Cenozoic microfossil datums in antarctic to subantarctic deep-sea ...
13. Biogeographic changes during the Neogene primarily have been in scope rather than styfe. For instance, major changes have occurred in biogenic productivity and inferred oceanic upwelling; in the geographic range of individual biogeographic provinces; and in the diversity of assemblages within these provinces. A steep diversity gradient between polar and tropical areas developed close to the beginning of the Miocene. This gradient has since been an essentially permanent feature. 14. Modern antarctic microfossil assemblages developed their basic characteristics by the early Pliocene; since then further changes have been biogeographically less important. 15. Although the Quaternary was a time of maximum global terrestrial and oceanic cooling for the Cenozoic, no major biogeographic crisis occurred in the marine realm. This is because global cimate changed in many steps over a long time, during the Cenozoic, each step having biogeographic repercussions. This research was supported by National Science Foundation grants DPP 77-06687 and OCE 76-81489.
Kennett, J . P. 1977. Cenozoic evolution of antarctic glaciation, the Circum -Antarctic Ocean, and their impact on global paleoceanography. Journal of Geophysical Research, 82(27): 3843-3860. Kennett,J. P. 1978. The development of planktonic biogeography in the Southern Ocean during the Cenozoic. Marine Micropaleontology, 3(4): 301-345. Kozlova, 0. G. 1970. Diatoms in suspension and in bottom sediments in the Southern Ocean and Pacific Oceans. In: Antarctic Ecology (Vol. 1, M. W. Hoidgate, ed.). Academic Press, New York. pp. 148-153. Sciater, J . G., D. Abbott, and J. Theide. 1977. Paleobathymetry and sediments of the Indian Ocean. In: Indian Ocean Geology and Biostratigraphy U. R Heirtzler et al., eds.). American Geophysical Union, Washington, D.C. Sciater, J . G., S. Hellinger, and C. Tapscott. 1977. The paleobathymetry of the Antlantic Ocean from the Jurassic to the present. Journal of Geology, 85: 509-552.
References Dunbar, M.J. 1977. The evolution of polar ecosystems. In: Adaptations Within Antarctic Ecosystems (G. A. Llano, ed.). Smithsonian Institu tion, Washington, D.C. pp. 1063-1076.
Cenozoic microfossil datums in antarctic to subantarctic deep-sea sedimentary sequences J . P. KENNErF
Graduate School of Oceanography University of Rhode Island Kingston, Rhode Island 02881 The southern ocean, here considered to represent the approximate present-day positions of the subantarctic and antarctic water masses, represents an enormous area of the earth's surface. Biostratigraphic information from this region is important for a complete understanding of global biogeographic and biostratigraphic patterns during the Cenozoic and of the paleoceanographic and paleoclimatic controls of dynamic changes that are known to have occurred in the planktonic realm. The high-latitude regions of the Southern Hemisphere are marked by an almost complete lack of marine sedimentary sections on land. The antarctic continent lacks useful marine sequences; a few shallow marine sediment sections occur on and around the antarctic continent but essentially lack planktonic m icrofossils. Therefore, information about the marine Cenozoic microfossil succession must be gained from deep-sea sediments. Piston-coring programs have provided the most useful biostratigraphic information on Quarternary102
Weissel,J. K., D. E. Hayes, and E. M. Herron. 1977. Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous. Marine Geology , 25: 231-277.
Pliocene sections (see figure). This biostratigraphy has been very successfully tied to the magnetostratigraphy, providing a detailed dated sequence of microfossil events. In fact it was the southern ocean area where magnetostratigraphy was first successfully applied to the enhanced correlation and dating of biostratigraphic successions (Goodell and Watkins, 1968; Hays and Opdyke, 1967; Opdyke et al., 1966). Although most piston cores taken in the southern ocean are of Quaternary and Pliocene age, only a few are older and have provided brief glimpses of Tertiary and Late Cretaceous planktonic biogeography of the region (Margolis and Kennett, 1970, 1971; Quilty, 1973). The development of any reasonable understanding of southern ocean biostratigraphy by necessity thus awaited deep-sea drilling operations, which began in 1972. Four deep-sea drilling legs involving the Glomar Challenger have provided much new information on the Cenozoic. Cruises 28 and 29 south of Australia and New Zealand provided the most complete sequences, while cruises 35 and 36, although important, were less successful because of poorer drilling conditions. Numerous biostratigraphers have analysed the materials, and the results have been for the most part published in the initial reports of each of the expeditions. Before now (Kennett, in press), no attempts have been made to synthesize this body of information which consists of two categories: (a) biostratigraphic ranges of individual microfossil species for correlation purposes within and outside of the southern ocean; and (b) biogeographic information related to general distribution patterns, relative importance of major planktonic microfossil groups and diversity patterns throughout the Cenozoic. My contribution (Kennett, in press) is a compilation of biostratigraphic datums (first and last appearances) of the major planktonic microfossil groups ANTARCTIC JOURNAL
U 1-URAF" CALC/ 0 NANN
A ell
IrnIA(.rLL4TES is
Location of piston cores that have been used in the development of Quaternary to Pliocene biostratigraphy of the southern ocean region. The type of planktonic microfossil group studied in any particular core is indicated. Cores studied by Hays (1965) for radiolarian biostratigraphy are scattered over much of the southern ocean and are not plotted on this map. Positions of antarctic and subtropical convergences are shown.
