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ItemLate Neogene stratigraphy and sedimentation across the Murray Basin, southeastern AustraliaMiranda, J. A. ( 2007)The Late Neogene sedimentary sequence of the Murray Basin provides an excellent opportunity to examine paleoenvironmental change across southeastern Australia. A detailed stratigraphic analysis of sediments deposited within the basin in the last 10 Ma was undertaken to assess the influence of tectonic and eustatic processes on deposition. Stratigraphic observations and radiogenic isotope analysis reveals the onset of deposition by 7.2 Ma with a transgressive episode that deposited the marine marls of the Bookpurnong Beds. Deposition was restricted to the central and eastern parts of the basin due to the Hamley Fault. In the west, subsurface elevation contours indicate the presence of incised paleodrainage channels above Miocene limestones, which facilitated the formation of a large estuary system at 5.3 Ma. The sediments of the Norwest Bend Formation were deposited within this western region, while further east, the Loxton-Parilla Sands strandplain deposited over 214 coastal ridges. The basal parts of this unit occur as lateral equivalents to the sediments of the Lower Norwest Bend Formation (in the west). Topographic and magnetic data reveal that tectonism was active during this period and resulted in the erosion and truncation of strandlines. Tectonic evidence and an estimated minimum 28,037 year cyclicity between strandline sets, suggests that the Loxton-Parilla Sands strandlines do not represent an unbroken record of glacioeustatic change. The subaerial exposure of these sediments at approximately 3.0 Ma caused the formation of a calcareous karst above the Norwest Bend Formation and a ferruginous and/or silicious cap (the Karoonda Surface) above the Loxton-Parilla Sands. The stratigraphic position of these surfaces are indicative of a regional widespread unconformity. The Douglas-Blackburn paleodrainage system in western Victoria was dammed during the Mid-Late Pliocene by uplift associated with the Padthaway High, which caused the formation of a 400,000 km2 lacustrine system, known as Lake Bungunnia. Topographic analysis indicates that Lake Bungunnia comprised at least four distinct sub-basins with water depths of up to 30 metres, with lake shorelines indicating that active tectonism occurred during this period. The resulting lack of sediment input to the coast caused the formation of the Kanawinka Escarpment, a large erosional scarp along the southern margin of the Padthaway High. The geomorphology of the modern Murray Basin can be directly attributed to the demise of the Lake Bungunnia system. Movement along the Morgan Fault in the west at approximately 700 Ka, resulted in the draining and progressive drying of Lake Bungunnia as a breach was created along the Padthaway High. The Murray River gorge as observed today was incised following this episode. The modern Murray River (and playa lakes such as Lake Tyrell) occupy the lowest elevations along the former sub-basins of Lake Bungunnia. The Late Neogene sedimentary sequence across the Murray Basin illustrates a complex interaction of eustatic and tectonic processes on deposition. Sedimentation within strandline, estuarine and lacustrine systems, particularly in the western Murray Basin, display evidence of significant tectonic control. This highlights the important role that neotectonic processes have played in shaping southeastern Australia.
ItemOrigin and formation of Ordovician sedimentary phosphates at Phosphate Hill, Mansfield, southern AustraliaMiranda, J. A. ( 2002)Ordovician sedimentary phosphatic rocks were mined for fertilizer use in the 1920’s from a small, highly deformed belt of Ordovician rocks at Phosphate Hill, near Mansfield (Warburton, 1:250,000 topographical map). The formation of the phosphate and the environmental conditions at the time of its formation have not been previously discussed in detail. It is an example of a sedimentary phosphate deposit that displays the results of both primary phosphogenesis and secondary, reworking processes. The black shale lithologies are host to the primary phosphate formation in the form of concretionary phosphates, derived from the high phosphorous content of bottom waters and the shells of the phyllocarid crustacean Caryocaris. Caryocaris has been misidentified in previous studies and their importance as the dominant bioclast at Phosphate Hill, has not been realised until now. Phosphorous enrichment within the depositional environment resulted from the extremely high organic contents within marine waters, which is illustrated by the bituminous character of some of the black shale lithologies. The depositional environment was relatively low energy, with low sedimentation rates, in isolated and stagnant marine waters. Thin alternating shale and phosphatic sand lithologies were derived from changes in the energy levels of the depositional environment which led to small turbidity currents reworking the concretionary phosphates and forming clastic phosphatic grains. Phosphate Hill has been extensively folded, fractured and faulted, with near-surface exposure resulting in the episodic precipitation of alumino-phosphatic, hydrous mineral phases. Phosphate Hill is compositionally similar to the Cheshunt sedimentary phosphate occurrence in Victoria, reflecting a similarity in environmental conditions within the Ordovician depositional marine environment.