Sedimentary petrology of Permian- Triassic fluvial rocks in Allan Hills ...

Cooper, R. A., J . B. Jago, A. J . Rowell, and P. Braddock. In press. Age and correlation of the Cambrian—Ordovician Bowers Supergroup, northern Victoria Land. In R. L. Oliver, J. B. Jago, and P. R. James (Eds.), Antarctic Earth Science, Australian Academy of Science, Canberra. Henderson, R. A., and D. I. MacKinnon. 1981. New Cambrian inarticulate Brachiopoda from Australasia and the age of the Tasman Formation. Alcheringia, 5, 289-309. Laird, M. C. 1981. Lower Paleozoic rocks of Antarctica. In C. H. Holland (Ed.), Lower Paleozoic of the Middle East, Eastern and Southern Africa, and Antarctica. New York: John Wiley and Sons. Laird, M. C., J . D. Bradshaw, and A. Wodzicki. 1982. Stratigraphy of the Upper Precambrian and Lower Paleozoic Bowers Supergroup,

northern Victoria Land, Antarctica. In C. Craddock (Ed.), Antarctic Madison: University of Wisconsin Press. Laird, M. C., and J . D. Bradshaw. In press. New data on the Early Paleozoic Bowers Supergroup. In R. L. Oliver, J . B. Jago, and P. R. James (Eds.) Antarctic Earth Science, Australian Academy of Science, Canberra. Shergold, J . H., R. A. Cooper, D. I. MacKinnon, and F. L. Yochelson. 1976. Late Cambrian Brachiopoda, Mollusca, and Trilobita from northern Victoria Land, Antarctica. Palaeontology, 19, 247-291. Tessensohn, E, K. Duphorn, K. Jordan, G. Kleinschmidt, D. Skinner, U. Vetter, T. 0. Wright, and D. Wyborn. 1981. Geological comparison of basement units in North Victoria Land, Antarctica. Geologisches

Sedimentary petrology of PermianTriassic fluvial rocks in Allan Hills, central Victoria Land

sandstone units (figure 3). Fifty or more measurements of trough crossbedding and of ripple mark directions within each sandstone unit at several localities indicate flow generally toward the west; dispersal varies as much as 180° from one cycle to another. Large coalified and silicified logs and Glossopteris leaves are locally abundant. Coal beds, up to 2 meters thick, are of high rank because of heating by the intrusion of Jurassic Ferrar Dolerite. An abrupt change in lithology and one to three well-developed ferricrete horizons at the top of the Weller Formation suggest that a major disconformity separates the Weller and the overlying Feather Sandstone. The lower Feather Sandstone (120 meters thick), which is a massive cliff-forming, medium-grained, quartzose sandstone, was deposited by sand-dominated braided streams. Verticaltube (Skolithos) burrows, also noted by Ballance (1977), 0.5 centimeters in diameter and 20-30 centimeters long, are abundant.

JAMES W. COLLINSON and DEANA CHAPMAN PENNINGTON Institute of Polar Studies The Ohio State University Columbus, Ohio 43210

NOEL R. KEMP Tasmanian Museum and Art Gallery Hobart, Tasmania, Australia 7001

Sedimentologic studies of Permian-Triassic fluvial rocks in Allan Hills (76°42'S 159°50'E) were conducted to compare the sequence there with equivalent sequences elsewhere in Victoria Land. The Allan Hills sequence, first described and mapped by Ballance (1977), was found to be greatly similar to the stratigraphic section described by McElroy (1969), Barrett, Grindley, and Webb (1972), and McKelvey et al. (1972) in southern Victoria Land, but dissimilar to equivalent rocks in northern Victoria Land (Collinson and Kemp 1982). The stratigraphic sequence in Allan Hills is shown in figure 1. A five-person field party, including Collinson and Kemp, B. L. Roberts (geologic field assistant), and W. H. Hammer and J. M. Zawiskie (vertebrate paleontologists), worked from 8-15 January 1982 from a tent camp emplaced by helicopter from McMurdo Station. Stratigraphic sections of each formation were measured, described, and sampled. More than 750 crossbedding directions were analyzed to determine paleocurrent dispersal. Figure 2 was compiled from modal analysis by Chapman of 29 thin sections. The lowest stratigraphic unit exposed in the Allan Hills, the upper 73 meters of the Permian Weller Formation, is represented by meandering stream, floodplain, and floodbasin deposits. These consist of 5 to 10 meter-thick fining-upward cycles of feldspathic sandstone, carbonaceous shale and coal. Pointbar accretion beds dip 10-15° and extend from top to bottom of 20

geoscience.

Jahrbuch, 41, 31-88.