in the southern ocean (planktonic foraminifera, calcareous nannofossils, diatoms, radiolaria, and silicoflagellates) studied by a large number of investigators. This compilation provides a sequence of integrated datums and hence a historical sequence of microfossil events through the Cenozoic. The datum levels of Quaternary and Pliocene age, their nature, relative position, and age are now well known because of integration with magnetostratigraphy, better sediment records, and more numerous studies. In contrast, the relative position of Miocene and older datum levels is much less firmly established. There are several reasons for this: poorer stratigraphic control because of a lack of a magnetostrati graphic framework; few sections that contain siliceous and calcareous microfossil assemblages in association to establish detailed relations; and poor stratigraphic representation especially in the Early Cenozoic. In general, most datum levels do not extend over both the antarctic and subantarctic regions. Instead, microfossil events, if they occur in both regions, can be highly diachronous between the two water-mass regimes. This is most strongly developed in the calcareous Cenozoic microfossil groups which, as in the present day, show major assemblage differences across the boundaries of these water masses. During the Cenozoic, major changes occurred in planktonic microfossil biogeography in the southern ocean as reflected in the biostratigraphic sequences. These changes were created in response to evolution of the southern ocean circulation system through the Cenozoic and the development of the antarctic and subantarctic water masses and high latitude climate. Eocene sediments are marked by a relatively high diverse and important calcareous microfossil October 1978
assemblage, even closely adjacent to the antarctic continent. During the Cenozoic, this calcareous microfossil province moved northward as cooler conditions developed and was replaced by a rich siliceous microfossil province. In the northern subantarctic area of the present day, calcareous microfossil diversity has remained relatively high throughout the Cenozoic compared with the antarctic area, providing a sequence of numerous datum levels very similar to that described in the temperate New Zealand area. Some enormous gaps in our knowledge still exist in the biostratigraphy of the southern ocean. Siliceous microfossil biostratigraphy of the subantarctic Miocene and older is still poorly known and has not yet been integrated into the calcareous microfossil sequence. Eocene biostratigraphy is also poorly known for the antarctic region, and the Late Miocene datum level sequence still requires much attention. Further deep-sea drilling is essential to provide the required marine sections. This research was supported by National Science Foundation grants DPP 77-06687 and OCE 76-81489.
References
Goodell, H. G., and N. D. Watkins. 1968. The paleomagnetic stratigraphy of the Southern Ocean: 200 West to 160° East longitude. Deep Sea Research, 15: 89-112. Hays,J. D. 1965. Radiolaria and late Tertiary and Quaternary history of antarctic seas. In: Biology of the Antarctic Seas II (American Geoph 'sical Union Antarctic Research Series), 5: 125-184. 103
Hays, J. D., and N. D. Opdyke. 1967. Antarctic radiolaria, magnetic reversals, and climatic change. Science, 158(3804): 1001-1011. Kennet, J . P. In press. Cenozoic microfossil datums in antarctic to subantarctic deep-sea sediment sequences. In: Integrated Cenozoic microfossil datums of the global ocean (N. de B. Hornibrook, ed.). Margolis, S. V., andJ. P. Kennett. 1970. Antarctic glaciation during the Tertiary recorded in subantarctic deep-sea cores. Science, 170: 1085-1087.
Paleontology and paleoenvironment of the Southwest Atlantic Ocean basin SHERWOOD W. WISE,JR.
Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
Over the past 4 years, research at the Antarctic Marine Geology Research Facility (Florida State University) has been concentrated on the southwest Atlantic sector of the southern ocean. Field work has involved one Deep Sea Drilling Project (DSDP) cruise and four ARA Islas Orcadas cruises to the area. This article is a condensation of an extended paleontological and paleoenviron mental summary by Wise ci al. (in press; also available from the author in preprint form) which covers some of the research to date. The summary is based on three DSDP leg 36 drill core sequences and a reconnaissance study of more than 75 piston cores taken on the Falkland (Malvinas) Plateau. Data from the three drill cores and selected piston cores are summarized in figures 1 and 2. Much of the drill core data was extracted from Barker ci al. (1977). The older sedimentary record (figure 1) suggests that a Middle (?) to Late Jurassic inland sea transgressed the southwest portion of Gondwanaland, then became progressively more restricted until by Oxfordian times predominantly pelagic and nektonic fossils were preserved, particularly coccol iths, belemnite rostra and arm hooks (onychites), and rare decapod remains. Stagnant conditions continued into the Early Cretaceous, when well-preserved marine palynomorphs and phytoplankton contributed to the high organic content of Aptian black sapropelic claystones deposited in a shallow, quiet water environment. Abrupt loss of all palynomorphs near the Aptian-Albian boundary coincides with the ventilation of this segment of the incipient South Atlantic Basin. Sharp changes in the benthic foraminiferal populations in the late Albian are attributed to down flank subsidence of the Falkland Plateau as seafloor spreading widened the South Atlantic Basin. A prominent stratigraphic hiatus encompassing most of the Cenomanian-Santonian suggests erosion and dissolution of calcareous microfossils by cold bottom currents. A return to normal pelagic sedimentation and a sharp 104
Margolis, S. V., andJ. P. Kennett. 1971. Cenozoic paleoglacial history of Antarctica recorded in subantarctic deep-sea cores. American Journal of Science, 271: 1-36. Opdyke, N. D., B. Glass,J. D. Hays, andJ. Foster. 1966. Paleomagnetic study of antarctic deep-sea cores. Science, 154(3748): 349-357. Quilty, P. G. 1973. Cenomanian-Turonian and Neogene sediments from northeast of Kerguelen Ridge, Indian Ocean. Geological Society of Australia Journal, 20: 361-370.
lowering of the carbonate compensation depth (CCD) is evidenced by a thick upper Campanian-Maestrichtian chalk sequence deposited at paleodepths close to present day. The Tertiary sequence (figures 1 and 2) is characterized by sharp fluctuations of the CCD (low stands during the late Paleocene-early Eocene, Oligocene, Miocene, and late Quaternary) and strong erosional events (Cretaceous-Tertiary boundary, Miocene, Miocene-Pliocene boundary). Conspicuous reworking of microfossils throughout the Miocene sequence (figure 2) occurred as a result of an increase in current velocity associated with the opening of Drake Passage and the establishment of the circumpolar current. A sharp change from calcareous ooze to diatom ooze and glacial marine sedimentation near the Miocene-Pliocene bou ridary (figure 2) plus erosional loss of considerable section represents intensification of the circumpolar current during the severe late Miocene antarctic glaciation (terminal Miocene event described by Van Couvering ci al., 1976). The rather extraordinary erosional history of the eastern portion of the Falkland Plateau is recounted by Ciesielski and Wise (1977), who found that during the latest Miocene, circumpolar deep water impinging from the southwest stripped away the upper sediment cover, exposing units at least as old as Maestrichtian (figure 3). They speculated that a concomitant northward expansion of the antarctic water mass forced the Polar Front (antarctic convergence) well to the north of the Maurice Ewing Bank, thus shutting off appreciable carbonate deposition until the southern limit of the Polar Front zone migrated to its present position along the southern margin of the plateau in late Pleistocene or perhaps Holocene times (figure 3). The late Miocene hiatus, so well developed on the Maurice Ewing Bank (Ciesielski ci al., 1977; Ciesielski and Wise, 1977), has now been documented in many sectors of the southern ocean and its environs (figure 4). The agent of erosion, however, was not always the same. In the Ross Sea the unconformity was formed by the grounded ice sheet (Hayes and Frakes, 1975); along the adjacent abyssal margin (Deep Sea Drilling Project sites 266 and 274), by bottom current winnowing (Frakes, 1975); in New Zealand, by glacial eustatic regression (Kennett and Watkins, 1974, as reinterpreted by Van Couvering ci al., 1976, and Weaver, 1976); and along the Agulhas Plateau, by climatically induced intensification of local or antarctic bottom currents (Tucholke and Carpenter, 1977). At DSDP site 328 (Malvinas Outer Basin adjacent to the Falkland Plateau) (Gombos, 1977), the agent probably was antarctic deep water or bottom water rather than circumpolar deep water. Correlations of this terminal Miocene event elsewhere in the world are given by Van Couvering ci al. (1976) and Peck ci al. (1976). ANTARCTIC JOURNAL
