-06 seismic reflections profiles (from Ciesielski and Wise, in press). Portions of the section are adapted from or have been modified from Barker (1977, figures 9 and 10).
October 1977
69
flanks. Miocene coccolith ooze deposition ceased when the center of high carbonate productivity (and the Miocene Polar Front?) migrated north of the bank in response to deteriorating climate associated with a severe late Miocene continental glaciation on the antarctic continent. Strong bottom currents associated with the Cretaceous Tertiary event, the early Miocene opening of the Drake Passage, and the late Miocene antarctic glaciation caused scouring and removal of significant volumes of sediment, thus contributing to the complex erosional history of the bank. During the late Miocene the last and most severe of these events truncated the previously deposited upper Cretaceous to Miocene sequence of sediments of the bank. The primary erosional agent during the late Miocene was probably a northerly component of the circumantarctic deep water that impinged on the bank from the southwest. Erosion of older pelagic oozes ceased with the deposition of Plio- Pleistocene glacial marine clastics that essentially "armored" the sediment surface with a protective, erosionresistant cover. Carbonate deposition in the study area resumed during the latest Pleistocene, when the polar front migrated to a mean position near the southern margin of the bank. This research was supported by National Science Foundation grant DPP 74-20109. Operational support by the Argentine Naval Hydrographic Service is gratefully acknowledged.
Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 575-688. Gordon, A.L., D.T. Georgi, and H.W. Taylor. In press. Antarctic polar front zone in the western Scotia Sea - summer I 975.Journal of Physical Oceanography. Leonardi, A., and M. Ewing. 1971. Bathyrnetry chart of the Argentine Basin (map, no scale). In: Physics and Chemistry of the Earth, London, Pergamon Press. 8. McCollum, D.W. 1975. Antarctic Cenozoic diatoms: leg 28, Deep Sea Drilling Project. In L.A. Frakes, D.E. Hayes, et al., Initial Reports of the Deep Sea Drilling Project, 28. U.S. Government Printing Office, Washington, D.C. 515-572. Sclater, T.G., D. Woodroffe, H. Dick, D. Georgi, S.W. Wise, and P. Ciesielski. 1977. Scientific report for Islas Orcadas cruise 11. AntarcticJournalof the U.S., XII(4): 62-65. Warnke, D.A., P. Bruchhausen, J . LaBrecque, P.F. Ciesielski and A. Federman. 1976. ARA Islas Orcadas cruise 7. Antarctic Jour. nalof the U.S., XI(2): 70-73. Weaver, F.M. 1976. Late Miocene and Pliocene radiolarian paleobiogeography and biostratigraphy of the southern ocean. Tallahassee, Department of Geology, Florida State University, Ph.D. Dissertation (unpublished). 175 p. Wise, SW., and F.H. Wind. 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP leg 36 drilling on the Falkland Plateau, Atlantic sector of the southern ocean. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 269-492.
References
Barker, P.F. 1977. Correlations between sites on the Eastern Falkland Plateau by means of seismic reflection profiles, Leg 36, Deep Sea Drilling Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 971-900. Barker, P.F., I.W.D. Dalziel, D.H. Elliot, C.C. von der Borch, R.W. Thompson, G. Plafker, R.C. Tjalsma, S.W. Wise, M.G. Dinkelman, A.M. Gombos, A. Lonardi, andJ. Tarney. 1977. Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 1080 p. Busen, Karen L., and S.W. Wise. 1977. Silicoflagellate biostratigraphy, Deep Sea Drilling Project leg 36. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 697-744. Ciesielski, P.F., 1975. Biostratigraphy and paleoecology of Neogene and Oligocene silicoflagellates from cores recovered during antarctic leg 28, Deep Sea Drilling Project. In: D.E. Hayes, L.A. Frakes, et al., Initial Reports of the Deep Sea Drilling Project, 28. U.S. Government Printing Office, Washington, D.C. 625-691. Ciesielski, P.F. and S.W. Wise. In press. Geologic history of the Maunice Ewing Bank of the Falkland Plateau (Southwest Atlantic section of the southern ocean) based on piston and drill cores. Marine Geology (special issue; D.E. Hayes, editor). aesielski, P.F., W.V. Sliter, F.H. Wind, and S.W. Wise. In press. Paleoenvironmental analysis and correlation of a Cretaceous Islas Orcadas core from the Falkland Plateau, Southwest Atlantic. Marine Micropaleontology, 2. Ciesielski, P.E., W.V. Sliter, F.H. Wind, and S.W. Wise. 1977. A Cretaceous Islas Orcadas core from the Falkland (Malvinas) Plateau, southwest Atlantic. Antarctic Journal of the U. S., XII(4): 65-67. Combos, A . M. 1977. Paleogene and Neogene diatoms from the Falkland Plateau and Malvinas Outer Basin, Deep Sea Drilling 70
Basal sediment ages of Islas Orcadas cruise 7 piston cores PAUL F. CIESIELSKI and SHERWOOD W. WISE, JR.
Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
As an aid to other investigators wishing to study ARA Islas Orcadas cores, we present here preliminary basal sediment ages for 45 piston cores taken on cruise 7, the first multidisciplinary cruise (marine geology, physical oceanography, and geophysics) of the ship to the southwest Atlantic sector of the southern ocean. This cruise, which began and ended at Buenos Aires, concentrated on the Falkland (Malvinas) Plateau, the South Georgia Basin, and peripheral areas (Warnke et al. 1976). The table lists piston core numbers, latitude, longitude, water depth, sample interval, age, and sediment lithology of the basal sedimentary unit. Sampling. Fully recovered cores, stored in plastic liners, were sampled within 1 to 6 centimeters of their base; those with disturbed basal sedimentary sequences were sampled above the disturbed sequence as well. For all such cores sampled in this manner, both samples gave similar ages. Seven cores comprise a second group, from which samples ANTARCTIC JOURNAL
Constans, RE., and S.W. Wise. 1974. Fluctuations in the carbonate compensation depth recorded in deep sea cores. Geological Society ofAmerica. Abstracts with Programs, 7: 1036. Combos, Jr., A.M. 1977. Paleogene and Neogene diatoms from the Falkland Plateau and Malvinas Outer basin: Leg 36, Deep Sea Drilling Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. p.575-688. Kaharoeddin F,A., F.M. Weaver, and S.W. Wise. 1973. Cretaceous and Paleogene cores from the Kerguelen Plateau, southern ocean. AntarctzcJournal of the U.S., VIII(5): 297-298. Qsilty, P.G. 1973. Cenomanian . Turonian and Neogene sediments from Northwest of Kerguelen Ridge, Indian Ocean, Journal of the Geological Society ofAustralia, 20: 361-371.
Late Mesozoic-Cenozoic history of the eastern Falkland (Malvinas) Plateau based on piston and drill cores SHERWOOD W. WISEJR. and PAUL F. CIESIELSKI Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
An objective of Florida State University's participation in
ARA Islas Orcadas coring has been the acquisition of piston
cores near Deep Sea Drilling Project (DSDP) leg 36 drill sites (Barker et al., 1977) on the eastern Falkland Plateau (figure 1). Older (pre-Pliocene) geologic strata crop out near the surface over much of this portion of the Plateau. Piston core samples from these older units can be correlated with reference sections established at the drill sites. Study of the stratigraphic sequence, which dates back to the Jurassic at the drill sites, provides most of the evidence on which present reconstructions of the sedimentologic and paleoclimatologic history of this sector of the southern ocean are based. A piston core survey of the older units has been done during the past two field seasons during Islas Orcadas cruises 7 and 11 (Warnke et al., 1976; Sclater et al., 1977). We selected piston core stations in partial collaboration with Peter F. Barker (geophysicist, University of Birmingham, England) who was with Wise on DSDP leg 36. These stations were located along Glomar Challenger and Robert Conrad October 1977
seismic profiles across the plateau at points where the probability of encountering older sediment appeared most likely. Waters of the eastern portion of the plateau are shallow (800 to 1500 fathoms), and cores can be raised in less than an hour by the highly efficient Argentine deck crew of the Islas Orcadas. Most of the cores for our study were taken during cruise 7 under the shipboard supervision of Ciesielskj. On cruise 11, Ciesielski and Wise were chief geologists. Core catcher samples were age-dated aboard ship by microscopy. Thus, age determinations made by any given station could be used in final site selection for subsequent stations. The scientific results of the study to date are in press (Ciesielski and Wise, in press; Ciesielski et al., in press, 1977); a summary follows. Paleontologic and sedimentologic analyses of 55 piston cores (including 26 from the Lamont-Doherty Geological Observatory and Eltanin collections; see figure 2) and three DSDP leg 36 drill core sequences combined with seismic