Glacial geology of Seymour Island
Glacial geology_________________________ Glacial geology of Seymour Island DAVID
H. ELLIOT
Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210
Seymour Island lies southeast of the northern end of the Antarctic Peninsula (figure 1). The islands in that region lie in a precipitation shadow and thus have more snow and ice-free land than has any other part of the peninsula area except the South Shetland Islands. The upper Cretaceous and lower Tertiary bedrock (Elliot et al. 1975) consists of very poorly consolidated sediments; less than 5 percent of the sequence forms resistant bedrock ledges. The sediments between the resistant ledges may be more lithified than their surface outcrop suggests because of freeze and thaw effects during midsummer. The rate of erosion of the sediments doubtless is high. The island can be divided into two physiographic provinces: (1) in the northeast a meseta that is the remnant of an erosion surface and is covered by glacial drift, and (2) in the southwest a ridge and valley topography underlain by a homoclinal sequence of Cretaceous sediments and lacking a cover, or evidence of a former cover, of glacial drift (figure 2). Whether the erosion surface extended southwest over the Cretaceous beds remains uncertain. The top of the meseta is covered, probably thinly, by glacial drift with a lag gravel at the surface; the maximum depth of the drift is uncertain but probably does not exceed 1 meter. The seasonal thaw and washing out of fines by summer melt, together with wind action (deflation), contribute to the drift thinning around much of the margin of the meseta. Pebble counts (table) of clasts in the drift show a range of rock types that can be matched with the bedrock exposed in the northern Antarctic Peninsula. The basalt pebbles are similar to the Pliocene to Pleistocene alkali-basaltic rocks of the James Ross Island Volcanic Group (Nelson 1975; Rex 1976). Low-grade metasedimentary clasts include sandstones, siltstones, and quartzites that can be matched with the upper Paleozoic-lower Mesozoic Trinity Peninsula Group (Aitkenhead 1975; Hyden and Tanner 1981). Granitic and dioritic plutonic rocks are like those of the Andean Intrusive Suite, of late Mesozoic to early Cenozoic age; the intermediate to silicic 66
volcanic rocks resemble those of the upper Mesozoic (Upper Jurassic to ?Cretaceous) Antarctic Peninsula Volcanic Group (Aitkenhead 1975). The coarse sandstones, listed separately, may be derived from the Cretaceous sequence now exposed on James Ross, Snow, and Seymour islands (Bibby 1966), or possibly from older rocks not now exposed. Large glacial erratics were first noted by Andersson (1906). The boulders, up to 3 x 2.5 x 1.5 meters, are scattered over the top of the meseta, on or in the glacial drift, and on the
•
_
Erebus and Zzo
Terror
Gulf i.
James Ross Island
64°S
41k.
PSeymour
.....Island Weddell Sea Snow Hill Island 0 km 50 58W -'__._rI_. 56W
Upper Tertiary James Ross Island Volcanic Group
1111111111 Lower Tertiary Seymour Island Group Jurassic to Cretaceous sedimentary rocks Upper Mesozoic Antarctic Peninsula Volcanic Group
Em
Upper Paleozoic to Lower Mesozoic Trinity Peninsula Group Upper Mesozoic Andean plutons
Figure 1. Location map for Seymour island, and geological sketch map of the northern Antarctic Peninsula. The extent of rock outcrop Is greatly exaggerated except for Seymour island and the northeastern point of Snow Hill island.