Kennett, J . P. 1977. Cenozoic evolution of antarctic glaciation, the Circum -Antarctic Ocean, and their impact on global paleoceanography. Journal of Geophysical Research, 82(27): 3843-3860. Kennett,J. P. 1978. The development of planktonic biogeography in the Southern Ocean during the Cenozoic. Marine Micropaleontology, 3(4): 301-345. Kozlova, 0. G. 1970. Diatoms in suspension and in bottom sediments in the Southern Ocean and Pacific Oceans. In: Antarctic Ecology (Vol. 1, M. W. Hoidgate, ed.). Academic Press, New York. pp. 148-153. Sciater, J . G., D. Abbott, and J. Theide. 1977. Paleobathymetry and sediments of the Indian Ocean. In: Indian Ocean Geology and Biostratigraphy U. R Heirtzler et al., eds.). American Geophysical Union, Washington, D.C. Sciater, J . G., S. Hellinger, and C. Tapscott. 1977. The paleobathymetry of the Antlantic Ocean from the Jurassic to the present. Journal of Geology, 85: 509-552.
References Dunbar, M.J. 1977. The evolution of polar ecosystems. In: Adaptations Within Antarctic Ecosystems (G. A. Llano, ed.). Smithsonian Institu tion, Washington, D.C. pp. 1063-1076.
Cenozoic microfossil datums in antarctic to subantarctic deep-sea sedimentary sequences J . P. KENNErF
Graduate School of Oceanography University of Rhode Island Kingston, Rhode Island 02881 The southern ocean, here considered to represent the approximate present-day positions of the subantarctic and antarctic water masses, represents an enormous area of the earth's surface. Biostratigraphic information from this region is important for a complete understanding of global biogeographic and biostratigraphic patterns during the Cenozoic and of the paleoceanographic and paleoclimatic controls of dynamic changes that are known to have occurred in the planktonic realm. The high-latitude regions of the Southern Hemisphere are marked by an almost complete lack of marine sedimentary sections on land. The antarctic continent lacks useful marine sequences; a few shallow marine sediment sections occur on and around the antarctic continent but essentially lack planktonic m icrofossils. Therefore, information about the marine Cenozoic microfossil succession must be gained from deep-sea sediments. Piston-coring programs have provided the most useful biostratigraphic information on Quarternary102
Weissel,J. K., D. E. Hayes, and E. M. Herron. 1977. Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous. Marine Geology , 25: 231-277.
Pliocene sections (see figure). This biostratigraphy has been very successfully tied to the magnetostratigraphy, providing a detailed dated sequence of microfossil events. In fact it was the southern ocean area where magnetostratigraphy was first successfully applied to the enhanced correlation and dating of biostratigraphic successions (Goodell and Watkins, 1968; Hays and Opdyke, 1967; Opdyke et al., 1966). Although most piston cores taken in the southern ocean are of Quaternary and Pliocene age, only a few are older and have provided brief glimpses of Tertiary and Late Cretaceous planktonic biogeography of the region (Margolis and Kennett, 1970, 1971; Quilty, 1973). The development of any reasonable understanding of southern ocean biostratigraphy by necessity thus awaited deep-sea drilling operations, which began in 1972. Four deep-sea drilling legs involving the Glomar Challenger have provided much new information on the Cenozoic. Cruises 28 and 29 south of Australia and New Zealand provided the most complete sequences, while cruises 35 and 36, although important, were less successful because of poorer drilling conditions. Numerous biostratigraphers have analysed the materials, and the results have been for the most part published in the initial reports of each of the expeditions. Before now (Kennett, in press), no attempts have been made to synthesize this body of information which consists of two categories: (a) biostratigraphic ranges of individual microfossil species for correlation purposes within and outside of the southern ocean; and (b) biogeographic information related to general distribution patterns, relative importance of major planktonic microfossil groups and diversity patterns throughout the Cenozoic. My contribution (Kennett, in press) is a compilation of biostratigraphic datums (first and last appearances) of the major planktonic microfossil groups ANTARCTIC JOURNAL
U 1-URAF" CALC/ 0 NANN
A ell
IrnIA(.rLL4TES is
Location of piston cores that have been used in the development of Quaternary to Pliocene biostratigraphy of the southern ocean region. The type of planktonic microfossil group studied in any particular core is indicated. Cores studied by Hays (1965) for radiolarian biostratigraphy are scattered over much of the southern ocean and are not plotted on this map. Positions of antarctic and subtropical convergences are shown.
