Lithofacies analysis of the Triassic Fremouw Formation ...

Lithofacies analysis of the Triassic Fremouw Formation at the Gordon Valley vertebrate site, Antarctica JOHN

L. ISBELL

Byrd Polar Research Center and Department of Geological Sciences Ohio State University Columbus, Ohio 43210 DAVID I.M. MACDONALD

British Antarctic Survey Natural Environment Research Council Cambridge CB3 OEL UK

During the austral summer of 1990-1991, we examined the Triassic Fremouw Formation at the Gordon Valley vertebrate sites in the Queen Alexandra Range (figure 1). Work was done in conjunction with paleontologist from Augustana College and geologists from Ohio State University. The main task of our work was to place the fossiliferous horizons into a sedimentological context. The vertebrate sites are located along the north side of Gordon Valley. Exposed rocks consist of the middle (90 meters) and upper (80 meters) members of the Fremouw Formation and thin dolerite sills of the Jurassic Ferrar Group (cf. Barrett and Elliot 1973; Barrett, Elliot, and Lindsay 1986; Collinson and Isbell 1986). The middle Fremouw member contains a vertebrate trackway (Macdonald, Isbell, and Hammer, Antarctic Journal,

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this issue), and the vertebrate bone-bearing beds occur in the upper Fremouw member (Hammer, Collinson, and Ryan 1990). The Fremouw Formation is 600 to 700 meters thick in the Beardmore Glacier area and consists of interstratified sandstone and mudstone. Thin volcaniclastic sandstone and thick mudstone beds characterize the middle member. The upper member consists predominantly of volcaniclastic sandstone with quartzose sandstone occurring at the base (Barrett et al. 1986; Hammer et al. 1990). Barrett et al. (1986) interpreted the Fremouw Formation as deposits of low-sinuosity alluvial streams. We measured a detailed stratigraphic section of 167 meters on the north-facing side of the ridge bounding the north side of Gordon Valley (see Macdonald et al., Antarctic Journal, this issue). Middle and upper Fremouw members constitute this section. The middle member of the Fremouw Formation is dominated by greenish-gray siltstone, very fine sandstone, and silty mudstone, with subordinate laminated, gray mudstone, and rare black shale containing Neocalamites (figure 2). Abundant, branching, white, hairline tubes penetrate many of these deposits. These tubes represent altered rootlets and suggest the presence of numerous paleosol horizons (Homer and Krissek, Antarctic Journal, this issue). Interbedded with these finegrained deposits are 1- to 4-meter-thick, medium- to coarsegrained, lenticular sandstones. These are cross-bedded, with sharp, commonly erosive, pebbly bases. In the direction of paleocurrent indicators, foreset-like beds occur, which grade laterally into fine-grained sediments. Commonly, multiple foreset beds are present, and individual foresets are draped by finegrained deposits. Channel-form cross-bedded sandstone bodies 0.5 to 2 meters thick and 10 to 40 meters wide also occur. We interpret the middle Fremouw member at this site as a fluvial overbank deposit, with crevasse-splay and crevassechannel sandstones. The vertebrate trackway horizon occurs at a stratigraphic height of 67 meters, in the middle member. Macdonald et al. (Antarctic Journal, this issue) describes the environmental setting of this bed. The upper member of the Fremouw Formation is predominantly a very coarse-to pebbly granular-grained, quartzose sandstone (figure 3). Sandstone bodies are 7 to 20 meters thick and contain the following:

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Figure 1. Location map showing the location of measured section GV-2 along the north side of Gordon Valley. Stippled areas are exposed rock or moraine. 1991 REVIEW

Figure 2. Fine-grained and sandstone deposits of the middle member of the Fremouw Formation at Gordon Valley. Photo shows part of section GV-2 from approximately 40 to 80 meters above the base. Person for scale (arrow). 15

• abundant large-scale trough cross-beds, • down-current descending planar-tabular cross-beds (cf. Haszeldine 1983), • sandstone-filled channel-form scours, • low paleocurrent variability, and • no obvious large-scale lateral-accretion bedding. Paleocurrent orientations are toward the north. Four sandstone bodies constitute the upper member at this site. These sandstones are interstratified with fine-grained sandstone, siltstone, and silty mudstone similar to the fine-grained part of the middie member. We interpret the upper member as a series of stacked fluvial channels deposited by low-sinuosity braided streams. The main vertebrate bone-bearing bed lies at 161 meters above the base of the section and is contained within a multistoried sandstone containing three amalgamated erosionalbased sandstone bodies. The bone bed is a matrix supported intraclast conglomerate, 0 to 1 meter thick. Conglomerate clasts consist of rounded siltstone blocks. Vertebrate bones are con-

tamed within the sandstone matrix of the conglomerate. We interpret the conglomerate and bones as a basal channel lag. Krissek and Homer (Antarctic Journal, this issue) provide a more detailed description and an alternative interpretation of the conglomerate bone bed. Rock samples are stored at the British Antarctic Survey and the Byrd Polar Research Center. The Antarctic Support Associates staff at Beardmore South Camp, Helicopters New Zealand Ltd. and U.S. Navy Squadron VXE-6 provided logistic support for this work. This research is supported by National Science Foundation grant DPP 89-17413 and by the British Antarctic Survey.

References Barrett, P.J., and D.H. Elliot. 1973. Reconnaissance geologic map of the

Buckley Island Quadrangle, Transantarctic Mountains, Antarctica

Figure 3. Contact between the middle and upper members of the Fremouw Formation at Gordon Valley. The prominent sandstone cliff is approximately 15 meters thick.

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(U.S.Geological Survey Map A-3). Washington, D.C.: U.S. Government Printing Office. Barrett, P.J., D. H. Elliot, and I.E Lindsay. 1986. The Beacon Supergroup (Devonian-Triassic) and Ferrar Group (Jurassic) in the Beardmore Glacier area, Antarctica. In M.D. Turner and J.E Splettstoesser (Eds), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36) Washington, D.C.: American Geophysical Union. Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5), 29-30. Hammer, W.R., J.W. Collinson, and WJ. Ryan. 1990. A new vertebrate fauna from Antarctica and its depositional setting. Antarctic Science, 2(2), 163-167 Haszeldine, R.S. 1983. Fluvial bars reconstructed from a deep, straight channel, Upper Carboniferous coalfield of northeast England. Journal of Sedimentary Petrology, 53, 1233-1247 Homer, T.C., and L.A. Krissek. 1991. Permian and Triassic paleosols from the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., 26(5). Krissek L.A., and T.C. Homer. 1991. Sedimentology of a vertebrate bone-bearing bed in the Triassic Fremouw Formation at Gordon Valley, Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 26(5). Macdonald, DIM., J.L. Isbell, and W.R. Hammer. 1991. Vertebrate trackways from the Triassic Fremouw Formation, Queen Alexandra Range, Antarctica. Antarctic Journal of the U.S., 26(5).

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