A Cretaceous Islas Orcadas core from the Falkland (Malvinas) Plateau ...
Coring. The coring program had three objectives: (1) to explore for and sample older (pre-Pliocene) sediments along the northeast flank of the Maurice Ewing Bank (eastern Falkland Plateau) and on the Northeast Georgia Rise in order to elucidate the geologic history of these features, (2) to extend the circumpolar bottom sediment survey into the southeast Atlantic sector of the southern ocean, and (3) to obtain closely spaced cores on a north-south transect beneath the polar front in order to study the late Neogene history of this important oceanographic feature. Core stations beneath the polar front were taken in conjunction with physical oceanographic stations to provide optimum integration of modern sediment and hydrologic data. Fifty piston and trigger cores were taken; sediment was recovered from 45 piston and 18 trigger cores. Four of the piston cores were presented to the shipboard representative of the Argentine Museum of Natural Science. Piston core sediment recovery totaled 443.27 meters (average 8.93 meters per core) and trigger core recovery was 7.40 meters (average 14.8 centimeters per core). Lithologies sampled ranged from calcareous nanofossil ooze, diatom ooze, foraminiferal ooze, and red clay to sands, gravels, glacial marine clastics, volcanic ash, and oceanic basalt. The siliceous microfossils in core catcher samples were age-dated aboard ship using available microscopic equipment. Core data will be available from the Department of Geology, Florida State University, Tallahassee, Florida 32306. Of six cores taken on the Maurice Ewing Bank, four penetrated to older sediments ranging from Eocene to Miocene in age. This suite of cores complements a series of 22 cores taken on Islas Orcadas cruise 7 and adds important new information concerning the depositional and erosional history of the Falkland Plateau. Nine cores across the northeast Georgia Rise recovered Plio- Pleistocene sediment, but did not penetrate into older units which seismic profile records show exposed just beneath 'lative thin mantle of glacial marine sediments and oo Thirteen cores were taken at hydrographic stations or during geophysical surveying in the Malvinas Outer Basin, the South Sandwich Island Arc, and along the easterly track from the South Sandwich Trench to 8°W. longitude. The final 21 cores were taken at 30' to 45' intervals along a north-northeast track from 58 0 to 45°S., which passed across the African-Antarctic midocean ridge and the polar front at 50 0 -49 0 S. Examination of core top samples indicates that existing sediment facies maps for the area need revision. Basaltic ocean crust was apparently sampled by one and possibly two of the cores taken adjacent to the ridge axis. Although core density along the transect was limited by bad weather and mechanical problems, the transect provides the most detailed sampling yet available for study of the sedimentological facies of the region, their relationship to bottom topography and, through time, the paleoposition of the polar front. Acknowledgments. We thank the Argentine Navy Hydrographic Service and Captain Lestrade and the officers and crew of Islas Orcadas for their help and cooperation, which made cruise 11 so successful. We are grateful to Lieutenant Parrodi, the oceanographic officers, the oceanographic watch, and Paul Dudley-Hart for their help in maintaining operation and supervision of our on-deck operations in often very unpleasant weather. For help with preparation of this manuscript we thank October 1977
Alan Brown for photographic assistance and Marjorie Knapp for drafting the figures.
A Cretaceous Islas Orcadas core from the Falkland (Malvinas) Plateau, southwest Atlantic PAUL F. CIE5IEL5KI', WILLIAM V. SLITER2, FRANK H. WIND', and SHERWOOD W. WISE, JR.'
