a Tillite, glacial striae, and hyaloclastite associations on Hobbs Coast ...
Table 1. Tentative correlation of glacial events in Pensacola Mountains with those of McMurdo Sound McMurdo Sound (Denton et al., 1971)
Pensacola Mts. (this study)
Absolute Time Scale
Taylor I
T I:
present time
Co
Alpine I
02
I)
ca
>
0
into E
Taylor II Taylor Ill
U)
W:t .0 U)
12,200 yr BP
.9 D
1.6-2.1 m.y.
a
>
I
Taylor IV
0) Cd E
I> 0)0
Taylor V
CL
0
cc
i
2.7-3.5 m.y. >4 m.y.
>a)O
cc Jô 5 A
Miocene
7
The glacial record in Davis Valley is worthy of a much more detailed investigation than was possible during this brief study of only a few days. The lake and glaciofluvial features are particularly noteworthy, as evidence for such processes in this part of the continent has not previously been reported. One of the greatest problems will be in establishing the absolute age of these features and of the moraines with which they are associated. Correlations as shown in table 1 can at present only be considered as highly speculative. This report is a product of grant DPP 77-22765 made to the U.S. Geological Survey. References Andersen, B. G. 1963. Preliminary report on glaciology and glacial geology of the Thiel Mountains, Antarctica. In U.S. Geological Survey Professional Paper 475-B, 37: B140-43.
Tillite, glacial striae, and hyaloclastite associations on Hobbs Coast, Marie Byrd Land
GEORGE W. GRINDLEY New Zealand Geological Survey Lower Hutt, New Zealand
WESLEY E. LEMASURIER
WILLIAM C. MCINTOSH
Natural and Physical Sciences Division University of Colorado at Denver Denver, Colorado 80202
University of Colorado at Boulder Boulder, Colorado 80302
OLLE MELANDER Department of Physical Geography University of Stockholm Sweden 48
Aughenbaugh, N. B. 1961. Preliminary report on the geology of the Dufek Massif. In IGY World Data Center A, Glaciology, Glaciology Report, 4: 155-93. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1970. Late Cenozoic glaciation in Antarctica: The record in the McMurdo Sound region. Antarctic Journal of the United States, 5: 15-21. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. Late Cenozoic glacial history of Antarctica. In Late Cenozoic GlacialAges, ed. K. K. Turekian, pp. 276-306. New Haven: Yale University Press. Ford, A. B., R. L. Reynolds, Carl Huie, and S. J . Boyer. 1979. Geological investigation of the Dufek intrusion, Pensacola Mountains, 1978-79. Antarctic Journal of the United States (this issue). Grindley, G. W. 1967. The geomorphology of the Miller Range, Transantarctic mountains: with note on the glacial history and neotectonics of East Antarctica. New Zealand Journal of Geology and Geophysics, 10: 557-98.
