Glacial geology______

Glacial geology of Seymour Island 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 DAVID H. ELLIOT on James Ross, Snow, and Seymour islands (Bibby 1966), or possibly from older rocks not now exposed. Institute of Polar Studies and Large glacial erratics were first noted by Andersson (1906). Department of Geology and Mineralogy The boulders, up to 3 x 2.5 x 1.5 meters, are scattered over The Ohio State University the top of the meseta, on or in the glacial drift, and on the 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 lith- ified than their surface outcrop suggests because of freeze and Erebus thaw effects during midsummer. The rate of erosion of the and sediments doubtless is high. Zzo Terror The island can be divided into two physiographic provinces: 64°S (1) in the northeast a meseta that is the remnant of an erosion Gulf surface and is covered by glacial drift, and (2) in the southwest James Ross a ridge and valley topography underlain by a homoclinal Island i. 41k. sequence of Cretaceous sediments and lacking a cover, or PSeymour evidence of a former cover, of glacial drift (figure 2). Whether .....Island the erosion surface extended southwest over the Cretaceous Weddell Sea beds remains uncertain. Snow Hill The top of the meseta is covered, probably thinly, by glacial Island drift with a lag gravel at the surface; the maximum depth of 0 km 50 the drift is uncertain but probably does not exceed 1 meter. 58W -__._rI_. 56W The seasonal thaw and washing out of fines by summer melt, together with wind action (deflation), contribute to the drift Upper Tertiary James Ross Island Volcanic Group thinning around much of the margin of the meseta. 1111111111 Lower Tertiary Seymour Island Group Pebble counts (table) of clasts in the drift show a range of Jurassic to Cretaceous sedimentary rocks rock types that can be matched with the bedrock exposed in the northern Antarctic Peninsula. The basalt pebbles are simi- Upper Mesozoic Antarctic Peninsula Volcanic Group lar to the Pliocene to Pleistocene alkali-basaltic rocks of the Em Upper Paleozoic to Lower Mesozoic Trinity Peninsula Group James Ross Island Volcanic Group (Nelson 1975; Rex 1976). Upper Mesozoic Andean plutons Low-grade metasedimentary clasts include sandstones, silt- stones, and quartzites that can be matched with the upper Paleozoic-lower Mesozoic Trinity Peninsula Group (Aitken- Figure 1. Location map for Seymour island, and geological sketch head 1975; Hyden and Tanner 1981). Granitic and dioritic map of the northern Antarctic Peninsula. The extent of rock out- plutonic rocks are like those of the Andean Intrusive Suite, of crop Is greatly exaggerated except for Seymour island and the late Mesozoic to early Cenozoic age; the intermediate to silicic northeastern point of Snow Hill island.

66 ANTARCTIC JOURNAL Pebble counts of the coarse fraction of the glacial drift at three sites on the southern end of the meseta, Seymour Island

- Site 1 Site 2 Site 3 - Pebble size 2.5_10cma 10-60 cm 2.5-10cm 10-60 cm 2.5-10cm 10-65 cm

Basalt 56 33 53 17 51 36

Metasedimentary rocks 17 31 15 25 18 19

Granitic rocks 6 13 7 22 8 19

Dioritic rocks 0 9 2 7 6 5

Silicic volcanic rocks 19 13 20 25 20 15

Sandstone 2 1 3 4 3 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 long subaerial exposure and/or high rates of erosion of the soft washed away. Most of these scattered large erratics consist of bedrock. plutonic and metasedimentary rocks similar to those in the 2. At the time of drift deposition (as an ice-contact deposit) pebble counts, but some are of pebbly mudstone, a rock type the local icecap on Snow Hill Island was much larger and not found in the pebble counts but which is sparsely distrib- diverted mainland ice to the north and east. Only the northern uted in the Trinity Peninsula Group on both the southeast and part of Seymour Island was covered by ice derived from the northwest flanks of the peninsula (Aitkenhead 1975; Elliot mainland. 1965, 1966). 3. The glacial drift on the northern part of Seymour Island The occurrence of these large erratics around the flanks of was deposited from floating ice. the meseta, which itself is covered with glacial drift, suggests Although the first two alternatives cannot be completely that the northern part of Seymour Island has a glacial history discounted, the third alternative is favored. It implies that the different from the southern part. Snow Hill Island, to the northern part of the island has been uplifted at least 200 meters; southwest of Seymour Island, is largely covered by an icecap, the rapid rate of coastal erosion (Zinsmeister 1979) and prom- but at the northern tip has exposed rock which apparently inent marine terraces (Zinsmeister 1980) support the inference lacks a cover of glacial drift. of recent uplift. The composition of the clasts in the glacial drift on the Fieldwork was supported by National Science Foundation northern part of Seymour Island implies transport by ice from grant OPP 74-21509. the mainland. At least three broad alternatives, which enlarge on the explanations offered by Andersson (1906), can be pro- posed: References 1. The southern part of Seymour Island and the adjacent part of Snow Hill Island were covered by ice, but all glacial Aitkenhead, N. 1975. The geology of the Duse Bay-Larsen Inlet area, north-east Graham Land (with particular reference to the Trinity Pen- drift derived from the mainland has been removed because of insula 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.

Upper Tertiary drift Cape Bibby, J. S. 1966. The stratigraphy of part of north-east Graham Land and (Scientific Report 53). Cambridge: British Wiman the James Ross Island group I71 1 Lower Tertiary strata Antarctic Survey. Cretaceous strata Elliot, D. H. 1965. Geology of north-west Trinity Peninsula, Graham [] Land. British Antarctic Survey Bulletin, 7, 1-24. . Pebble count sites Elliot, D. H. 1966. Geology of the Nordenskjbld Coast and comparison with north-west Trinity Peninsula, Graham Land. British Antarctic Bodman Pt. Survey Bulletin, 10, 1-43. Elliot, D. H., Rinaldi, C., Zinsmeister, W. J . , Trautman, T. A., Bryant, 64°15S 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 lur fore-arc basin sedimentary rocks at the Pacific margin in western Antarctica. Geologische Rundschau, 70, 529-541. island 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 0 km 5 Antarctic Peninsula. British Antarctic Survey Bulletin, 43, 49-58. 560454 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, Antarc- Figure 2. Geological sketch map of Seymour Island. tic Peninsula. Antarctic Journal of the U.S., 15(5), 25-26.

1981 REVIEW 67