whole-van_de_geer-thesis.pdf (8.03 MB)
Late quaternary marine and freshwater swamp deposits of Northwestern Tasmania
thesisposted on 2023-05-27, 06:38 authored by Van de Geer, G
A study of marine and freshwater swamp deposits and landforms in northwestern Tasmania reveals that profound palaeoenvironmental changes occurred during the late Quaternary as a consequence of eustatic sea level changes, tectonic and/or hydro-isostatic uplift, and palaeoclimatic changes. Prograded bay sand barriers and lagoonal inlets constitute the most complex and extensively developed Holocene landforms and deposits on this coast. The barriers clearly depict marine transgression, followed by a major phase of barrier progradation and an episode of blowout and parabolic dune development. In the lagoonal inlets, the effects of strong tidal current action, halophytic vegetation, and wave and wind action have resulted in the development of distinctive depositional environments and landforms. Pre-Holocene depositional marine landforms and sand deposits which locally contain a well-preserved fauna of mollusca and foraminifera, and fossil shore platforms covered with beach cobble deposits occur extensively in the area. These deposits occur from below sea level up to 15 to 20 m. The local and wider stratigraphic relationships of the marine material in relation to glacial, freshwater and aeolian deposits, and the 14c dating of some of these deposits consistently point to a Last Interglacial age for the fossil marine features.Oxygen isotope and chronostratigraphic studies elsewhere suggest that the maximum level attained by the sea during the Last Interglacial transgression was 5 to 10 m above present sea level. Although there is presently no direct evidence for or information on tectonic deformation or theoretically calculated data on hydro-isostatic deformation in northwestern Tasmania or elsewhere on the island, the higher levels recorded in this study suggest that such uplift and deformation occurred in the area during the late Quaternary. Stratigraphic, sedimentary, palynologic, faunal, and conventiona l and isotopically enriched 1~C analysis of the swamp and lacustrine deposits formed under the influence of fluctuating artesian springs provides evidence from which a general pal aeoenvironmental and palaeoclimatic record of approximately 100,000 years may be constructed. During the Holocene Stage (10,000-0 BP) climate was warm and wet, and woody vegetation was dominant throughout the area. Locally, sand lunettes developed along lee shores of shallow lagoons. During the late Last Glacial Stage (25,000-10,000 BP) the climate became progressively drier and grassy open environments were more widespread. The driest part of this period occurred between~ 17,000 and 10,000 BP, when spring activity was very low and temperatures in western Tasmania were markedly reduced by highland glaciation. Predominantly wet conditions resulting from high precipitation and/or low evaporation rates occurred during the middle Last Glacial Stage (25,000-50,000 BP). The wettest part of this period occurred after about 35,000 BP during which the springs were very active, and extensive deposition of coarse river bed loads and alluvial fan gravels occurred elsewhere in the area. Considerably drier conditions occurred between approximately 55,000 and 45,000 BP during which woody vegetation was much more important than herbaceous vegetation and aquatic vegetation was virtually absent from the swamps. Prior to-55,000 BP, predominantly wet conditions prevailed on the swamps. These were periodically interrupted by relatively brief, drier phases during which woody scrub communities were somewhat more important and herbaceous and aquatic communities were less important than during the preceding and succeeding periods of the early Last Glacial Stage. The direction of general climatic changes presented in this dissertation appears to be broadly sympathetic with climatic changes inferred from other southern Australian localities.
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