in the southern ocean (planktonic foraminifera, calcareous nannofossils, diatoms, radiolaria, and silicoflagellates) studied by a large number of investigators. This compilation provides a sequence of integrated datums and hence a historical sequence of microfossil events through the Cenozoic. The datum levels of Quaternary and Pliocene age, their nature, relative position, and age are now well known because of integration with magnetostratigraphy, better sediment records, and more numerous studies. In contrast, the relative position of Miocene and older datum levels is much less firmly established. There are several reasons for this: poorer stratigraphic control because of a lack of a magnetostrati graphic framework; few sections that contain siliceous and calcareous microfossil assemblages in association to establish detailed relations; and poor stratigraphic representation especially in the Early Cenozoic. In general, most datum levels do not extend over both the antarctic and subantarctic regions. Instead, microfossil events, if they occur in both regions, can be highly diachronous between the two water-mass regimes. This is most strongly developed in the calcareous Cenozoic microfossil groups which, as in the present day, show major assemblage differences across the boundaries of these water masses. During the Cenozoic, major changes occurred in planktonic microfossil biogeography in the southern ocean as reflected in the biostratigraphic sequences. These changes were created in response to evolution of the southern ocean circulation system through the Cenozoic and the development of the antarctic and subantarctic water masses and high latitude climate. Eocene sediments are marked by a relatively high diverse and important calcareous microfossil October 1978
assemblage, even closely adjacent to the antarctic continent. During the Cenozoic, this calcareous microfossil province moved northward as cooler conditions developed and was replaced by a rich siliceous microfossil province. In the northern subantarctic area of the present day, calcareous microfossil diversity has remained relatively high throughout the Cenozoic compared with the antarctic area, providing a sequence of numerous datum levels very similar to that described in the temperate New Zealand area. Some enormous gaps in our knowledge still exist in the biostratigraphy of the southern ocean. Siliceous microfossil biostratigraphy of the subantarctic Miocene and older is still poorly known and has not yet been integrated into the calcareous microfossil sequence. Eocene biostratigraphy is also poorly known for the antarctic region, and the Late Miocene datum level sequence still requires much attention. Further deep-sea drilling is essential to provide the required marine sections. This research was supported by National Science Foundation grants DPP 77-06687 and OCE 76-81489.
References
Goodell, H. G., and N. D. Watkins. 1968. The paleomagnetic stratigraphy of the Southern Ocean: 200 West to 160° East longitude. Deep Sea Research, 15: 89-112. Hays,J. D. 1965. Radiolaria and late Tertiary and Quaternary history of antarctic seas. In: Biology of the Antarctic Seas II (American Geoph 'sical Union Antarctic Research Series), 5: 125-184. 103
Hays, J. D., and N. D. Opdyke. 1967. Antarctic radiolaria, magnetic reversals, and climatic change. Science, 158(3804): 1001-1011. Kennet, J . P. In press. Cenozoic microfossil datums in antarctic to subantarctic deep-sea sediment sequences. In: Integrated Cenozoic microfossil datums of the global ocean (N. de B. Hornibrook, ed.). Margolis, S. V., andJ. P. Kennett. 1970. Antarctic glaciation during the Tertiary recorded in subantarctic deep-sea cores. Science, 170: 1085-1087.
Paleontology and paleoenvironment of the Southwest Atlantic Ocean basin SHERWOOD W. WISE,JR.
