ABSTRACT Conspicuous chert horizons occur as discontinuous bands and isolated nodules in dolostones in the Eocene Dammam Formation, which is exposed in the southeast of Kuwait. The Formation has never been deeply buried, and so chert formation is likely to have resulted from silicification processes at or near the land surface. Erosional reworking of the chert constrains its formation to a time period between the late Eocene and the Mio-Pliocene. As there is no significant source of silica in the dolostones, the chert was formed from silica imported from other sources. This process, together with the specificity of chert to particular non-bedding horizons, suggests that silicification is related to discrete locations of the groundwater table during landscape incision and resultant groundwater discharge in the region. Detailed petrographical studies demonstrate that “chertification” was initiated by precipitation of nanoglobules of silica (opal-A) from supersaturated groundwater solutions flowing through voids formed concomitantly by dissolution of dolomite. Subsequently, silica was precipitated as more crystalline forms of chalcedony, microquartz, and megaquartz from successively less saturated groundwater. The most likely mechanism for triggering the precipitation of silica is considered to be significant cooling of the groundwater as it neared the landsurface and came into contact with a cold regolith terrain. Precipitation of disordered forms of silica (opal-A) occurred at the cold front: progressively more crystalline phases formed as the host rock was warmed by the inflowing groundwater and its degree of supersaturation diminished. If our interpretation is correct, this “chertification” process could have been initiated during global cooling related to one of the glaciations recorded during Oligocene and Miocene times.
Abstract A preliminary evaluation of the extent of racemization of a range of amino acids contained in molluscs from fossiliferous marine sediments associated with calcretes has been made for the ultimate purpose of establishing a chronology of calcrete formation in southern Australia. Amino acids detected in molluscs from Holocene and Late Pleistocene marine sediments intersected in trenches cut into alluvial terraces in the valley of the Hindmarsh River at Victor Harbor exhibit distinct d/l ratio differences, as expected from the geological and palaeontological assessment. In addition, the rates of racemization, which are known to vary from amino acid to amino acid, appear to have followed the general order described in the literature. Based on comparitive d/l ratio data for radiocarbon-dated molluscs from a stranded Holocene marine deposit exposed in an excavation 8 km ESE of Robe, an age of approximately 4000 years bp is estimated for the Holocene marine incursion into the Hindmarsh River Valley. The incursion deposited sediments equated palaeontologically with the St Kilda Formation which post-dates the major development of calcretes in South Australia. At this stage we have insufficient amino acid racemization data to speculate on the absolute age of the Late Pleistocene marine incursion into the valley and resulting sediments equated with the Glanville Formation, which commonly in southern Australia exhibits an indurated carbonate crust or is interbedded with calcretes. Further work on the extent of amino acid racemization in molluscs from the Glanville Formation, and from the Early Pleistocene marine deposits (Point Ellen Formation and Burnham Limestone) which predate major calcrete formation in southern South Australia, is in progress. Key words : Amino acid racemization molluscs South Australia calcrete Glanville Formation St Kilda Formation Pleistocene Holocene dating
Geomorphological features on the southwestern margins of the Lake Eyre Basin provide a basis for interpreting the evolution of old landscapes containing pedogenic and groundwater silcretes. and thick bleached and weathered profiles. Recurrent sequences of cut-and-fill and duricrust formation have been identified and related to major sea level changes and tectonic movements in the Lake Eyre Basin. An extensive high pediment had formed around a basement inlier by the late Eocene. An armour of pedogenic silcrete developed on this pediment under alternating dry and wet climates during the late Eocene and Oligocene. The characteristic kaolinite + opal + alunite + gypsum assemblage of the bleached profiles formed in acid saline groundwaters during the Miocene at a time of regional low water table and arid climate. Groundwater silcretes formed in the bleached profile in response to dissection of the high pediment. They are related to a period of high groundwater tables. humid climate. and gradual sinking of the Lake Eyre basin. In the meantime, widespread low pediments (glacis) formed in Lake Eyre catchment, possibly in the early Pliocene. There is a good correspondence between a geomorphic approach, taking in to account the relationship between weathering features and palaeolandscapes, and the information provided from stratigraphic studies in the region.
The Troubridge Basin in South Australia is a late Paleozoic sedimentary basin, extending from the Coorong area across Fleurieu Peninsula and Kangaroo Island to Yorke Peninsula (Alley and Bourman 1995; Fig. 1). The basin contains the Cape Jervis Formation, which crops out extensively in these areas. The formation is also widely distributed in the subsurface below Yorke and Fleurieu peninsulas, Gulf St Vincent, Investigator Strait, Backstairs Passage, the Coorong area, Kangaroo Island and on the continental shelf to the east of Kangaroo Island (Alley and Bourman 1995). In some areas the formation forms a blanket of sediment and, in others, significant thicknesses are preserved in glacially eroded troughs or local tectonic depressions.
Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.
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