ANTARCTIC JOURNAL
Pebble counts of the coarse fraction of the glacial drift at three sites on the southern end of the meseta, Seymour Island
Pebble size Basalt Metasedimentary rocks Granitic rocks Dioritic rocks Silicic volcanic rocks Sandstone
Site 3 Site 2 Site 1 2.5_10cma 10-60 cm 2.5-10cm 10-60 cm 2.5-10cm 10-65 cm 56 17 6 0 19 2
33 31 13 9 13 1
53 15 7 2 20 3
17 25 22 7 25 4
51 18 8 6 20 3
36 19 19 5 15 6
Note. All counts based on 100 pebbles each. Each count includes the maximum clast size encountered. a cm = centimeters.
meseta flanks where they are isolated, the fines having been washed away. Most of these scattered large erratics consist of plutonic and metasedimentary rocks similar to those in the pebble counts, but some are of pebbly mudstone, a rock type not found in the pebble counts but which is sparsely distributed in the Trinity Peninsula Group on both the southeast and northwest flanks of the peninsula (Aitkenhead 1975; Elliot 1965, 1966). The occurrence of these large erratics around the flanks of the meseta, which itself is covered with glacial drift, suggests that the northern part of Seymour Island has a glacial history different from the southern part. Snow Hill Island, to the southwest of Seymour Island, is largely covered by an icecap, but at the northern tip has exposed rock which apparently lacks a cover of glacial drift. The composition of the clasts in the glacial drift on the northern part of Seymour Island implies transport by ice from the mainland. At least three broad alternatives, which enlarge on the explanations offered by Andersson (1906), can be proposed: 1. The southern part of Seymour Island and the adjacent part of Snow Hill Island were covered by ice, but all glacial drift derived from the mainland has been removed because of
Upper Tertiary drift
1 Lower Tertiary strata
I71 []
Cape Wiman
Cretaceous strata
. Pebble count sites
64°15'S
Bodman Pt.
lur island 560454
0 km 5
Figure 2. Geological sketch map of Seymour Island.
1981 REVIEW
long subaerial exposure and/or high rates of erosion of the soft bedrock. 2. At the time of drift deposition (as an ice-contact deposit) the local icecap on Snow Hill Island was much larger and diverted mainland ice to the north and east. Only the northern part of Seymour Island was covered by ice derived from the mainland. 3. The glacial drift on the northern part of Seymour Island was deposited from floating ice. Although the first two alternatives cannot be completely discounted, the third alternative is favored. It implies that the northern part of the island has been uplifted at least 200 meters; the rapid rate of coastal erosion (Zinsmeister 1979) and prominent marine terraces (Zinsmeister 1980) support the inference of recent uplift. Fieldwork was supported by National Science Foundation grant OPP 74-21509. References Aitkenhead, N. 1975. The geology of the Duse Bay-Larsen Inlet area, north-east Graham Land (with particular reference to the Trinity Peninsula Series) (Scientific Report 51). Cambridge: British Antarctic Survey. Andersson, J. G. 1906. On the geology of Graham Land. Bulletin of the Geological Institution of the University of Upsala, 7, 19-71. Bibby, J. S. 1966. The stratigraphy of part of north-east Graham Land and the James Ross Island group (Scientific Report 53). Cambridge: British Antarctic Survey. Elliot, D. H. 1965. Geology of north-west Trinity Peninsula, Graham Land. British Antarctic Survey Bulletin, 7, 1-24. Elliot, D. H. 1966. Geology of the Nordenskjbld Coast and comparison with north-west Trinity Peninsula, Graham Land. British Antarctic Survey Bulletin, 10, 1-43. Elliot, D. H., Rinaldi, C., Zinsmeister, W. J . , Trautman, T. A., Bryant, W. A., and del Valle, R. 1975. Geological investigations on Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 10(4), 183-186. Hyden,G., and Tanner, P. W. G. 1981. Late Palaeozoic-early Mesozoic fore-arc basin sedimentary rocks at the Pacific margin in western Antarctica. Geologische Rundschau, 70, 529-541. Nelson, P. H. H. 1975. The James Ross Island Volcanic Group of northeast Graham Land (Scientific Report 54). Cambridge: British Antarctic Survey. Rex, D. C. 1976. Geochronology in relation to the stratigraphy of the Antarctic Peninsula. British Antarctic Survey Bulletin, 43, 49-58. Zinsmeister, W. J . 1979. Coastal erosion on Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 14(5), 16-17. Zinsmeister, W. J. 1980. Marine Terraces of Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 15(5), 25-26.
