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C.C. VON DER BORCH A. E. GRADY R. ALDAM D. MILLER R. NEUMANN A. ROVIRA K. EICKHOFF 《Sedimentology》1985,32(4):507-518
The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn-rift or early post-rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation. The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low-stand of sea-level. They were subsequently filled by a fining-upwards suite of sediments which reflects subsequent relative rise of sea-level and carbonate platform development. Ultimately the canyon complex was buried by north-westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle. It is considered possible that the series of isolated outcrops of canyon cross-sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge-like Patsy Springs Canyon lies in more proximal regions of the basin-slope, whereas 40 km to the north-east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel-fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg. The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel-fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys. 相似文献
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Spherular modern dolomite from the Coorong area, South Australia 总被引：1，自引：0，他引：1
Scanning electron micrographs show that the youngest and apparently least altered of the Coorong dolomite is in the form of spherular bodies about 0.2–1.0 μm in size which themselves are composed of spherules about 100 nm in diameter. Older and more lithified sediments show sharply defined dolomite crystals suggesting an origin as primary dolomite spherules followed by aggregation and diagenetic alteration to well crystallized dolomites. 相似文献
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Environmental setting and microstructure of subfossil lithified stromatolites associated with evaporites, Marion Lake, South Australia 总被引：2，自引：0，他引：2
A variety of finely laminated, subfossil, aragonitic stromatolites and oncolites occur on a regressive marginal flat surrounding Marion Lake, South Australia. These algal forms overlie a substrate of coarse, highly porous, moldic aragonitic limestone which passes progressively towards the take centre through a zone of interstatified aragonite and gypsum and ultimately to pure crystalline gypsum. All of these facies overlie Holocene marine carbonate bank sediments which unconformably overlie at least one upper Pleistocene marine unit. Detailed petrographic and stratigraphic studies, combined with comparative studies of related nearby lakes containing a variety of living aragonitic cryptalgalaminates, provide a model for development of the Holocene sedimentary sequence. Marion Lake last became inundated by the sea around 6500 years ago during the Holocene transgression, when a protected marine environment was initiated. Lateral sediment accretion sealed marine passes into the resulting lagoon system soon after sealvel stabilized, and a variety of gypsum and gypsum-carbonate-algal facies evolved. Pure gypsum was deposited in waters 2–3 m deep in the central basin area concurrently with formation of seasonally alternating gypsum and aragonite layers towards basin margins. Blue-green filamentous algae thrived in the shallower marginal areas and at least partly controlled carbonate deposition, which must have occurred during seasonal outflow of carbonate-rich ground water from the calcareous dune aquifer over denser gypsum-saturated waters. These systems eventually migrated towards the centre of the lake to produce the relationships preserved today. The fresher waters also leached the gypsum from the marginal gypsum-carbonate facies. Collapse due to gypsum dissolution, along with aragonite crystallization, combined to form a lake-marginal mega-polygonal facies. Teepee structures formed around polygon margins, with optimum conditions for stromatolite development occurring on the teepee crests. The actual stromatolites which occur around Marion Lake are strongly indurated and involve a variety of morphologies, the most common of which are laterally linked hemispheroids. Stacked hemispheroids and oncolites are also relatively common, along with irregular forms, many of which encrust a variety of substrate irregularities. Vertical relief of the stromatolites varies from centimetres to tens of centimetres and all forms are characterized by extremely fine internal interlaminations of alternate light and dark grey laminae which typically occur several per millimetre. The microstructure comprises micritic aragonite crystals with fibrous habit associated with organic matter, and occasional zones of abundant algal filament molds which are generally oriented normal to the laminae. 相似文献
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C. C. von der BORCH A. E. GRADY K. H. EICKHOFF P. DIBONA N. CHRISTIEBLICK 《Sedimentology》1989,36(5):777-792
A significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre-deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin-slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon-filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon-fill, has a stable isotope signature which may imply a non-marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon-filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base-level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally-induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian-style evaporitic lowering of base-level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations. 相似文献
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Geological significance of Coorong dolomites 总被引：2，自引：0，他引：2
Microcrystalline dolomite and related carbonate minerals have been forming throughout the Quaternary in shallow ephemeral alkaline lakes on the coastal plain of the Coorong area in southern Australia. These Coorong dolomites differ significantly from sabkha-type dolomites. They form in areas where evaporation rates during summer months exceed groundwater inflow rates to a series of alkaline lakes. This results in the lakes becoming desiccated during summer months. Brines resulting from this drying phase are then refluxed out of the system into seaward-flowing groundwaters of an unconfined coastal aquifer. Dolomites and other fine-grained carbonates remain behind, whilst saline and sulphate evaporite minerals are flushed out of the system. Progressive restriction by sedimentation in and around the Holocene coastal dolomite lakes results in an upward-shoaling sedimentary cycle. Basal sediments which formed in a restricted marine environment pass upwards to lacustrine dolomites or magnesites exhibiting desiccation and groundwater resurgence structures such as mudcracks and teepees. The upper Proterozoic Skillogallee Dolomite Formation, an early rift basin unit of the Adelaide Supergroup, contains dolomites which show many of the features characteristic of the peculiar groundwater hydrology which plays an important role in Coorong dolomite genesis. These features include aphanitic dolomites which lack relict saline or sulphate evaporite minerals. The Skillogallee Dolomite Formation in some areas overlies an earlier dolomitic unit, informally named the Callanna Beds, typified by abundant pseudomorphs after sulphate minerals. Sabkha style dolomites characterizing the Callanna Beds are replaced up-section by the Coorong-type dolomite of the Skillogallee Dolomite Formation. This implies the development of an increasingly more active groundwater regime. The ultimate source and mode of concentration of the necessary Mg required to form both the modern and ancient dolomites remain imperfectly understood. 相似文献
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