摘要:

The growth rates and patterns of stromatolites are of interest in that fine‐scale laminations in some Precambrian examples have been interpreted as daily growth layers whose orientations reflect the annual transit of the sun. By contrast, the modern and Holocene stromatolites at Shark Bay occur in a high‐energy environment where accumulation and growth only marginally exceed erosion. Long‐term vertical growth rates of ≤0.4 mm/year were determined by radiocarbon dating and supported by uranium‐series methods. These rates are up to 250 times slower than those growth rates estimated from the laminae patterns of some Precambrian stromatolites. Thus, daily bands are not recognized and individual subtidal stromatolite columns from Shark Bay take up to 1000 years to reach their present heights of ∼ 350 mm. The earliest exposed stromatolites at Shark Bay grew during the interval 1250–1000 years ago and are now dead and stranded (in response to falling relative sea level) in terraces some 0.3 m above present high tide.

ISSN:0812-0099    

DOI:10.1080/08120099008727913

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Sea floor spreading between Antarctica and Australia was resolved into two stages: (1) fast (27 mm/year), from the present to 49 Ma on a northerly azimuth constrained by well mapped fracture zones; and (2) slow (4.5 mm/year), from 49 Ma to break‐up at 96 Ma. A northwesterly azimuth was inferred by interpolation between the position of the continents at 49 Ma and the initial fit of the continents at break‐up at 96 Ma; during this stage, jumps to Australia of the spreading ridge west of the Spencer‐George V Fracture Zone were postulated to have transferred parts of the Australian Plate to Antarctica. Recently acquired satellite gravity trends confirm the inferred northwesterly azimuth and ridge jumps of the early spreading stage.

ISSN:0812-0099    

DOI:10.1080/08120099008727914

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Magnetotelluric data have been obtained over the obscured contact between the Precambrian Willyama Complex and Palaeozoic, Mesozoic and Cainozoic sediments of the Murray‐Darling Basin. Major structural elements can be identified from the resulting resistivity pseudosections for two traverses to the northeast and to the south of Broken Hill. Resistivities exceeding 1000 ohm m are obtained for the Precambrian basement, contrasting with values in the range 1–200 ohm m obtained for the sediments of the Murray‐Darling basin. Two‐dimensional forward modelling suggests a semi‐vertical contact for these units consistent with previous magnetotelluric surveys in the same area. Resistivities of 100 ohm m in the lower crust are attributed to partial melting of hydrated granites at depths of 20–30 km.

ISSN:0812-0099    

DOI:10.1080/08120099008727915

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A shelf to basin‐slope transition is vertically and laterally exposed within the Late Proterozoic Wonoka Formation in the northern Flinders Ranges of South Australia. The shelf to basin‐slope transition can be divided into four units (C to F) which are defined on the basis of facies, sedimentary structures, contacts, stratal geometry, and the type and abundance of down‐slope mass movement. The lowest unit (C) is mudstone dominated and parallel laminated with rare synsedimentary slides. Unit D, a thin, resedimented siliciclastic‐carbonate unit deposited on a sequence boundary at the end of unit C progradation, displays a lateral facies change from well bedded ‘outer shelf deposits in the east to basin‐slope debris flows in the west. Unit E forms a shallowing and coarsening upward succession from ‘outer shelf siltstone to ‘inner shelf storm wave influenced sandstone deposits. The unit thickens westwards, in the interpreted down‐slope direction, where it becomes finer grained and thinner bedded and displays an increasing abundance of synsedimentary slides. Unit F, deposited on an inferred shelf to basin‐slope transition, coarsens and shallows upward, thickens to the west and contains the highest percentage of sandstone and synsedimentary slides. Unit G, deposited at shelf depths, also shallows and coarsens upward from a thin, basal carbonate‐siliciclastic member, with sandstone increasing upsection to a gradational contact with the Pound Subgroup.