During the 1977-78 field season, we discovered an excellent exposure of tillite interbedded with hyaloclastite at Shibuya Peak (75°9'S/133°39'W) and fresh glacial striae on granitic bedrock directly beneath hyaloclastite at Bowyer Butte (74°57'S/134°45'W). The relationships we observed are similar to those described from the
Jones Mountains, 1,150 kilometers to the east along the same coastline (Rutford et at., 1972). These discoveries provide direct evidence for a subglacial (as opposed to submarine) environment of formation for the basaltic hyaloclastites at these localities, and they also provide a basis for distinguishing between glacial erratics and xenoliths in other hyaloclastite localities. Those discoveries, first announced in this journal last year (LeMasurier, McIntosh, and Tewksbury, 1978; Karlén and Melander, 1978) are described in more detail in this article. Shibuya Peak and Bowyer Butte are isolated nunataks located about 40 kilometers apart along the Hobbs Coast. They lie within a group of north-south-trending fault block nunataks that are composed of pre-Cenozoic granitic and metamorphic rocks and overlain by Upper Tertiary basaltic hyaloclastites and lava flows. The unconformity is an erosion surface of generally low relief that is exposed at different levels in different nunataks. At Bowyer Butte, the unconformity lies approximately 800 meters above sea level and 400 meters above the level of the ice sheet. The basement rock here is Unweathered granodiorite porphyry. Very fresh glacial striations on the granodiorite pass beneath the hyaloclastite (Karlén and Melander, 1978); at one spot, hyaloclastite slabs can be peeled off the unconformity to expose the underlying striations. The unconformity is overlain by approximately 5 meters of hyaloclastite, which in turn is overlain by basalt flow rock of a thickness that is undetermined but known to be less than 100 meters. We searched, unsuccessfully, for tillite and glacial erratics at this locality. Basalt collected in 1967 from a nearby locality on Bowyer Butte yielded an apparent potassium-argon age of 13± 2 million years (LeMasurier and Rex, in press). The potassium-argon age of basalt from the striation site is now being determined. At Shibuya Peak, basement rock lies below ice level, and the entire nunatak is composed of subhorizontally
:._
exposure of tillite-rich hyaloclastite on the north-facing slope of Shibuya Peak, looking east. Light-colored clasts are granitic basement rock.
stratified basaltic hyaloclastite, with an exposed thickness of roughly 240 meters. Tillite-rich hyaloclastite occurs within roughly the lower one-third of this section. The tillite-rich hyaloclastite is distinguished by an abundance of rounded boulders of plutonic and metamorphic basement rock imbedded in a matrix of unsorted and very poorly stratified hyaloclastite (see figure). The largest boulders are roughly 40-50 centimeters in diameter, and they range downward in size to rock fragments and crystal clasts that are visible in thin sections of the matrix. A rough visual estimate indicated that these clasts make up 10-30 percent of the deposit. The matrix hyaloclastite is composed predominantly of fragments of basaltic glass less than 0.5 centimeter in diameter. However, larger clasts are common, and they almost invariably are more crystalline than the smaller clasts and are essentially holocrystalline if larger than about 5 centimeters in diameter. It is quite possible that some of these clasts were transported by ice from other localities and deposited with the granitic boulders. This caused us concern in selecting samples to date the deposit. Only the freshest, most common, and least rounded clasts were selected for dating. A sample collected uphill from the tillite in 1967 yielded a potassium-argon age of 4.4± 0.2 million years (LeMasurier and Rex, in press). Multiple samples are now being dated from the tillite locality. A very few of the granitic boulders are sheathed in a thin layer of lava, which raises the question of how to distinguish between glacial till and xenoliths. Our conclusion that very few of the non-volcanic clasts are likely to be xenoliths will be documented in a future report. The relationships in this deposit suggest that the eruption of basalt took place beneath an ice sheet that was carrying a substantial amount of moraine. The eruption evidently melted the glacier and incorporated its load into hyaloclastite that was forming as lava came in contact with the ice and glacial meltwater. The lower onethird of the deposit (roughly 80 meters of thickness) appears to represent this interaction. In the upper part of the deposit, the hyaloclastite becomes finer grained and better stratified, and plutonic cobbles become smaller and more widely scattered. Although the total thickness of tillite-rich hyaloclastite may seem large, the total thickness of tillite without the hyaloclastite matrix would probably be on the order of 20 meters. This still suggests a rather large glacier, particularly in view of the fact that it probably represents almost instantaneous deposition of moraine, rather than accumulation around a stationary or slowly retreating ice margin. In summary, the Bowyer Butte and Shibuya Peak discoveries add further documentation to earlier interpretations that hyaloclastites in Marie Byrd Land and Ellsworth Land are a kind of glacial deposit—the product of subglacial volcanic eruptions like those in Iceland (Rutford et al., 1972; LeMasurier, 1972; LeMasurier and Rex, in press). The Shibuya Peak discovery suggests the possibility that the rounded, fresh granitic boulders occasionally observed in other hyaloclastite deposits in this region may be glacial erratics rather than xenoliths. Our field work in Marie Byrd Land has been supported by National Science Foundation grants DPP 7604396 to the University of Colorado and DPP 76-24209 to the University of Maine. G. W. Grindley participated 49
in our fieldwork as a visiting scientist representing the New Zealand Geological Survey.
