Reconstructing the pre-Quaternary landscape in Agnew–Lawlers area, Western Australia with emphasis on the Permo-Carboniferous glaciation and post-glacial weathering

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• 作者：Walid Salama ; Ravi R. Anand
• 关键词：Permo ; Carboniferous ; Mineralogy ; Geochemistry ; Palaeoclimate ; Yilgarn Craton
• 刊名：International Journal of Earth Sciences
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：106
• 期：1
• 页码：311-339
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Geology; Geophysics/Geodesy; Sedimentology; Structural Geology; Mineral Resources; Geochemistry;
• 出版者：Springer Berlin Heidelberg
• ISSN：1437-3262
• 卷排序：106

文摘

The pre-Quaternary stratigraphic section in Agnew–Lawlers area consists of Permo-Carboniferous glacial sediments, unconformably overlain by Tertiary clastics of palaeochannel sequence and Quaternary alluvial and colluvial sediments. The unique Permo-Carboniferous glacial sedimentary succession in the Yilgarn Craton has not been studied in detail in terms of vertical chemostratigraphic variations, basin and landscape evolution, diagenesis and post-Permian weathering overprints. In Agnew–Lawlers district, continental facies of glacial diamictites, glaciofluvial sandstones and glaciolacustrine rhythmites vary in thickness from 28 m to 181 m and is preserved beneath Cenozoic sediments as relict landforms in highly irregular, asymmetrical and poorly drained basins. The Permo-Carboniferous sediments were mechanically weathered and eroded by glaciers from the surrounding Archaean basement palaeohighs under a cold arid climate. Textural relationships of diamictites indicate that they were derived from proximal and distal source rocks. The denuded palaeotopography of the basement palaeohighs (source rocks) and Permo-Carboniferous sediments were subjected simultaneously to an intensive, post-Permian chemical weathering. The exposed basement rocks were deeply weathered into residual saprolite, whereas the Permo-Carboniferous sediments were differentiated into three chemostratigraphic units. The diamictite and rhythmite of the lower unit are unweathered and preserves the signature of the glacial/interglacial weathering. Mineralogical and geochemical changes dominated in this unit are related mainly to diagenesis at shallow depth (eogenesis) with no significant compaction. The formation of a paragenetic diagenetic sequence of chlorite, ferroan dolomite and pyrite indicates diagenesis has occurred under reducing and alkaline conditions below the permafrost and modern water table. Mineralogical and geochemical changes dominated in the middle unit are linked mainly to eogenetic redox reactions. The formation of smectite and ferruginous cements in this unit is attributed to weathering of ferromagnesian and opaque heavy minerals under oxic and slightly alkaline conditions. Mineralogical and geochemical changes dominated in the upper unit is related mainly to Post-Permian chemical weathering, where almost all rock-forming minerals are altered to kaolinite and alkali and alkaline earth elements are leached forming bleached white saprolitic diamictites. A change in clay minerals from kaolinite in the weathered Permo-Carboniferous succession to smectite in the overlying Cenozoic succession demarcates the unconformity and probably reflects a change to an arid climatic condition. Chemically, the Cenozoic succession shows a sharp increase in Ti/Al and La/Ce ratios and a decrease in Ce anomaly that remain constant in the Permo-Carboniferous section. These mineralogical and chemical variations through the Phanerozoic stratigraphic section can be used to follow the landscape and climatic evolution since the late Palaeozoic glaciation.