Paleomagnetism of Red Beds of the Late Devonian Worange Point Formation, Se Australia

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Geophysical Journal International
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Gently folded strata of the Late Devonian Merrimbula Group along the south coast of New South Wales are similar to numerous deposits of Late Devonian to Early Carboniferous, subaerial to shallow marine, quartzose sandstone that are known as the Lambie Facies of SE Australia. Because the Lambie sands overlap the early Palaeozoic tectonic elements of the Lachlan Fold Belt, large displacement since the Late Devonian of any Lachlan terranes, with respect to interior Australia, is precluded. We collected oriented core samples from 37 sites primarily in reddish, quartzose litharenites of the Worange Point Formation. The remanent magnetization is carried by haematite. Incremental thermal demagnetization reveals a dominant, well-defined, steep-upward-north component of magnetization that post-dates the mid-Carboniferous folding. The south pole position (146.4-degrees-E, 68.6-degrees-S, A95 3.1-degrees) derived from the overprinted specimens is closed to both the Late Carboniferous and mid-Cretaceous reference poles as well as the spin axis of today. The overprint is attributed to both viscous partial thermoremanent and chemical remanent magnetization (VPTRM and CRM). Its exclusively normal polarity is consistent with a mid-Cretaceous acquisition, perhaps related to the rifting of the SE Australian margin.A characteristic component of magnetization is isolated between approximately 660-degrees and approximately 680-degrees-C in approximately 30 per cent of the samples. Although a regional fold test of the overall result is inconclusive because the directions of characteristic magnetization are generally close to the axes of folding, two sets of samples from opposite limbs of a local syncline pass a fold test at 90 per cent confidence, indicating a pre-folding origin of the remanence. The presence of both polarities in thin horizons, some of which are palaeosols, and diagenetic haematite associated with incipient cleavage formed in an early stage of deformation before folding, indicates that the acquisition of remanent magnetization occurred over a protracted period. An analysis that combines great circle demagnetization paths and set points, facilitates the incorporation of more data but gives a biassed result. Although normal and reverse polarity subsets are roughly antipodal, residual post-folding components cause the normal subset to fail a fold test. A south pole position (19.7-degrees-E, 70.8-degrees-S, A95 7.1-degrees) is derived from the specimens with characteristic component of reverse polarity in which the isolation of a characteristic component is clear.The pole position is close to that of a previous palaeomagnetic study from a widely separated occurrence of Lambie Facies sediments; both results are applicable not only to the Lachlan Fold belt, but also to the rest of Australia, and Gondwana as a whole. The position of the two poles derived from the Lambie Facies overlap assemblage in the Lachlan Fold Belt supports the interpretation that the progression of Australian poles derived from mid to Late Devonian rocks reflects Gondwanan drift from mid-Devonian to mid-Carboniferous, and does not require rotation of SE Australia relative to the Australian craton since the mid-Devonian.


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