67
H. ELLIOT
Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210
Seymour Island lies southeast of the northern end of the Antarctic Peninsula (figure 1). The islands in that region lie in a precipitation shadow and thus have more snow and ice-free land than has any other part of the peninsula area except the South Shetland Islands. The upper Cretaceous and lower Tertiary bedrock (Elliot et al. 1975) consists of very poorly consolidated sediments; less than 5 percent of the sequence forms resistant bedrock ledges. The sediments between the resistant ledges may be more lithified than their surface outcrop suggests because of freeze and thaw effects during midsummer. The rate of erosion of the sediments doubtless is high. The island can be divided into two physiographic provinces: (1) in the northeast a meseta that is the remnant of an erosion surface and is covered by glacial drift, and (2) in the southwest a ridge and valley topography underlain by a homoclinal sequence of Cretaceous sediments and lacking a cover, or evidence of a former cover, of glacial drift (figure 2). Whether the erosion surface extended southwest over the Cretaceous beds remains uncertain. The top of the meseta is covered, probably thinly, by glacial drift with a lag gravel at the surface; the maximum depth of the drift is uncertain but probably does not exceed 1 meter. The seasonal thaw and washing out of fines by summer melt, together with wind action (deflation), contribute to the drift thinning around much of the margin of the meseta. Pebble counts (table) of clasts in the drift show a range of rock types that can be matched with the bedrock exposed in the northern Antarctic Peninsula. The basalt pebbles are similar to the Pliocene to Pleistocene alkali-basaltic rocks of the James Ross Island Volcanic Group (Nelson 1975; Rex 1976). Low-grade metasedimentary clasts include sandstones, siltstones, and quartzites that can be matched with the upper Paleozoic-lower Mesozoic Trinity Peninsula Group (Aitkenhead 1975; Hyden and Tanner 1981). Granitic and dioritic plutonic rocks are like those of the Andean Intrusive Suite, of late Mesozoic to early Cenozoic age; the intermediate to silicic 66
volcanic rocks resemble those of the upper Mesozoic (Upper Jurassic to ?Cretaceous) Antarctic Peninsula Volcanic Group (Aitkenhead 1975). The coarse sandstones, listed separately, may be derived from the Cretaceous sequence now exposed on James Ross, Snow, and Seymour islands (Bibby 1966), or possibly from older rocks not now exposed. Large glacial erratics were first noted by Andersson (1906). The boulders, up to 3 x 2.5 x 1.5 meters, are scattered over the top of the meseta, on or in the glacial drift, and on the
•
_
Erebus and Zzo
Terror
Gulf i.
James Ross Island
64°S
41k.
PSeymour
.....Island Weddell Sea Snow Hill Island 0 km 50 58W -'__._rI_. 56W
Upper Tertiary James Ross Island Volcanic Group
1111111111 Lower Tertiary Seymour Island Group Jurassic to Cretaceous sedimentary rocks Upper Mesozoic Antarctic Peninsula Volcanic Group
Em
Upper Paleozoic to Lower Mesozoic Trinity Peninsula Group Upper Mesozoic Andean plutons
Figure 1. Location map for Seymour island, and geological sketch map of the northern Antarctic Peninsula. The extent of rock outcrop Is greatly exaggerated except for Seymour island and the northeastern point of Snow Hill island.
ANTARCTIC JOURNAL
Pebble counts of the coarse fraction of the glacial drift at three sites on the southern end of the meseta, Seymour Island
Pebble size Basalt Metasedimentary rocks Granitic rocks Dioritic rocks Silicic volcanic rocks Sandstone
Site 3 Site 2 Site 1 2.5_10cma 10-60 cm 2.5-10cm 10-60 cm 2.5-10cm 10-65 cm 56 17 6 0 19 2
33 31 13 9 13 1
53 15 7 2 20 3
17 25 22 7 25 4
51 18 8 6 20 3
36 19 19 5 15 6
Note. All counts based on 100 pebbles each. Each count includes the maximum clast size encountered. a cm = centimeters.
