Pre-Devonian Paleozoic rocks of the central Transantarctic Mountains
Pre-Devonian Paleozoic rocks of the central Transantarctic Mountains A.J. ROWELL Department of Geology
and
Museum of Invertebrate Paleontology University of Kansas Lawrence, Kansas 66045
M. N. REES Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 89154 P. BRADDOCK
Queen Charlotte College Picton, New Zealand
The Lower Cambrian Shackleton Limestone and possible Silurian strata are the only pre-Devonian Paleozoic rocks that have been reported previously from the central Transantarctic Mountains in the 400-kilometer segment between the southern Holyoake Range and the Beardmore Glacier. The areal distribution of Cambrian strata is well known through earlier mapping projects (Grindley 1963; Laird 1963; Young and Ryburn 1968; Laird, Mansergh, and Chappell 1971). The existence of fossiliferous marine Silurian rocks from this part of Antarctica, or indeed from anywhere on the continent, is less firmly established. Our investigation during the 1985-1986 austral summer season was a logical continuation of fieldwork in the northern Holyoake Range during the previous summer (Rees, Rowell, and Pratt 1985). Although our overall objective was to understand better the early Paleozoic evolution of the margin of the east antarctic craton and its setting in Gondwana, we recognized three immediate targets: to document the age of the Shackleton Limestone, its depositional setting, and tectonic history; to determine the extent of the post-Shackleton episode of uplift that recently was recognized as the cause for the postorogenic deposition of the Douglas Conglomerate (Rees, Rowell, and Pratt 1985); and to re-evaluate the evidence for fossiliferous marine Silurian rocks from Antarctica. Outstanding support by members of VXE-6 squadron, helicopter flight coordination by John E Splettstoesser, the location of the Beardmore Camp, and good weather enabled us to contribute significantly to all three points, as is briefly reported below. The field party consisted of Rowell, Rees, and Braddock who were in Antarctica from middle November 1985 until early January 1986. Work was done from four tent-camps put in by helicopter in the southern Holyoake Range (82°15'S 160°20'E), the northern (82°51'S 159°26'E) and southern (82°55'S 159°11'E) Cotton Plateau, and The Cloudmaker moraines of the Beardmore Glacier (84°15'S 168°55'E). Additional fieldwork was aided by helicopter close support in the southern Holyoake 48
Range and in the Beardmore Glacier area around Mount Bowers (85°02'S 162°51'E) and the moraines of Buckley Island (84°55'S 164°09'E), Plunket Point (85°04'S 166°32'E), and Lizard Point (84°49'S 162°47'E). We were ably assisted by Bill Ryan from the University of Michigan at the latter locality. Although the Shackleton Limestone does not crop out for 200 kilometers between the Mount Rabot area (83°15'S 161°20'E) and Mount Bowers, small exposures at the latter locality, together with an abundance of erratics in the most recent moraines of the upper Beardmore Glacier, show that the limestone was laterally extensive. The Shackleton was deposited on a broad, shallow-subtidal to peritidal platform that probably covered the entire area of investigation and extended northward at least to the Byrd Glacier (80°30'S 158°00'E). Cryptalgal laminites and cyanobacteria-archaeocyathan boundstones are associated with rare bioclastic grainstones and oolites. Burrowed carbonate wackestones are locally characteristic. These limestones carry a fauna indicating a middle and late Early Cambrian age (Debrenne and Kruse in press; Rowell et al. in preparation). Sandstones and interbedded sandstone and limestone sequences, from exposures near limestone outcrops, are also included in the Shackleton and are commonly considered its basal beds (Laird, Mansergh, and Chappell 1971). However, at all localities examined, these sand-rich sequences are faulted against pre-Cambrian strata and no fauna has been recovered from them. Their sedimentary structures, although typically obscured by deformation, locally suggest accumulation as subtidal bars and on a shallow shelf. Collectively, these characteristics and relationships suggest that the sand-rich sequences are slightly older than the limestones and are at least partially Early Cambrian in age. The Shackleton Limestone is very strongly deformed and the effects of at least two phases of folding are visible in major cliff exposures (figure 1). Such deformation, combined with the effects of persistent faulting, precludes accurate determination of the true thickness of the unit. The principal deformation of the Shackleton Limestone predates deposition of the overlying Douglas Conglomerate as evidenced, in