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During the 1995-1996 field season, we measured and described stratigraphic sections, collected paleocurrent data, and searched for vertebrate fossils at the eight Triassic localities listed in the table and located in figure 1. Several localities in the Cumulus Hills where we had worked in 1970 and 1977, including Mount Kenyon and Shenk Peak, were inaccessible because of heavy December snows.
The most interesting stratigraphic sections are in the lower Fremouw Formation on Collinson Ridge and Kitching Ridge on opposite sides of the Shackleton Glacier (see figure 2). A major dolerite sill follows the basal contact of the Fremouw Formation throughout the region and only thin sequences of highly baked Permian Buckley Formation are locally preserved in contact with the Triassic. Permian rocks can be identified by their carbonaceous nature and by impressions of Glossopteris leaves. The base of the Triassic is typically a prominent channel-form sandstone.
As noted by Hammer, Hickerson, and Collinson (Antarctic Journal, in this issue), vertebrate fossils occur at several horizons in lower Fremouw sections (figure 2). At Kitching Ridge and Collinson Ridge, the lowest occurrences are water-worn bone fragments at the base of stream channel deposits. The bones were probably eroded out of surrounding floodplain deposits and redeposited along with mudrock and quartz clasts downstream. Higher vertebrate horizons contain partial skeletons at the tops of major channel sandstones and within the basal part of the overlying mudrock sequences. The upper parts of the same mudrock sequences contain abundant vertical traces of roots and other paleosol indicators. The animals appear to have been buried during major floods after stream avulsion. Relatively rapid burial by floodplain sediments, probably over weeks or days, accounts for the excellent preservation. Skeletal material is typically scattered over a small area, possibly owing to scavenging by animals before burial.
Silicified logs and upright tree stumps occur in the lower part of the sequence at Collinson Ridge. One log is 6.3 meters (m) long and gradually tapers from a diameter of 10 centimeters (cm) to 12 cm and abruptly flares to 40 cm in diameter at the base. Logs have suffered little compression from burial, suggesting early replacement by silica. Stumps are up to 0.8 m in diameter and have the thick growth rings characteristic of Permo-Triassic wood in Antarctica (Taylor and Taylor 1993). Stumps and logs that have the basal flare preserved have roots that extend laterally into the surrounding sandstone matrix, suggesting shallow root systems. In one case, a quartz clast, 4.5 cm in diameter, is imbedded in the roots of an overturned stump. Scattered quartz pebbles are common in this part of the sequence and may have been transported by uprooted trees. The sandstone, which is fine- to medium-grained and cross-bedded, represents channel bars in a braided stream sequence (Collinson, Stanley, and Vavra 1981). If the stumps are in place, the trees grew on submerged bars in the stream. Alternatively, upright stumps and logs were transported a short distance during floods.
On Collinson Ridge, a lens of silicified peat, 6 m across and 0.6 m thick, occurs within the fossil wood-bearing sequence. Cross-sections of fossil plants including a fragment of a Dicroidium frond were observed within this deposit. Other partly silicified coaly lenses occur in the mudrock above the wood-bearing sequence. The silicified peat, the first of its kind reported from the Lower Triassic, is being studied by Edith L. and Thomas N. Taylor at the University of Kansas.
Fossil wood and leaves are rare in rocks of Early Triassic age in Antarctica. Root traces are ubiquitous, however, in fine-grained, greenish-gray mudrock of this age, suggesting that the occurrence of fossil wood is more a problem of preservation than original tree distribution.
Vertical and horizontal trace fossils of the type described by Miller and Collinson (1994) occur in many of the sandstones (figure 3). Larger burrows near the top of the Kitching Ridge section are described by Miller and Smail (Antarctic Journal, in this issue). Most of these burrows are attributed to crustaceans, but at least one is large enough to have accommodated a small vertebrate.
In a previous Triassic study of this region, Collinson and Elliot (1984) conjectured a southwesterly paleoslope for the Fremouw Formation on the basis of a few readings at a few localities. In this study, the paleoslope direction was confirmed by more than 400 paleocurrent readings from several localities, most of which were toward the southwest quadrant. Readings (83) from the Upper Triassic Falla Formation were much less consistent and were bimodal toward the north and the south.
This research was supported by National Science Foundation grants OPP 93-15830 and OPP 93-15826 to Augustana College. Fieldwork was dependent on the logistic support of U.S. Navy squadron VXE-6 and Helicopters New Zealand. We thank Rob Andress for help in measuring sections.
References
Collinson, J.W., and D.H. Elliot. 1984. Triassic stratigraphy of the Shackleton Glacier region, Transantarctic Mountains. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains (Antarctic Research Series, Vol. 36). Washington, D.C.: American Geophysical Union.
Collinson, J.W., K.O. 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 five. Rotterdam: A.A. Balkema.
Hammer, W.R., W.J. Hickerson, and J.W. Collinson. 1996. Preliminary analysis of Triassic vertebrates from the Shackleton Glacier region. Antarctic Journal of the U.S., 31(2).
Miller, M.F., and J.W. Collinson. 1994. Trace fossils from Permian and Triassic sandy braided stream deposits, central Transantarctic Mountains. Palaios, 9(6), 605-610.
Miller, M.F., and S.E. Smail. 1996. Permian and Triassic biogenic structures, Shackleton Glacier and Mount Weaver areas, Transantarctic Mountains. Antarctic Journal of the U.S., 31(2).
Taylor, E.L., and T.N. Taylor. 1993. Fossil tree rings and paleoclimate from the Triassic of Antarctica. In S.G. Lucas and M. Morales (Eds.), The nonmarine Triassic (New Mexico Museum of Natural History and Science Bulletin No. 3). New Mexico: New Mexico Museum of Natural History and Science.