References Karlén, Wibjörn, and 011e Melander. 1978. Reconnaissance of the glacial geology of Hobbs Coast and Ruppert Coast, Marie Byrd Land. Antarctic Journal of the United States, 13: 46-47. LeMasurier, W. E. 1972. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In Antarctic Geology and Geophysics, ed. R. J. Adie, pp. 251-60. Oslo: Universitetsforlaget.
Cape Spirit mirabilite beds
LeMasurier, W. E., W. C. McIntosh, and D. A. Tewksbury. 1978. Volcanoes in the Hobbs Coast and Ruppert Coast sectors of Marie Byrd Land. Antarctic Journal of the United States, 13: 31-32. LeMasurier, W. E., and D. C. Rex. In press. Volcanic record of Cenozoic glacial history in Marie Byrd Land and western Ellsworth Land II: Revised chronology and evaluation of tectonic factors. In Proceedings of the Third Symposium on Antarctic Geology and Geophysics, Madison, Wisconsin, 22-27 August 1977. Rutford, R., C. Craddock, C. White, and R. Armstrong. 1972. Tertiary glaciations in the Jones Mountains. In Antarctic Geology and Geophysics, ed. R. J . Adie, pp. 239-43. Oslo: Universitetsforlaget.
MARK LECKIE
Geology Department Northern Illinois Univesity De Kalb, Illinois 60115
HOWARD THOMAS BRADY School of of Biological Sciences Macquarie University North Ryde, New South Wales, Australia
and RICHARD WHITE
In January 1979, further mapping was carried out on a large system of mirabilite beds on the Ross Ice Shelf
30
40
50
7800
10
20 162
163
164 - 165
166
167
Figure 1. Location map of major mirablilte beds, McMurdo Sound region. 50
168
Pensacola Mts. (this study)
Absolute Time Scale
Taylor I
T I:
present time
Co
Alpine I
02
I)
ca
>
0
into E
Taylor II Taylor Ill
U)
W:t .0 U)
12,200 yr BP
.9 D
1.6-2.1 m.y.
a
>
I
Taylor IV
0) Cd E
I> 0)0
Taylor V
CL
0
cc
i
2.7-3.5 m.y. >4 m.y.
>a)O
cc Jô 5 A
Miocene
7
The glacial record in Davis Valley is worthy of a much more detailed investigation than was possible during this brief study of only a few days. The lake and glaciofluvial features are particularly noteworthy, as evidence for such processes in this part of the continent has not previously been reported. One of the greatest problems will be in establishing the absolute age of these features and of the moraines with which they are associated. Correlations as shown in table 1 can at present only be considered as highly speculative. This report is a product of grant DPP 77-22765 made to the U.S. Geological Survey. References Andersen, B. G. 1963. Preliminary report on glaciology and glacial geology of the Thiel Mountains, Antarctica. In U.S. Geological Survey Professional Paper 475-B, 37: B140-43.