part, by calcite-veined clasts of the Shackleton, with its typical archaeocyathan fauna, incorporated in the Douglas (figure 2). Similar conglomerates, previously included in the Shackleton at its type section, now are considered to be down-dropped fault blocks of the much younger Douglas Conglomerate (figure 3). The Douglas Conglomerate and correlative strata were formerly known only between the Byrd Glacier and the southern Holyoake Range where it unconformably overlies the Shackleton and represents deposition, at least in part, in alluvial fan settings (Rees, Rowell, and Pratt 1985). Our 1985-1986 investigation showed, however, that the conglomerate was deposited over a much greater area because comparable rocks occur some 400 kilometers to the south in the Beardmore moraines upstream from The Cloudmaker. The diversity of lithologies present in the morainic blocks is closely comparable to that in the Holyoake Range outcrops and includes conglomerates with archaeocyathan-bearing clasts. The source of this morainic material is unknown but probably is concealed by ice close to the upper reaches of the Beardmore Glacier. At Lizard Point, near the head of the glacier, blocks of Douglas Conglomerate exceed 3 meters in maximum dimension. The widespread distribution of these conglomerates suggests that uplift and subaerial exposure subsequent to deformation of the Shackleton Limestone was a feature of much of the central Transantarctic Mountains. The Douglas Conglomerate and underlying rocks were strongly tilted by a second phase of movement and were extenANTARCTIC JOURNAL
4
,
Figure 2. Part of a Shackleton Limestone clast in the Douglas Conglomerate, southern Holyoake Range. Clast with archaeocyathans (arrow) was veined with calcite before deposition of the Douglas.
r
Figure 3. Douglas Conglomerate, i )reground, faulted against Shackleton Limestone at the type locdty of the latter formation in the southern Holyoake Range (82 012'S 160014'E). Figure 1. Complex folding of the Shackleton Limestone. A. Upper cliffs of the southeast face of Cambrian Bluff (82 051'S 160035'E), Holoyake Range, viewed from the air. B. Northwest facing slope at the southern end of Hochstein Ridge (82 051'S 159038'E), Queen Elizabeth Range.
sively eroded prior to the Devonian (Rees, Rowe!!, and Pratt 1985). Thus, the age, origin, and depositional setting of the Douglas Conglomerate together with its later history imply that during much or all of the Ordovician and Silurian, this segment of the Transantarctic Mountains was subjected to tectonism and erosion and that coarse-clastic sediment was deposited. Fossi!iferous marine Silurian strata, however, have been claimed to occur in the region based on erratics reputedly collected by Frank Wild from The Cloudmaker moraines on the Beardmore Glacier (Jell 1981). As is discussed in more detail elsewhere (Rowell, Rees, and Braddock in preparation), we failed to confirm the report and conclude that although the material on which the claim is based is probably Silurian, its locality infor1986 REVIEW
mation was incorrectly recorded during curation. It is unlikely to have been collected by Wild and probably did not come from Antarctica. This research was supported by National Science Foundation grant DI'P 83-17966 to the University of Kansas. References Debrenne, E, and P. Kruse. In press. Shackleton Limestone archaeocyath s A icheringa. Grindley, G.W. 1963. The geology of the Queen Alexandra Range, Beardmore Glacier, Ross dependency, Antarctica. New Zealand Journal of Geology and Geophysics, 6(3), 307-347. Jell, J.S. 1981. Silurian tabulate corals from the Cloudmaker Moraine, Beardmore Glacier, Antarctica. Alcheringa, 5, 311-316. Laird, M.G. 1963. Geomorphology and stratigraphy of the Nimrod Glacier-Beaumont Bay region southern Victoria Land, Antarctica. New Zealand Journal of Geology and G1: Jiysics, 6(3), 465-484.
49
Laird, M.G., G.D. Mansergh, an J.M. Chappell. 1971. Geology of the central Nimrod Glacier area, Antarctica. New Zealand Journal of Geology and Geophysics, 14(3), 427-468. Rees, M.N., A.J. Rowell, and B.R. Pratt. 1985. The Byrd Group of the Holyoake Range, central Transantarctic Mountains. Antarctic Journal of the U.S., 20(5), 3-5. Rowell, A.J., M.N. Rees, and P. Braddock. In preparation. Silurian
marine fauna not confirmed from Antarctica. Alcht'ringa. Rowell, A.J., M.N. Rees, R.A. Cooper, and B.R. Pratt. In preparation. Early Paleozoic history of the central Transantarctic Mountains: Evidence from the Holyoake Range, Antarctica. Geology. Young, D.J., and R.J. Ryburn. 1968. The geology of Buckley and Darwin Nunataks, Beardmore Glacier, Ross dependency, Antarctica. New Zealand Journal of Geology and Geophysics, 11(4), 922-939.