meseta flanks where they are isolated, the fines having been washed away. Most of these scattered large erratics consist of plutonic and metasedimentary rocks similar to those in the pebble counts, but some are of pebbly mudstone, a rock type not found in the pebble counts but which is sparsely distributed in the Trinity Peninsula Group on both the southeast and northwest flanks of the peninsula (Aitkenhead 1975; Elliot 1965, 1966). The occurrence of these large erratics around the flanks of the meseta, which itself is covered with glacial drift, suggests that the northern part of Seymour Island has a glacial history different from the southern part. Snow Hill Island, to the southwest of Seymour Island, is largely covered by an icecap, but at the northern tip has exposed rock which apparently lacks a cover of glacial drift. The composition of the clasts in the glacial drift on the northern part of Seymour Island implies transport by ice from the mainland. At least three broad alternatives, which enlarge on the explanations offered by Andersson (1906), can be proposed: 1. The southern part of Seymour Island and the adjacent part of Snow Hill Island were covered by ice, but all glacial drift derived from the mainland has been removed because of
Upper Tertiary drift
1 Lower Tertiary strata
I71 []
Cape Wiman
Cretaceous strata
. Pebble count sites
64°15'S
Bodman Pt.
lur island 560454
0 km 5
Figure 2. Geological sketch map of Seymour Island.
1981 REVIEW
long subaerial exposure and/or high rates of erosion of the soft bedrock. 2. At the time of drift deposition (as an ice-contact deposit) the local icecap on Snow Hill Island was much larger and diverted mainland ice to the north and east. Only the northern part of Seymour Island was covered by ice derived from the mainland. 3. The glacial drift on the northern part of Seymour Island was deposited from floating ice. Although the first two alternatives cannot be completely discounted, the third alternative is favored. It implies that the northern part of the island has been uplifted at least 200 meters; the rapid rate of coastal erosion (Zinsmeister 1979) and prominent marine terraces (Zinsmeister 1980) support the inference of recent uplift. Fieldwork was supported by National Science Foundation grant OPP 74-21509. References Aitkenhead, N. 1975. The geology of the Duse Bay-Larsen Inlet area, north-east Graham Land (with particular reference to the Trinity Peninsula Series) (Scientific Report 51). Cambridge: British Antarctic Survey. Andersson, J. G. 1906. On the geology of Graham Land. Bulletin of the Geological Institution of the University of Upsala, 7, 19-71. Bibby, J. S. 1966. The stratigraphy of part of north-east Graham Land and the James Ross Island group (Scientific Report 53). Cambridge: British Antarctic Survey. Elliot, D. H. 1965. Geology of north-west Trinity Peninsula, Graham Land. British Antarctic Survey Bulletin, 7, 1-24. Elliot, D. H. 1966. Geology of the Nordenskjbld Coast and comparison with north-west Trinity Peninsula, Graham Land. British Antarctic Survey Bulletin, 10, 1-43. Elliot, D. H., Rinaldi, C., Zinsmeister, W. J . , Trautman, T. A., Bryant, W. A., and del Valle, R. 1975. Geological investigations on Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 10(4), 183-186. Hyden,G., and Tanner, P. W. G. 1981. Late Palaeozoic-early Mesozoic fore-arc basin sedimentary rocks at the Pacific margin in western Antarctica. Geologische Rundschau, 70, 529-541. Nelson, P. H. H. 1975. The James Ross Island Volcanic Group of northeast Graham Land (Scientific Report 54). Cambridge: British Antarctic Survey. Rex, D. C. 1976. Geochronology in relation to the stratigraphy of the Antarctic Peninsula. British Antarctic Survey Bulletin, 43, 49-58. Zinsmeister, W. J . 1979. Coastal erosion on Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 14(5), 16-17. Zinsmeister, W. J. 1980. Marine Terraces of Seymour Island, Antarctic Peninsula. Antarctic Journal of the U.S., 15(5), 25-26.
67