Tillite, glacial striae, and hyaloclastite associations on Hobbs Coast, Marie Byrd Land
GEORGE W. GRINDLEY New Zealand Geological Survey Lower Hutt, New Zealand
WESLEY E. LEMASURIER
WILLIAM C. MCINTOSH
Natural and Physical Sciences Division University of Colorado at Denver Denver, Colorado 80202
University of Colorado at Boulder Boulder, Colorado 80302
OLLE MELANDER Department of Physical Geography University of Stockholm Sweden 48
Aughenbaugh, N. B. 1961. Preliminary report on the geology of the Dufek Massif. In IGY World Data Center A, Glaciology, Glaciology Report, 4: 155-93. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1970. Late Cenozoic glaciation in Antarctica: The record in the McMurdo Sound region. Antarctic Journal of the United States, 5: 15-21. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. Late Cenozoic glacial history of Antarctica. In Late Cenozoic GlacialAges, ed. K. K. Turekian, pp. 276-306. New Haven: Yale University Press. Ford, A. B., R. L. Reynolds, Carl Huie, and S. J . Boyer. 1979. Geological investigation of the Dufek intrusion, Pensacola Mountains, 1978-79. Antarctic Journal of the United States (this issue). Grindley, G. W. 1967. The geomorphology of the Miller Range, Transantarctic mountains: with note on the glacial history and neotectonics of East Antarctica. New Zealand Journal of Geology and Geophysics, 10: 557-98.
During the 1977-78 field season, we discovered an excellent exposure of tillite interbedded with hyaloclastite at Shibuya Peak (75°9'S/133°39'W) and fresh glacial striae on granitic bedrock directly beneath hyaloclastite at Bowyer Butte (74°57'S/134°45'W). The relationships we observed are similar to those described from the
Jones Mountains, 1,150 kilometers to the east along the same coastline (Rutford et at., 1972). These discoveries provide direct evidence for a subglacial (as opposed to submarine) environment of formation for the basaltic hyaloclastites at these localities, and they also provide a basis for distinguishing between glacial erratics and xenoliths in other hyaloclastite localities. Those discoveries, first announced in this journal last year (LeMasurier, McIntosh, and Tewksbury, 1978; Karlén and Melander, 1978) are described in more detail in this article. Shibuya Peak and Bowyer Butte are isolated nunataks located about 40 kilometers apart along the Hobbs Coast. They lie within a group of north-south-trending fault block nunataks that are composed of pre-Cenozoic granitic and metamorphic rocks and overlain by Upper Tertiary basaltic hyaloclastites and lava flows. The unconformity is an erosion surface of generally low relief that is exposed at different levels in different nunataks. At Bowyer Butte, the unconformity lies approximately 800 meters above sea level and 400 meters above the level of the ice sheet. The basement rock here is Unweathered granodiorite porphyry. Very fresh glacial striations on the granodiorite pass beneath the hyaloclastite (Karlén and Melander, 1978); at one spot, hyaloclastite slabs can be peeled off the unconformity to expose the underlying striations. The unconformity is overlain by approximately 5 meters of hyaloclastite, which in turn is overlain by basalt flow rock of a thickness that is undetermined but known to be less than 100 meters. We searched, unsuccessfully, for tillite and glacial erratics at this locality. Basalt collected in 1967 from a nearby locality on Bowyer Butte yielded an apparent potassium-argon age of 13± 2 million years (LeMasurier and Rex, in press). The potassium-argon age of basalt from the striation site is now being determined. At Shibuya Peak, basement rock lies below ice level, and the entire nunatak is composed of subhorizontally
:._
exposure of tillite-rich hyaloclastite on the north-facing slope of Shibuya Peak, looking east. Light-colored clasts are granitic basement rock.