reflection profiler data provide the basis for the study. Micropaleontologic dating of the cores has been done using the high latitude biostratigraphic zonations summarized by these workers: Calcareous nannofossils: Wise and Wind (1977). Silicoflagellates: Ciesielskj (1975), Busen and Wise (1977). Diatoms: McCollum (1975), Gombos (1977), Weaver (1976). Our primary data is presented in a geologic map (figure 3) and cross-section (figure 4) of older (pre-Pliocene) units sampled beneath a thin veneer (1 to 2 meters) of Plio/Pleistocene siliceous ooze and glacial marine clastics that mantle the Plateau. Thirty-one of the piston cores penetrated pre-Pliocene sediments, the oldest being Campanian in age. Comparison of faunal and floral assemblages of Cretaceous sediment from opposite sides of the Maurice Ewing 67
701w
61W
60'W
55W
OW
45'N
40'W
35'W
mo
45S
50S
A
55'S 0
PASSAGE
,
XrW
Figure 1. Location of the Maurice Ewing Bank study area (black retangle) at the eastern end of the Falkland Plateau, Southwest Atlantic sector of the southern ocean. The polar front position is from Gordon of al. (in press). Bathymetry (in fathoms) for this and all subsequent map figures is from Leonardi and Ewing (1 971). 48
42
40 38' - -.--- -. 49'
50'
51'
52'
53.
Figure 2. Core location map (from Ciesielski and Wise, 1977). 54
Bank shows significant paleoecological differences which compensation depth, coupled with a significant variation in suggest that the Falkland Plateau served as an important topography across the bank, contributed to marked changes barrier between water masses in the South Atlantic during in lithologic facies ranging from coccolith ooze near the this time. During the Tertiary, a relatively high carbonate apex of the bank to diatom ooze and zeolitic clay on its 68
ANTARCTIC JOURNAL
48
46
44
42
38
40
49,
5O
GIL
Figure 3. Geologic map of older (pre-Pliocene) strata sampled in the area directly beneath a thin (1 to 2 meter) areawide cover of P1 jo/Pleistocene siliceous ooze and glacial marine clastics (from Ciesielski and Wise, in press). Superimposed are Glomar Challenger leg 36 and Robert Conrad 16-06 seismic reflection profile tracks used in constructing the geologic crosssection of figure 4.
ILL
54. R/V ROBERT COGAD CRUISE 16
LARGER RIDGE
MIOCENE
PALEOCENE
DIV GLOMAR CHALLENGER LEG 36
FAULT
OLIGOCENE/ EOCENE
CRETACEOUS
Figure 4. Geologic cross-section across the Maurice Ewing Bank as interpreted from piston and drill core data and from
Glomar Challenger leg 36 and Robert Conrad 16-06 seismic reflections profiles (from Ciesielski and Wise, in press). Portions of the section are adapted from or have been modified from Barker (1977, figures 9 and 10).
October 1977
69
flanks. Miocene coccolith ooze deposition ceased when the center of high carbonate productivity (and the Miocene Polar Front?) migrated north of the bank in response to deteriorating climate associated with a severe late Miocene continental glaciation on the antarctic continent. Strong bottom currents associated with the Cretaceous Tertiary event, the early Miocene opening of the Drake Passage, and the late Miocene antarctic glaciation caused scouring and removal of significant volumes of sediment, thus contributing to the complex erosional history of the bank. During the late Miocene the last and most severe of these events truncated the previously deposited upper Cretaceous to Miocene sequence of sediments of the bank. The primary erosional agent during the late Miocene was probably a northerly component of the circumantarctic deep water that impinged on the bank from the southwest. Erosion of older pelagic oozes ceased with the deposition of Plio- Pleistocene glacial marine clastics that essentially "armored" the sediment surface with a protective, erosionresistant cover. Carbonate deposition in the study area resumed during the latest Pleistocene, when the polar front migrated to a mean position near the southern margin of the bank. This research was supported by National Science Foundation grant DPP 74-20109. Operational support by the Argentine Naval Hydrographic Service is gratefully acknowledged.
Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 575-688. Gordon, A.L., D.T. Georgi, and H.W. Taylor. In press. Antarctic polar front zone in the western Scotia Sea - summer I 975.Journal of Physical Oceanography. Leonardi, A., and M. Ewing. 1971. Bathyrnetry chart of the Argentine Basin (map, no scale). In: Physics and Chemistry of the Earth, London, Pergamon Press. 8. McCollum, D.W. 1975. Antarctic Cenozoic diatoms: leg 28, Deep Sea Drilling Project. In L.A. Frakes, D.E. Hayes, et al., Initial Reports of the Deep Sea Drilling Project, 28. U.S. Government Printing Office, Washington, D.C. 515-572. Sclater, T.G., D. Woodroffe, H. Dick, D. Georgi, S.W. Wise, and P. Ciesielski. 1977. Scientific report for Islas Orcadas cruise 11. AntarcticJournalof the U.S., XII(4): 62-65. Warnke, D.A., P. Bruchhausen, J . LaBrecque, P.F. Ciesielski and A. Federman. 1976. ARA Islas Orcadas cruise 7. Antarctic Jour. nalof the U.S., XI(2): 70-73. Weaver, F.M. 1976. Late Miocene and Pliocene radiolarian paleobiogeography and biostratigraphy of the southern ocean. Tallahassee, Department of Geology, Florida State University, Ph.D. Dissertation (unpublished). 175 p. Wise, SW., and F.H. Wind. 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP leg 36 drilling on the Falkland Plateau, Atlantic sector of the southern ocean. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 269-492.
References
Barker, P.F. 1977. Correlations between sites on the Eastern Falkland Plateau by means of seismic reflection profiles, Leg 36, Deep Sea Drilling Project. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 971-900. Barker, P.F., I.W.D. Dalziel, D.H. Elliot, C.C. von der Borch, R.W. Thompson, G. Plafker, R.C. Tjalsma, S.W. Wise, M.G. Dinkelman, A.M. Gombos, A. Lonardi, andJ. Tarney. 1977. Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 1080 p. Busen, Karen L., and S.W. Wise. 1977. Silicoflagellate biostratigraphy, Deep Sea Drilling Project leg 36. In: P.F. Barker, I.W.D. Dalziel, et al., Initial Reports of the Deep Sea Drilling Project, 36. U.S. Government Printing Office, Washington, D.C. 697-744. Ciesielski, P.F., 1975. Biostratigraphy and paleoecology of Neogene and Oligocene silicoflagellates from cores recovered during antarctic leg 28, Deep Sea Drilling Project. In: D.E. Hayes, L.A. Frakes, et al., Initial Reports of the Deep Sea Drilling Project, 28. U.S. Government Printing Office, Washington, D.C. 625-691. Ciesielski, P.F. and S.W. Wise. In press. Geologic history of the Maunice Ewing Bank of the Falkland Plateau (Southwest Atlantic section of the southern ocean) based on piston and drill cores. Marine Geology (special issue; D.E. Hayes, editor). aesielski, P.F., W.V. Sliter, F.H. Wind, and S.W. Wise. In press. Paleoenvironmental analysis and correlation of a Cretaceous Islas Orcadas core from the Falkland Plateau, Southwest Atlantic. Marine Micropaleontology, 2. Ciesielski, P.E., W.V. Sliter, F.H. Wind, and S.W. Wise. 1977. A Cretaceous Islas Orcadas core from the Falkland (Malvinas) Plateau, southwest Atlantic. Antarctic Journal of the U. S., XII(4): 65-67. Combos, A . M. 1977. Paleogene and Neogene diatoms from the Falkland Plateau and Malvinas Outer Basin, Deep Sea Drilling 70
Basal sediment ages of Islas Orcadas cruise 7 piston cores PAUL F. CIESIELSKI and SHERWOOD W. WISE, JR.
Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
As an aid to other investigators wishing to study ARA Islas Orcadas cores, we present here preliminary basal sediment ages for 45 piston cores taken on cruise 7, the first multidisciplinary cruise (marine geology, physical oceanography, and geophysics) of the ship to the southwest Atlantic sector of the southern ocean. This cruise, which began and ended at Buenos Aires, concentrated on the Falkland (Malvinas) Plateau, the South Georgia Basin, and peripheral areas (Warnke et al. 1976). The table lists piston core numbers, latitude, longitude, water depth, sample interval, age, and sediment lithology of the basal sedimentary unit. Sampling. Fully recovered cores, stored in plastic liners, were sampled within 1 to 6 centimeters of their base; those with disturbed basal sedimentary sequences were sampled above the disturbed sequence as well. For all such cores sampled in this manner, both samples gave similar ages. Seven cores comprise a second group, from which samples ANTARCTIC JOURNAL