Antarctic Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306
Over the past 4 years, research at the Antarctic Marine Geology Research Facility (Florida State University) has been concentrated on the southwest Atlantic sector of the southern ocean. Field work has involved one Deep Sea Drilling Project (DSDP) cruise and four ARA Islas Orcadas cruises to the area. This article is a condensation of an extended paleontological and paleoenviron mental summary by Wise ci al. (in press; also available from the author in preprint form) which covers some of the research to date. The summary is based on three DSDP leg 36 drill core sequences and a reconnaissance study of more than 75 piston cores taken on the Falkland (Malvinas) Plateau. Data from the three drill cores and selected piston cores are summarized in figures 1 and 2. Much of the drill core data was extracted from Barker ci al. (1977). The older sedimentary record (figure 1) suggests that a Middle (?) to Late Jurassic inland sea transgressed the southwest portion of Gondwanaland, then became progressively more restricted until by Oxfordian times predominantly pelagic and nektonic fossils were preserved, particularly coccol iths, belemnite rostra and arm hooks (onychites), and rare decapod remains. Stagnant conditions continued into the Early Cretaceous, when well-preserved marine palynomorphs and phytoplankton contributed to the high organic content of Aptian black sapropelic claystones deposited in a shallow, quiet water environment. Abrupt loss of all palynomorphs near the Aptian-Albian boundary coincides with the ventilation of this segment of the incipient South Atlantic Basin. Sharp changes in the benthic foraminiferal populations in the late Albian are attributed to down flank subsidence of the Falkland Plateau as seafloor spreading widened the South Atlantic Basin. A prominent stratigraphic hiatus encompassing most of the Cenomanian-Santonian suggests erosion and dissolution of calcareous microfossils by cold bottom currents. A return to normal pelagic sedimentation and a sharp 104
Margolis, S. V., andJ. P. Kennett. 1971. Cenozoic paleoglacial history of Antarctica recorded in subantarctic deep-sea cores. American Journal of Science, 271: 1-36. Opdyke, N. D., B. Glass,J. D. Hays, andJ. Foster. 1966. Paleomagnetic study of antarctic deep-sea cores. Science, 154(3748): 349-357. Quilty, P. G. 1973. Cenomanian-Turonian and Neogene sediments from northeast of Kerguelen Ridge, Indian Ocean. Geological Society of Australia Journal, 20: 361-370.
lowering of the carbonate compensation depth (CCD) is evidenced by a thick upper Campanian-Maestrichtian chalk sequence deposited at paleodepths close to present day. The Tertiary sequence (figures 1 and 2) is characterized by sharp fluctuations of the CCD (low stands during the late Paleocene-early Eocene, Oligocene, Miocene, and late Quaternary) and strong erosional events (Cretaceous-Tertiary boundary, Miocene, Miocene-Pliocene boundary). Conspicuous reworking of microfossils throughout the Miocene sequence (figure 2) occurred as a result of an increase in current velocity associated with the opening of Drake Passage and the establishment of the circumpolar current. A sharp change from calcareous ooze to diatom ooze and glacial marine sedimentation near the Miocene-Pliocene bou ridary (figure 2) plus erosional loss of considerable section represents intensification of the circumpolar current during the severe late Miocene antarctic glaciation (terminal Miocene event described by Van Couvering ci al., 1976). The rather extraordinary erosional history of the eastern portion of the Falkland Plateau is recounted by Ciesielski and Wise (1977), who found that during the latest Miocene, circumpolar deep water impinging from the southwest stripped away the upper sediment cover, exposing units at least as old as Maestrichtian (figure 3). They speculated that a concomitant northward expansion of the antarctic water mass forced the Polar Front (antarctic convergence) well to the north of the Maurice Ewing Bank, thus shutting off appreciable carbonate deposition until the southern limit of the Polar Front zone migrated to its present position along the southern margin of the plateau in late Pleistocene or perhaps Holocene times (figure 3). The late Miocene hiatus, so well developed on the Maurice Ewing Bank (Ciesielski ci al., 1977; Ciesielski and Wise, 1977), has now been documented in many sectors of the southern ocean and its environs (figure 4). The agent of erosion, however, was not always the same. In the Ross Sea the unconformity was formed by the grounded ice sheet (Hayes and Frakes, 1975); along the adjacent abyssal margin (Deep Sea Drilling Project sites 266 and 274), by bottom current winnowing (Frakes, 1975); in New Zealand, by glacial eustatic regression (Kennett and Watkins, 1974, as reinterpreted by Van Couvering ci al., 1976, and Weaver, 1976); and along the Agulhas Plateau, by climatically induced intensification of local or antarctic bottom currents (Tucholke and Carpenter, 1977). At DSDP site 328 (Malvinas Outer Basin adjacent to the Falkland Plateau) (Gombos, 1977), the agent probably was antarctic deep water or bottom water rather than circumpolar deep water. Correlations of this terminal Miocene event elsewhere in the world are given by Van Couvering ci al. (1976) and Peck ci al. (1976). ANTARCTIC JOURNAL