'A ntarctzc Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306 2 U. S. Geological Survey Menlo Park, California 94025 The few older (pre-Pliocene) cores in the Eltanin collection were taken primarily along the flanks of intermediate depth aseismic features such as the Kerguelen Plateau and the Naturaliste Plateau (examples, see Kaharoeddin, Weaver and Wise, 1973; Quilty, 1973; Constans and Wise, 1974). These cores have been important in reconstructing the geologic history of these regions. In some cases they have allowed important seismic reflectors to be dated (such as a mid-Cretaceous reflector on the Kerguelen Plateau). In other cases, they provide the only direct evidence of the character and age of the sediment and microfossils assemblages present, thereby shedding light on the paleoenvironmental history of the region. Due to the unusual value of these older cores, an effort has been made during planning of the Islas Orcadas cruises to survey the several prominent intermediate depth aseismic features that lie in the South Atlantic sector of the southern ocean. These efforts have yielded 22 older cores, one of which is of Cretaceous age, the third Mesozoic core taken since the circumantarctic sediment survey began in 1963. Islas Orcadas core 07-75-44 was taken 60 kilometers northeast of DSDP drill site 327 on the eastern Falkland Plateau (figure 1). A stratigraphic and paleoenvironmental analysis of this core and a comparison with similar aged material from DSDP hole 327A is given by Ciesielski et al. (1977). A summary of that paper follows. Islas Orcadas core 07-75-44 penetrated CampanianMaestrichtian carbonate ooze and chert (figure 2). Despite its relatively close proximity to DSDP site 327, a significant change in the planktic faunas and floras (particularly the nannofloras), believed to be temperature dependent, is noted between the two sites. Noted in particular is a sharp reduction in the numbers of the planktic foraminifera species Globotruncana arca (Cushman) going from north to south across the Plateau. Similarly, at DSDP site 327 the tropical coccolith species Biscutum constans (Gorka) (common at the more northerly site) is replaced by the species Biscutum magnum Wind and Wise and B. coronum Wind 65
60
45•
30
Figure 1. Location of Islas Orcadas piston core 07•75•44 and Glomar Challenger drill hole 327A on the eastern Falkland Plateau, Southwest Atlantic (from Ciesielski et al., 1977).
and Wise. These latter species appear to prefer cooler waters. Because of the relatively short latitudinal distance between the two sites (62 kilometers), the sharp faunal and floral changes observed are believed to have resulted not from changes across a normal surface temperature gradient but rather from a steepened gradient produced at the confluence of two separate water masses. As shown in figure 3, two primary paleocurrent systems are suggested to have converged along the Falkland Plateau during Late Cretaceous times. These would have served as a topographic barrier separating the major water masses. The warmer northern current would be the South Atlantic gyre, whereas the current to the south would be analogous to the present West Wind Drift. Assuming that the South American-Antarctic Peninsula isthmus was essentially closed at this time (see Barker et al., 1977), the southerly current could have caused upwelling along the plateau, thus promoting the formation of the biogenic silica and/or chert now found in the Late Cretaceous sediments. This research was supported by National Science Foundation grant OPP 74 . 20109. Operational support by the Argentine Naval Hydrographic Service is gratefully acknowledged. References
Barker, P.F., I.W.D. Dalziel, D.H. Elliott, C.C. von der Broch. 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. Ciesielski, P.F. 1975. Biostratigraphy and paleoecology of Neogene and Oligene 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. p. 625-691. Ciesielski, P.F., W.V. Sliter, F.H. Wind, and S.W. Wise, 1977. Paleoenvironmental analysis and correlation of a Cretaceous Islas Orcadas core from the Falkland Plateau, Southwest Atlantic. Marine Micropaleontology, 2(1): 27-34. 66
Figure 2. Lithologic section and occurrence of selected diatoms, silicoflagellates, and calcareous nannoplankton in Islas Orcadas core 07.75.44 (from Ciesielskl et al., 1977). The diatom and silicoflagellate zones and taxa are described in Gombos (1977) and Ciesielski (1975). ANTARCTIC JOURNAL
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
Alan Brown for photographic assistance and Marjorie Knapp for drafting the figures.
A Cretaceous Islas Orcadas core from the Falkland (Malvinas) Plateau, southwest Atlantic PAUL F. CIE5IEL5KI', WILLIAM V. SLITER2, FRANK H. WIND', and SHERWOOD W. WISE, JR.'
