The Tamar Trough revisited:correlations berween sedimentary beds, basalts, their ages and valley evolution, North Tasmania
Papers and Proceedings of the Royal Society of Tasmania(2006)
摘要
The Tamar Trough, an Early Palaeogene fault structure, contains sedimentaty beds and interleaved basaltic flows that infill the structure
along its 70 km length. These infills represent a complex interplay between sedimentation, channel erosion, eruptive dislocations, and
even 'out of trough' diyersions of the ancestral Tamar drainage. Several areas of resistant basalt flows remain in the south, upper, middle
and lower Tamar reaches. Although some palynological control was known, radiometric dating of previously untested basalts now allows
close integration and age-pegging for observed palynological biozones. The K-Ar and Ar-Ar ages of the basalt bodies indicate eruptive
events at 47, 33-37 and 25 Ma, correlating with Proteacidites asperopolus-Malvacipollis diversus, Nothofagites asperus and Proteacidites tuberculatus
biozone age sedimentary beds respectively. Basanite, alkali basalt and hawaiite flows dominate basalt lithology with lesser olivine
nephelinite, transitional olivine basalt, olivine tholeiite and quartz tholeiite. Basalt geochemistry suggests derivation from different degrees
of partial mande melting (from 7 to 35%), with alkaline and tholeiitic basalts being derived from separate source regions. Most alkaline
basalts have high-jl (HIMU) related trace element signatures, which are absent in the tholeiitic rocks. A basalt plug on the trough margin
at Loira gave a Jurassic age and has Jurassic dolerite-like geochemistry. The Tamar sequence suggests that the initial fluvio-Iacustrine and
later channel-fill sedimentation from 65(?) to 24(?) Ma was then punctuated in places by periods of alkaline volcanism between 47 to 33(?)
Ma, and alkaline and tholeiitic volcanism between 33 to 24(?) Ma. No Neogene fossils are known, so this later period was probably one of
net erosion. These contrasting quiet sedimentary and more volcanic intervals are related here to a tectonic model that involves northerly
drift of Victorian and Tasmanian lithosphere over several former Tasman metasomatised mantle plume sources.
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geography,botany,geology,zoology,taxonomy,ecology,natural history
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