Beardmore project, 1985-1986
fossils, and (3) the tectonic evolution of the Transantarctic Mountains.
D.H. ELLIOT
I.?
auticalMs
Institute of Polar Studies
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
J.
SPLETTSTOESSER
LQmbrian Bluff
4_5
165°E82
NiFnr0d
83°S Mi
! J Mta
Moo Range(2 NJG4S
j^
Minnesota Geological Survey University of Minnesota St. Paul, Minnesota 55114
D.B. WALDRIP
She/f
84°S
ITT/Antarctic Services, Inc. Paramus, New Jersey 07652
Beardmore South camp was constructed at 84°00'S 164°24'43"E (elevation 1,816.1 meters) (figure 1) during the 1984-1985 austral summer season by employees of ITT/Antarctic Services, Inc., to provide a meeting facility for the Antarctic Treaty Workshop which was held 7-13 January 1985. The camp was secured for the winter after the workshop concluded. It was reoccupied by ITT/ANS employees in late October 1985 to prepare the camp for science operation in the 1985-1986 season. A short project in atmospheric sciences was conducted from late October to early November by Austin Hogan, and the first group of Earth science researchers arrived on 12 November, the science projects beginning the next day. Science operations ended on 23 January 1986, and all scientists then in camp returned to McMurdo on 24 January. Beardmore camp, as it was called in the 1985-1986 season, was constructed about 35 kilometers north-northeast of the site of a camp near Coalsack Bluff that was supported by helicopters and used by geologists and others during the 1969-1970 austral summer (Elliot 1970). Science activities. A workshop held at the University of Maine in 1983 concluded that the Beardmore Glacier region offered the opportunity to address a number of important topics in the Earth sciences and identified three themes for concentrated research effort. The three themes were (1) the antarctic ice sheet and global ice ages, (2) the Gondwana sequence and its unique 50
0
Elizabeth
Ts7 rickp
Dominion Range
"
Mt. 'exler
/
RO
00 VV 023 /\Otway Massif ^ N afical 0 KUometers 50
Figure 1. Location map of the Beardmore Glacier area. The circle is the outer limit of the flying range (185 kilometers, or 100 nautical miles) of the UH-1 N helicopters. (The coordinates of the northeast corner of the science hut by Doppler satellite receiver are courtesy of the U.S. Geological Survey.) ANTARCTIC JOURNAL
and
Museum of Invertebrate Paleontology University of Kansas Lawrence, Kansas 66045
M. N. REES Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 89154 P. BRADDOCK
Queen Charlotte College Picton, New Zealand
The Lower Cambrian Shackleton Limestone and possible Silurian strata are the only pre-Devonian Paleozoic rocks that have been reported previously from the central Transantarctic Mountains in the 400-kilometer segment between the southern Holyoake Range and the Beardmore Glacier. The areal distribution of Cambrian strata is well known through earlier mapping projects (Grindley 1963; Laird 1963; Young and Ryburn 1968; Laird, Mansergh, and Chappell 1971). The existence of fossiliferous marine Silurian rocks from this part of Antarctica, or indeed from anywhere on the continent, is less firmly established. Our investigation during the 1985-1986 austral summer season was a logical continuation of fieldwork in the northern Holyoake Range during the previous summer (Rees, Rowell, and Pratt 1985). Although our overall objective was to understand better the early Paleozoic evolution of the margin of the east antarctic craton and its setting in Gondwana, we recognized three immediate targets: to document the age of the Shackleton Limestone, its depositional setting, and tectonic history; to determine the extent of the post-Shackleton episode of uplift that recently was recognized as the cause for the postorogenic deposition of the Douglas Conglomerate (Rees, Rowell, and Pratt 1985); and to re-evaluate the evidence for fossiliferous marine Silurian rocks from Antarctica. Outstanding support by members of VXE-6 squadron, helicopter flight coordination by John E Splettstoesser, the location of the Beardmore Camp, and good weather enabled us to contribute significantly to all three points, as is briefly reported below. The field party consisted of Rowell, Rees, and Braddock who were in Antarctica from middle November 1985 until early January 1986. Work was done from four tent-camps put in by helicopter in the southern Holyoake Range (82°15'S 160°20'E), the northern (82°51'S 159°26'E) and southern (82°55'S 159°11'E) Cotton Plateau, and The Cloudmaker moraines of the Beardmore Glacier (84°15'S 168°55'E). Additional fieldwork was aided by helicopter close support in the southern Holyoake 48
