Triassic Paleoecology of the Lashly Formation, Transantarctic Mountains, Antarctica. Helen Isobel Gabites (MSc thesis)

Abstract

The Lashly Formation, an alluvial plain sequence of Triassic age in South Victoria Land, contains a variety of fossil plant material and other features such as trace fossils and paleosols. This study examines the paleoecological record, and uses it to provide a scenario of geography and climate of the period.

Twenty varieties of megaflora are described. A further two specimens are identified to genus level, and seven are recorded as unidentified foliage, seeds, sporophytes or cones. Samples of silicified wood provided growth ring data for 'sensitivity' and 'complacency' statistical analysis. Five fresh water and terrestrial trace fossils are also described. They include Cylindricum, Skolithos, Scoyenia and Heimdallia. Root casts and impressions are found associated with paleosols and mudstone horizons. They are described in four categories: horizontal, taproot, adventitious and rhizomatous root systems. Seven types of developed paleosols were recognised. They are classified according to their morphological and sedimentological similarities with modern soil types. Paleoflora, paleosol and sedimentological records at specific sites are used to provide models for floral succession, and the effect of flood events on the floodplain vegetation.

Interpretation of paleoenvironment and paleofloral characteristics provides the basis for a model of paleolatitude of the field area through the Triassic. There is a close correlation between the apparent polar wander curve of Irving & Irving (1981) and the field evidence of this study. The model developed shows the field area at approximately 75°S in early Triassic, moving to beyond the Antarctic Circle to approximately 61°S by late Triassic.

Sedimentology of the Weller Coal Measures at Mt. Fleming Antarctica.

Alex R. Pyne (MSc Thesis)

Abstract

The weller Coal Measures are an alluvial plain sequence of early Permian age, and are particularly well exposed at Mt Fleming, where they are 186 m thick. In this study they are mapped, described and interpreted in terms of three facies associations. FA1 forms the basal and upper units and is a cyclic association of sandstone, shale and coal deposited by meandering channels. FA2, which overlies the basal occurrence of FA1, contains in the lower part interbedded sandstone and shale with channelling and sand lenses indicating depostion on a levee. This grades into shale with thin sandstone stringers and limestones deposited in a lacustrine setting on the floodplain. FA3, which overlies FA2, is dominantly sandstone with minor shale and coal lenses and was deposited in braided channels.

The sandstone beds of FA1 and FA3 include intervals of small and large planar and trough cross-stratification. The large scale trough cross-stratification is considered to form from migrating in-channel linguoid dunes and hence provides paleocurrent directions that are close to the channel direction. A study of FA1 based on 187 measurements shows a symmetrical bimodal distribution indicating high variability in channel deposited sandstone from within meander loops. The symmetrical bimodal paleocurrent pattern can be simply explained by the downstream migration of a circular meander loop. The 88 measurements from FA3 are also bimodal but have a lower angular range, giving a sinuosity of 1.6, close to the low sinuosity channel pattern inferred from facies analysis.

Regional paleoslope has been determined for the Weller Coal Measures by considering patterns of point bar migration and preservation as well as channel direction measured from large trough cross-stratification. Paleoslope is taken as the direction with least readings and one that bisects the two main modes. These data indicate an east-sloping floodplain throughout most of the deposition of the Weller Coal Measures, consistent with few flow directions for the Carboniferous-early Permian ice sheet, but covered the region immediately prior to Weller deposition. The study indicates that simple averaging is not a reliable way of estimating paleoslope for high sinuosity current systems.

Deposition of the Weller Coal Measures followed a climatic warming and the final retreat of the early Permian ice sheet. However, vertical facies changes in the coal measures are thought to have resulted from regional tectonic events influencing slope and sediment supply rather than further climatic change.

Modern Sedimentation in Ferrar Fjord, Antarctica.

David N. Kelley (BSc lions, thesis)

Abstract

The origin of sediment forming the floor of Ferrar Fjord, on the coast of South Victoria Land, Antarctica, has been studied using grain size distribution, composition and shape. The sediment has subequal proportions of mud and sand and a few stones.

Comparison with accessible sources indicates about 25% of the sediment is supraglacial sand and 25% is eolian sand, the remainder being mud whose properties are consistent with those of the fine fraction of basal debris from a wet-based glacier. A process model is proposed whereby sedimentation of mud takes place from turbidity currents, generated by slumping at the grounding line, and density currents, formed by the sinking of hypersaline, supercooled water produced during sea-ice formation. The sedimentation rate is estimated to be about 0.4 mm yr⁻¹.

A facies model, based on the processes considered to be important and which identifies five depositional zones in the present-day fjord, is also presented. This model is considered in conjunction with the recent glacial history of McMurdo Sound and used to infer an origin for the well-sorted sand and sand-mud couplets of CIROS-2 drill core.