stratified basaltic hyaloclastite, with an exposed thickness of roughly 240 meters. Tillite-rich hyaloclastite occurs within roughly the lower one-third of this section. The tillite-rich hyaloclastite is distinguished by an abundance of rounded boulders of plutonic and metamorphic basement rock imbedded in a matrix of unsorted and very poorly stratified hyaloclastite (see figure). The largest boulders are roughly 40-50 centimeters in diameter, and they range downward in size to rock fragments and crystal clasts that are visible in thin sections of the matrix. A rough visual estimate indicated that these clasts make up 10-30 percent of the deposit. The matrix hyaloclastite is composed predominantly of fragments of basaltic glass less than 0.5 centimeter in diameter. However, larger clasts are common, and they almost invariably are more crystalline than the smaller clasts and are essentially holocrystalline if larger than about 5 centimeters in diameter. It is quite possible that some of these clasts were transported by ice from other localities and deposited with the granitic boulders. This caused us concern in selecting samples to date the deposit. Only the freshest, most common, and least rounded clasts were selected for dating. A sample collected uphill from the tillite in 1967 yielded a potassium-argon age of 4.4± 0.2 million years (LeMasurier and Rex, in press). Multiple samples are now being dated from the tillite locality. A very few of the granitic boulders are sheathed in a thin layer of lava, which raises the question of how to distinguish between glacial till and xenoliths. Our conclusion that very few of the non-volcanic clasts are likely to be xenoliths will be documented in a future report. The relationships in this deposit suggest that the eruption of basalt took place beneath an ice sheet that was carrying a substantial amount of moraine. The eruption evidently melted the glacier and incorporated its load into hyaloclastite that was forming as lava came in contact with the ice and glacial meltwater. The lower onethird of the deposit (roughly 80 meters of thickness) appears to represent this interaction. In the upper part of the deposit, the hyaloclastite becomes finer grained and better stratified, and plutonic cobbles become smaller and more widely scattered. Although the total thickness of tillite-rich hyaloclastite may seem large, the total thickness of tillite without the hyaloclastite matrix would probably be on the order of 20 meters. This still suggests a rather large glacier, particularly in view of the fact that it probably represents almost instantaneous deposition of moraine, rather than accumulation around a stationary or slowly retreating ice margin. In summary, the Bowyer Butte and Shibuya Peak discoveries add further documentation to earlier interpretations that hyaloclastites in Marie Byrd Land and Ellsworth Land are a kind of glacial deposit—the product of subglacial volcanic eruptions like those in Iceland (Rutford et al., 1972; LeMasurier, 1972; LeMasurier and Rex, in press). The Shibuya Peak discovery suggests the possibility that the rounded, fresh granitic boulders occasionally observed in other hyaloclastite deposits in this region may be glacial erratics rather than xenoliths. Our field work in Marie Byrd Land has been supported by National Science Foundation grants DPP 7604396 to the University of Colorado and DPP 76-24209 to the University of Maine. G. W. Grindley participated 49
in our fieldwork as a visiting scientist representing the New Zealand Geological Survey.
References Karlén, Wibjörn, and 011e Melander. 1978. Reconnaissance of the glacial geology of Hobbs Coast and Ruppert Coast, Marie Byrd Land. Antarctic Journal of the United States, 13: 46-47. LeMasurier, W. E. 1972. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In Antarctic Geology and Geophysics, ed. R. J. Adie, pp. 251-60. Oslo: Universitetsforlaget.
Cape Spirit mirabilite beds
LeMasurier, W. E., W. C. McIntosh, and D. A. Tewksbury. 1978. Volcanoes in the Hobbs Coast and Ruppert Coast sectors of Marie Byrd Land. Antarctic Journal of the United States, 13: 31-32. LeMasurier, W. E., and D. C. Rex. In press. Volcanic record of Cenozoic glacial history in Marie Byrd Land and western Ellsworth Land II: Revised chronology and evaluation of tectonic factors. In Proceedings of the Third Symposium on Antarctic Geology and Geophysics, Madison, Wisconsin, 22-27 August 1977. Rutford, R., C. Craddock, C. White, and R. Armstrong. 1972. Tertiary glaciations in the Jones Mountains. In Antarctic Geology and Geophysics, ed. R. J . Adie, pp. 239-43. Oslo: Universitetsforlaget.
MARK LECKIE
Geology Department Northern Illinois Univesity De Kalb, Illinois 60115
HOWARD THOMAS BRADY School of of Biological Sciences Macquarie University North Ryde, New South Wales, Australia
and RICHARD WHITE
In January 1979, further mapping was carried out on a large system of mirabilite beds on the Ross Ice Shelf
30
40
50
7800
10
20 162
163
164 - 165
166
167
Figure 1. Location map of major mirablilte beds, McMurdo Sound region. 50
168