Range and in the Beardmore Glacier area around Mount Bowers (85°02'S 162°51'E) and the moraines of Buckley Island (84°55'S 164°09'E), Plunket Point (85°04'S 166°32'E), and Lizard Point (84°49'S 162°47'E). We were ably assisted by Bill Ryan from the University of Michigan at the latter locality. Although the Shackleton Limestone does not crop out for 200 kilometers between the Mount Rabot area (83°15'S 161°20'E) and Mount Bowers, small exposures at the latter locality, together with an abundance of erratics in the most recent moraines of the upper Beardmore Glacier, show that the limestone was laterally extensive. The Shackleton was deposited on a broad, shallow-subtidal to peritidal platform that probably covered the entire area of investigation and extended northward at least to the Byrd Glacier (80°30'S 158°00'E). Cryptalgal laminites and cyanobacteria-archaeocyathan boundstones are associated with rare bioclastic grainstones and oolites. Burrowed carbonate wackestones are locally characteristic. These limestones carry a fauna indicating a middle and late Early Cambrian age (Debrenne and Kruse in press; Rowell et al. in preparation). Sandstones and interbedded sandstone and limestone sequences, from exposures near limestone outcrops, are also included in the Shackleton and are commonly considered its basal beds (Laird, Mansergh, and Chappell 1971). However, at all localities examined, these sand-rich sequences are faulted against pre-Cambrian strata and no fauna has been recovered from them. Their sedimentary structures, although typically obscured by deformation, locally suggest accumulation as subtidal bars and on a shallow shelf. Collectively, these characteristics and relationships suggest that the sand-rich sequences are slightly older than the limestones and are at least partially Early Cambrian in age. The Shackleton Limestone is very strongly deformed and the effects of at least two phases of folding are visible in major cliff exposures (figure 1). Such deformation, combined with the effects of persistent faulting, precludes accurate determination of the true thickness of the unit. The principal deformation of the Shackleton Limestone predates deposition of the overlying Douglas Conglomerate as evidenced, in part, by calcite-veined clasts of the Shackleton, with its typical archaeocyathan fauna, incorporated in the Douglas (figure 2). Similar conglomerates, previously included in the Shackleton at its type section, now are considered to be down-dropped fault blocks of the much younger Douglas Conglomerate (figure 3). The Douglas Conglomerate and correlative strata were formerly known only between the Byrd Glacier and the southern Holyoake Range where it unconformably overlies the Shackleton and represents deposition, at least in part, in alluvial fan settings (Rees, Rowell, and Pratt 1985). Our 1985-1986 investigation showed, however, that the conglomerate was deposited over a much greater area because comparable rocks occur some 400 kilometers to the south in the Beardmore moraines upstream from The Cloudmaker. The diversity of lithologies present in the morainic blocks is closely comparable to that in the Holyoake Range outcrops and includes conglomerates with archaeocyathan-bearing clasts. The source of this morainic material is unknown but probably is concealed by ice close to the upper reaches of the Beardmore Glacier. At Lizard Point, near the head of the glacier, blocks of Douglas Conglomerate exceed 3 meters in maximum dimension. The widespread distribution of these conglomerates suggests that uplift and subaerial exposure subsequent to deformation of the Shackleton Limestone was a feature of much of the central Transantarctic Mountains. The Douglas Conglomerate and underlying rocks were strongly tilted by a second phase of movement and were extenANTARCTIC JOURNAL
4
,
Figure 2. Part of a Shackleton Limestone clast in the Douglas Conglomerate, southern Holyoake Range. Clast with archaeocyathans (arrow) was veined with calcite before deposition of the Douglas.
r
Figure 3. Douglas Conglomerate, i )reground, faulted against Shackleton Limestone at the type locdty of the latter formation in the southern Holyoake Range (82 012'S 160014'E). Figure 1. Complex folding of the Shackleton Limestone. A. Upper cliffs of the southeast face of Cambrian Bluff (82 051'S 160035'E), Holoyake Range, viewed from the air. B. Northwest facing slope at the southern end of Hochstein Ridge (82 051'S 159038'E), Queen Elizabeth Range.