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Figure 1. Stratigraphlc sequence in Allan Hills. ANTARCTIC JOURNAL



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Figure 2. Triangular composition diagram indicating mean sandstone composition of: "A" denotes Weller Formation; "B" denotes Lower Feather Formation; "C" denotes Upper Feather Formation, Fleming Member; Lashly Formation, units A + B; "E" denotes Lashly Formation, unit C; and "F" denotes Lashly Formation, unit D. Figure 3. Point-bar accretion beds in Weller Formation. Beds dip from right to left in center and foreground.

Similar structures have been reported in the Feather Formation in southern Victoria Land by Barrett, Grindley, and Webb (1972). A ferruginous horizon at the top of the lower Feather may represent a regional disconformity separating Permian from Triassic. This level marks the lowest appearance of volcanic detritus in the sequence, an abrupt change in mean paleocurrent direction from west-northwest to north, and a change from homogenous sandstone to cyclical sandstone and shale. The change in paleocurrent direction has been documented at several localities in southern Victoria Land by Barrett and Kohn (1975). The upper part of the Feather Formation (53 meters thick) is here assigned to the Fleming Member of the Feather Formation as suggested by Barrett et al. (1971) in southern Victoria Land. Kyle (1977) noted the occurrence of Lower Triassic palynomorphs in the upper part of the Fleming Member. The Lashly Formation (325 meters thick) can be divided into four members, which are similar to members A-D described by Barrett and Kohn (1975) for areas to the south. The base of the formation is marked by a decrease in grain-size and an abrupt increase in volcanic detritus, which gives the generally finegrained sandstone a greenish tint and slope-forming characteristics. The Lashly appears to be conformable with the underlying Feather, but a ferruginous horizon at the contact suggests the possibility of a disconformity. The northerly paleocurrent direction remains unchanged across the boundary but gradually swings to northeast higher in the Lashly. Member A (49 meters thick) is represented by fining-upward cycles of medium- to fine-grained volcaniclastic sandstone and greenish-gray siltstone and mudstone with abundant root casts. Mud drapes on some rippled sandstone surfaces contain mudcracks, indicating subaerial exposure. Sedimentary cycles in member A and in the underlying Fleming Member are similar to cycles described from the lower Fremouw Formation of similar age in the central Transantarctic Mountains by Barrett (1969) 1983 REVIEW

and Collinson, Stanley, and Vavra (1981). Deposition probably occurred in low sinuosity braided streams on a muddy floodplain. Member B (54 meters thick), a massive medium- to finegrained volcaniclastic sandstone containing abundant fossil logs, was deposited by sand-dominated, braided streams. Member C (137 meters thick) consists of fining-upward cycles of medium- to fine-grained volcaniclastic sandstone, carbonaceous siltsone and mudstone, and coal. Well-developed point-bar accretion beds indicate deposition by high sinousity streams. Carbonaceous beds contain abundant fossil plants of the Middle to Upper Triassic Dicroidium flora, which has been described from the Allan Hills by Townrow (1967). Member D (85 meters thick) represents a' major lithologic change from the volcaniclastic sandstones of members A-C, to quartzose sandstone (figure 2). The member consists of thick fining-upward cycles of resistant crossbedded sandstone and carbonaceous siltstone and shale with the Dicroidium flora. The highest part of the Lashly in the Allan Hills is a 36-meter-thick resistant coarse- to medium-grained sandstone. A spectacular unconformity (figure 1) with local relief of 500 meters occurs between the Lashly, Feather, and Weller Formations and the Mawson Formation of Jurassic age (Ballance and Watters 1971). As noted by Barrett and Kohn (1975) in southern Victoria Land, the Triassic sequence in the Allan Hills is strikingly similar to the Fremouw and Falla Formations in the central Transantarctic Mountains (Barrett 1969; Collinson, Stanley, and Vavra 1981). This similarity and similarities in dispersal directions suggest that Triassic fluvial systems may have occupied a single elongate depositional basin. This work was supported by National Science Foundation grant DPP 80-20098. 21

References Ballance, P. F. 1977. The Beacon Supergroup in the Allan Hills, central Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 20, 1003-1016. Ballance, P. F., and W. A. Watters. 1971. Mawson Diamictite and the Carapace Sandstone, formations of the Ferrar Group at Allan Hills and Carapace Nunatak, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 14, 512-527. Barrett, P. J. 1969. Stratigraphy and petrology of the mainly fluviatile Permian and Triassic Beacon Rocks, Beardmore Glacier Area, Antarctica. (Ohio State

University. Institute of Polar Studies. Report 34.) Columbus: The Ohio State University Press. Barrett, P. J . , C. W. Grindley, I. N. Webb. 1972. Beacon Supergroup of East Antarctica. In R. J . Adie (Ed.), Antarctic geology and geophysics symposium on Antarctic and solid earth geophysics. Oslo:

Universitetsforlaget. Barrett, P. 1 . ' and B. P. Kohn. 1975. Changing sediment transport directions from Devonian to Triassic in the Beacon Super-group of south Victoria Land. In K. S. W. Campbell (Ed.), Gondwana geology. Canberra: Australian National University Press. Barrett, P. J . , B. P. Kohn, R. A. Askin, and J. C. McPherson. 1971. Preliminary report on Beacon Supergroup studies between the Hatherton and Mackay Glaciers, Antarctica. New Zealand Journal of

Gondwana rocks of the Allan Hills SANKAR CHATTERJEE The Museum, Texas Tech University Lubbock, Texas 79409

HAROLD W. BORNS, JR. Institute for Quaternary Studies University of Maine Orono, Maine 04473

NICHOLAS HOTTON III National Museum of Natural History Smithsonian Institution, Washington, D.C. 20560

The Gondwana System exposed at Allan Hills, southern Victoria Land, consists of flat-lying rocks of continental provenance that range in age from Permian to Jurassic. Two units are recognized: a lower one, the Victoria Group, mainly fluvial clastics, and an upper, the Ferrar Group, of volcanic origin. During the 1982-1983 field season, our investigation centered on the stratigraphy and paleontology of the Victoria Group, as it was deposited while the ancient Gondwanaland was still united. Correlation of Gondwana rocks of southerly continental masses is traditionally based largely on fossils of land plants, but remains of terrestrial animals offer finer resolution as new 22

Geology and Geophysics, 14, 605-614. Collinson, J. W. and N. R. Kemp. 1982. Sedimentology of the Takrouna Formation, a Permian-Triassic fluvial deposit in northern Victoria Land. Antarctic Journal of the U.S., 17(5), 15-17. Collinson, J . W., K. 0. Stanley, and C. L. Vavra, 1981. Triassic fluvial depositional systems in the Fremouw Formation, Cumulus Hills, Antarctica. In M. M. Cresswell, and P. Vella (Eds.), Gondwana V. Rotterdam: A. A. Balkema. Kyle, R. A. 1977. Palynostratigraphy of the Victoria Group of South Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 20, 1081-1102.

McElroy, C. T. 1969. Comparative lithostratigraphy of Gondwana sequences in eastern Australia and Antarctica. In Gondwana stratigraphy. International Union of Geological Sciences Symposium. Buenos Aires, October 1-15, 1967. (UNESCO Earth Sciences 2), Paris, 441-461. McKelvey, B. C., P. N. Webb, M. P. Gorton, and B. P. Kohn. 1972. Stratigraphy of the Beacon Supergroup between the Olympus and Boomerang Ranges, Victoria Land. In R. J. Adie (Ed.), Antarctic geology and geophysics symposium on Antarctic and solid earth geophysics.

Oslo: Universitetsforlaget. Townrow J. A. 1967. Fossil plants from Allan and Carapace nunataks, and from the upper Mill and Shackleton Glaciers, Antarctica. New Zealand Journal of Geology and Geophysics, 10, 456-473.

occurrences are found. While we found no vertebrate fossils, we did succeed in collecting a comprehensive suite of plant fossils of very high quality. This material will enhance the current basis of biostratigraphic zonation and may also provide new details of gross morphology of the Gondwana flora of Antarctica. The stratigraphy of Allan Hills was discussed by Ballance (1977), Borns and Hall (1969), and Gunn and Warren (1962). (See figure 1.) The outcrop is in the shape of a crescent, the horns pointing nearly due north to enclose the blue ice of the bay (approximate coordinates 76°62'S 159°40'E). The arch of the crescent has a tail-like projection which is directed to the southwest. Topography produces a repetition of beds, the oldest in the central valley region, south and southwest of the bay. Paleocurrent vectors indicate flow to the northwest over most areas. The age of the different formations is based on plant fossils of the Glossopteris flora, which indicate Permian age, and the Dicroidium flora of Triassic age (Townrow 1967). The following is the sequence of Gondwana rocks in the Allan Hills as currently understood. 1. Feistmantel Formation (Permian). Unlike other Gondwana basins in Antarctica, the basal tillite bed (Metschel Formation) is absent here. The oldest mappable unit, recognized by Ballance (1977) is a 30-meter thick Fiestmantel Formation. The dominant lithology is laminated shale and fine sandstone which are commonly in rhythmic alternation. The beds are parallel or microcrosslaminated, with occasional large-scale cross-bedding. On weathered sandstone surfaces, dome-shaped, concentric structures from 0.5 to 2 meters across are common. They are usually brown in color and may represent algal mats. Leaf impressions of both Glossopteris and Gangamopteris are found in the shale. A thin conglomerate occurs at the base and contains faceted graANTARCTIC JOURNAL