ISSN:0812-0099    

DOI:10.1080/08120099008727916

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Rocks in the Brungle‐Darbalara area of the Tumut Trough form two distinct domains: basement (mainly Bullawyarra Schist), of Cambrian‐Ordovician age, and an Ordovician ‐ Early Silurian sedimentary and volcanic cover sequence. These two domains are separated by a sharp discontinuity that marks an abrupt change in rock type, structure, metamorphic grade and deformation style. Cover sequences have undergone only one major penetrative deformation during the Late Silurian, involving sub‐greenschist facies metamorphism and upright folding. In contrast, the basement also underwent at least two older deformations at greenschist facies and contains distinct high‐strain zones subconcordant with the basement‐cover contact. The high‐strain zones, characterized by a ubiquitous south‐southeast trending mineral lineation, record a discontinuous history of ductile followed by brittle behaviour, consistent with an extensional origin.

ISSN:0812-0099    

DOI:10.1080/08120099008727917

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

New earthquake risk maps, based on the Cornell‐McGuire methodology, have been prepared for the Australian continent and continental margins. The risk is depicted as contours of peak ground velocity, acceleration and ground intensity with a 10% probability of being exceeded in 50 years. The earthquake process was assumed to be Poissonian and consequently foreshocks and aftershocks were eliminated. The estimated maximum earthquake magnitude likely to be expected in each zone was assumed to be half a magnitude unit greater than the largest recorded Richter magnitude for that zone. For areas outside the defined source zones, normalized background seismicity levels were used in the risk estimates.

ISSN:0812-0099    

DOI:10.1080/08120099008727918

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In the Emperor mine area, widespread propylitic alteration was followed by limited K‐silicate alteration and mineralization within and along vein edges. The propylitic assemblage is characterized by chlorite, ankerite, magnetite and pyrite. The K‐silicate assemblage includes sericite, rare chlorite, quartz, K‐feldspar and pyrite. The propylitic rocks are depleted in Si, Na, and Mg, have minor increases in K, and have substantial additions of CO2. The K‐silicate rocks are depleted in Fe, Al, Na, Ca and Mg, and enriched in Si, K and CO2.

ISSN:0812-0099    

DOI:10.1080/08120099008727919

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The origin of the peculiar banded claystone known as ‘zebra rock’ from the Ord River area of Western Australia has long been shrouded in controversy. A re‐examination of the mineral composition indicates that the principal phases include kaolinite, dickite, quartz, sericite, alunite and hematite, and that the amount of each varies appreciably even within a single hand specimen. The unusual composition and structure of the rock are attributed to authigenesis involving an attack on original sericite and kaolinite by sulphate ions produced by the oxidation of pyrite. Alunite, aluminium and ferrous sulphate, and silicic acid were the initial products; but with subsequent leaching of the sulphate ions, aluminium combined with silicic acid to yield dickite. Simultaneously, the ferrous ions were converted to colloidal ferric oxide which periodically coagulated, resulting in diffusion or Liesegang banding.

ISSN:0812-0099    

DOI:10.1080/08120099008727920

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Small but laterally extensive buildups, composed of intergrown Bryozoa and serpulids, are ubiquitous along the eastern margin of the Coorong lagoon, SA. The anascan cheilostome bryozoan isMembranipora aciculata(MacGilli vray 1891). Neither the presence ofM. aciculatawithin South Australia nor the buildups within the Coorong lagoon have been reported previously. Dating by14C indicates that the main buildup phase, dominated by Bryozoa, took place some 700 years bp. Salinity increase and water level decrease has led to productivity reduction and subsequent diminished size of the active buildups, along with a reversal of the dominant organisms. Today only sparsely distributed colonies ofM. aciculataremain in this stressed marginal marine setting, indicating that the rate of natural environmental changes can be accelerated by man over a geologically meaningful length of time. Given the present interest in climatic change, this study provides documented evidence for a warmer global climate 700 years bp.

ISSN:0812-0099    

DOI:10.1080/08120099008727921

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A 2‐dimensional vertical section model has been developed to simulate the groundwater head distribution in three major aquifers of the Amadeus Basin, central Australia. The resulting model‐derived velocity profile indicates groundwater flow times along a 250 km long and 6000 m deep section to be several million years. The slow and uneven groundwater flow times in this arid basin result from structural and palaeo‐recharge influences. A simulation of previous wetter conditions confirms these influences and suggests that the present‐day system reflects palaeo‐recharge conditions pertaining to the early Tertiary period, at 50 Ma.

ISSN:0812-0099    

DOI:10.1080/08120099008727922

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor