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Upper Carboniferous to Triassic stratigraphic units in the central Transantarctic Mountains were deposited in diverse nonmarine environments that record climate amelioration from glacial to temperate conditions (table). Biogenic structures are widespread and locally abundant and include discrete tracks, trails, and burrows produced by benthic animals as well as generalized sediment disruption (bioturbation). In fossil-poor units of this sequence (e.g., Pagoda, Mackellar, and Fairchild Formations), biogenic structures give crucial, if limited, information about the biota. Where plant and vertebrate fossils are abundant (Buckley and Fremouw Formations), the biogenic structures provide the only record of the bottom-dwelling fauna that played an integral role in the contemporaneous aquatic ecosystem.
During the 1995-1996 field season, biogenic structures were sampled and described from 11 locations in the Shackleton Glacier area and observed on a brief reconnaissance trip to Mount Weaver. Bioturbation on bedding planes was assessed semiquantitatively using a new field technique (Miller and Smail in press). Preliminary results highlight the contribution of the biogenic structures both to constraining the Early Permian salinity conditions and thus paleogeography of the Shackleton and Mount Weaver areas and elucidating the changes in the nonmarine fauna during this pivotal period in its evolution.
The Mackellar Formation in the Beardmore Glacier area previously was interpreted as recording turbidite deposition under freshwater conditions within an inland sea that extended to the Nimrod-Byrd Glaciers area (Isbell, Seegers, and Mackenzie 1994; Miller and Collinson 1994a, pp. 215-233). Presence of marine trace fossils in the Ellsworth Mountains and intervening areas suggests that the inland sea was connected to the paleo-Pacific Ocean (Collinson et al. 1994). The Mackellar Formation in the Shackleton Glacier area lacks marine trace fossils and contains known nonmarine trace fossils (e.g., Mermia, Isopodichnus, Cochlichnus). These data indicate that freshwater conditions persisted in the early Permian inland sea from the present-day Nimrod-Beardmore Glaciers to the Shackleton Glacier areas, implying no significant connection to the paleo-Pacific Ocean in this region (figure 1). The Mackellar equivalent in the Mount Weaver area contains densely packed specimens of small Skolithos, more typical of marine conditions. Its presence at Mount Weaver provides some suggestion of marine influence and connection to the paleo-Pacific Ocean (figure 2).
Biogenic structures in Permian formations are small (<0.5 centimeters) and restricted to fine-grained facies. We found no evidence that higher energy fluvial and turbidite channel environments were inhabited by benthic infaunal animals.
Worms and arthropods are inferred to have produced the trace fossils. Possible arthropod producers include notostracans, conchostracans, and a variety of insect nymphs. Hypothesized producers of the common trace fossils Mermia, Helminthopsis, and Cochlichnus include nematomorphs and dipteran larvae. Nonmarine aquatic faunas expanded during the late Paleozoic with aquatic insects appearing in the Permian. Holometabolous insects (including diptera) probably were not present prior to the Jurassic (Gray 1988). The abundance of several types of arthropod and worm-produced trace fossils, however, underscores the importance of benthic animals in aquatic ecosystems by the Early Permian.
Permian biogenic structures in the Shackleton Glacier area are confined to thin layers. Shallow penetration is typical of late Paleozoic aquatic biogenic structures and is consistent with the inferred domination of the benthic fauna by surface grazers and shallow burrowers.
Triassic (Fremouw Formation) biogenic structures differ from their Permian counterparts in size, depth of penetration, and facies distribution. Biogenic structures are abundant in the channel-fill sandstones of the Fremouw Formation (Collinson and Elliot 1984), although they are absent from equivalent Permian facies. Up to 30 percent of samples on sandstone bedding planes (each sample equals 0.0625 per square meter) are disrupted by bioturbation. The most common trace fossils are large (>1 centimeter), morphologically variable burrows produced by an unknown arthropod that moved deeply within the sediment (Miller and Collinson 1994b).
Large and complex burrows occurring in floodplain deposits (lower Fremouw Formation) are interpreted as produced by crayfish based on close resemblance to crayfish burrows (figure 2), which predate the previous published first occurrence of crayfish burrows (Hasiotis 1993) and provide evidence of pre-Cenozoic crayfish in the Southern Hemisphere.
Late Carboniferous to Permian biogenic structures from the Shackleton Glacier area were produced by shallow burrowing arthropods and worms in quiet-water settings within diverse nonmarine environments. Limited data do not preclude higher salinities in the Early Permian inland sea in the Mount Weaver area. By the early Triassic, channel sands were inhabited by vigorously burrowing arthropods. Crayfish burrowed deeply into the channel margins and adjacent floodplains, demonstrating behaviors similar to those of their modern counterparts.
This research was supported by National Science Foundation grant OPP 94-17978. Fieldwork was done with R.A. Askin, J.W. Collinson, S. Giller, J.L. Isbell, J. Roberts, and G. Seeger.
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