'A ntarctzc Marine Geology Research Facility Department of Geology Florida State University Tallahassee, Florida 32306 2 U. S. Geological Survey Menlo Park, California 94025 The few older (pre-Pliocene) cores in the Eltanin collection were taken primarily along the flanks of intermediate depth aseismic features such as the Kerguelen Plateau and the Naturaliste Plateau (examples, see Kaharoeddin, Weaver and Wise, 1973; Quilty, 1973; Constans and Wise, 1974). These cores have been important in reconstructing the geologic history of these regions. In some cases they have allowed important seismic reflectors to be dated (such as a mid-Cretaceous reflector on the Kerguelen Plateau). In other cases, they provide the only direct evidence of the character and age of the sediment and microfossils assemblages present, thereby shedding light on the paleoenvironmental history of the region. Due to the unusual value of these older cores, an effort has been made during planning of the Islas Orcadas cruises to survey the several prominent intermediate depth aseismic features that lie in the South Atlantic sector of the southern ocean. These efforts have yielded 22 older cores, one of which is of Cretaceous age, the third Mesozoic core taken since the circumantarctic sediment survey began in 1963. Islas Orcadas core 07-75-44 was taken 60 kilometers northeast of DSDP drill site 327 on the eastern Falkland Plateau (figure 1). A stratigraphic and paleoenvironmental analysis of this core and a comparison with similar aged material from DSDP hole 327A is given by Ciesielski et al. (1977). A summary of that paper follows. Islas Orcadas core 07-75-44 penetrated CampanianMaestrichtian carbonate ooze and chert (figure 2). Despite its relatively close proximity to DSDP site 327, a significant change in the planktic faunas and floras (particularly the nannofloras), believed to be temperature dependent, is noted between the two sites. Noted in particular is a sharp reduction in the numbers of the planktic foraminifera species Globotruncana arca (Cushman) going from north to south across the Plateau. Similarly, at DSDP site 327 the tropical coccolith species Biscutum constans (Gorka) (common at the more northerly site) is replaced by the species Biscutum magnum Wind and Wise and B. coronum Wind 65
60
45•
30
Figure 1. Location of Islas Orcadas piston core 07•75•44 and Glomar Challenger drill hole 327A on the eastern Falkland Plateau, Southwest Atlantic (from Ciesielski et al., 1977).
and Wise. These latter species appear to prefer cooler waters. Because of the relatively short latitudinal distance between the two sites (62 kilometers), the sharp faunal and floral changes observed are believed to have resulted not from changes across a normal surface temperature gradient but rather from a steepened gradient produced at the confluence of two separate water masses. As shown in figure 3, two primary paleocurrent systems are suggested to have converged along the Falkland Plateau during Late Cretaceous times. These would have served as a topographic barrier separating the major water masses. The warmer northern current would be the South Atlantic gyre, whereas the current to the south would be analogous to the present West Wind Drift. Assuming that the South American-Antarctic Peninsula isthmus was essentially closed at this time (see Barker et al., 1977), the southerly current could have caused upwelling along the plateau, thus promoting the formation of the biogenic silica and/or chert now found in the Late Cretaceous sediments. This research was supported by National Science Foundation grant OPP 74 . 20109. Operational support by the Argentine Naval Hydrographic Service is gratefully acknowledged. References
Barker, P.F., I.W.D. Dalziel, D.H. Elliott, C.C. von der Broch. 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. Ciesielski, P.F. 1975. Biostratigraphy and paleoecology of Neogene and Oligene 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. p. 625-691. Ciesielski, P.F., W.V. Sliter, F.H. Wind, and S.W. Wise, 1977. Paleoenvironmental analysis and correlation of a Cretaceous Islas Orcadas core from the Falkland Plateau, Southwest Atlantic. Marine Micropaleontology, 2(1): 27-34. 66
Figure 2. Lithologic section and occurrence of selected diatoms, silicoflagellates, and calcareous nannoplankton in Islas Orcadas core 07.75.44 (from Ciesielskl et al., 1977). The diatom and silicoflagellate zones and taxa are described in Gombos (1977) and Ciesielski (1975). ANTARCTIC JOURNAL
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