sively eroded prior to the Devonian (Rees, Rowe!!, and Pratt 1985). Thus, the age, origin, and depositional setting of the Douglas Conglomerate together with its later history imply that during much or all of the Ordovician and Silurian, this segment of the Transantarctic Mountains was subjected to tectonism and erosion and that coarse-clastic sediment was deposited. Fossi!iferous marine Silurian strata, however, have been claimed to occur in the region based on erratics reputedly collected by Frank Wild from The Cloudmaker moraines on the Beardmore Glacier (Jell 1981). As is discussed in more detail elsewhere (Rowell, Rees, and Braddock in preparation), we failed to confirm the report and conclude that although the material on which the claim is based is probably Silurian, its locality infor1986 REVIEW
mation was incorrectly recorded during curation. It is unlikely to have been collected by Wild and probably did not come from Antarctica. This research was supported by National Science Foundation grant DI'P 83-17966 to the University of Kansas. References Debrenne, E, and P. Kruse. In press. Shackleton Limestone archaeocyath s A icheringa. Grindley, G.W. 1963. The geology of the Queen Alexandra Range, Beardmore Glacier, Ross dependency, Antarctica. New Zealand Journal of Geology and Geophysics, 6(3), 307-347. Jell, J.S. 1981. Silurian tabulate corals from the Cloudmaker Moraine, Beardmore Glacier, Antarctica. Alcheringa, 5, 311-316. Laird, M.G. 1963. Geomorphology and stratigraphy of the Nimrod Glacier-Beaumont Bay region southern Victoria Land, Antarctica. New Zealand Journal of Geology and G1: Jiysics, 6(3), 465-484.
49
Laird, M.G., G.D. Mansergh, an J.M. Chappell. 1971. Geology of the central Nimrod Glacier area, Antarctica. New Zealand Journal of Geology and Geophysics, 14(3), 427-468. Rees, M.N., A.J. Rowell, and B.R. Pratt. 1985. The Byrd Group of the Holyoake Range, central Transantarctic Mountains. Antarctic Journal of the U.S., 20(5), 3-5. Rowell, A.J., M.N. Rees, and P. Braddock. In preparation. Silurian
marine fauna not confirmed from Antarctica. Alcht'ringa. Rowell, A.J., M.N. Rees, R.A. Cooper, and B.R. Pratt. In preparation. Early Paleozoic history of the central Transantarctic Mountains: Evidence from the Holyoake Range, Antarctica. Geology. Young, D.J., and R.J. Ryburn. 1968. The geology of Buckley and Darwin Nunataks, Beardmore Glacier, Ross dependency, Antarctica. New Zealand Journal of Geology and Geophysics, 11(4), 922-939.
Beardmore project, 1985-1986
fossils, and (3) the tectonic evolution of the Transantarctic Mountains.
D.H. ELLIOT
I.?
auticalMs
Institute of Polar Studies
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
J.
SPLETTSTOESSER
LQmbrian Bluff
4_5
165°E82
NiFnr0d
83°S Mi
! J Mta
Moo Range(2 NJG4S
j^
Minnesota Geological Survey University of Minnesota St. Paul, Minnesota 55114
D.B. WALDRIP
She/f
84°S
ITT/Antarctic Services, Inc. Paramus, New Jersey 07652
Beardmore South camp was constructed at 84°00'S 164°24'43"E (elevation 1,816.1 meters) (figure 1) during the 1984-1985 austral summer season by employees of ITT/Antarctic Services, Inc., to provide a meeting facility for the Antarctic Treaty Workshop which was held 7-13 January 1985. The camp was secured for the winter after the workshop concluded. It was reoccupied by ITT/ANS employees in late October 1985 to prepare the camp for science operation in the 1985-1986 season. A short project in atmospheric sciences was conducted from late October to early November by Austin Hogan, and the first group of Earth science researchers arrived on 12 November, the science projects beginning the next day. Science operations ended on 23 January 1986, and all scientists then in camp returned to McMurdo on 24 January. Beardmore camp, as it was called in the 1985-1986 season, was constructed about 35 kilometers north-northeast of the site of a camp near Coalsack Bluff that was supported by helicopters and used by geologists and others during the 1969-1970 austral summer (Elliot 1970). Science activities. A workshop held at the University of Maine in 1983 concluded that the Beardmore Glacier region offered the opportunity to address a number of important topics in the Earth sciences and identified three themes for concentrated research effort. The three themes were (1) the antarctic ice sheet and global ice ages, (2) the Gondwana sequence and its unique 50
0
Elizabeth
Ts7 rickp
Dominion Range
"
Mt. 'exler
/
RO
00 VV 023 /\Otway Massif ^ N afical 0 KUometers 50
Figure 1. Location map of the Beardmore Glacier area. The circle is the outer limit of the flying range (185 kilometers, or 100 nautical miles) of the UH-1 N helicopters. (The coordinates of the northeast corner of the science hut by Doppler satellite receiver are courtesy of the U.S. Geological Survey.) ANTARCTIC JOURNAL
