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Jurassic volcaniclastic-basaltic andesite-dolerite sequence in Tasmania: New age constraints for fossil plants from Lune River

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Bromfield, KE, Burrett, CF, Leslie, RA and Meffre, S 2007 , 'Jurassic volcaniclastic-basaltic andesite-dolerite sequence in Tasmania: New age constraints for fossil plants from Lune River' , Australian Journal of Earth Sciences, vol. 54, no. 7 , pp. 965-974 , doi: 10.1080/08120090701488297.

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Abstract

Jurassic plants excavated from a 1265 m site, at Lune River, southern Tasmania, include an araucarian
tree and numerous pteridophytes, belonging to the orders Osmundales, Filicales and Bennettitales. The
fossils occur in 2 – 3 m of immature volcanilithic sandstone beds. The sandstone consists primarily of clasts
from granitic basement rocks underlying much of southeast Tasmania and mafic clasts containing
feldspathicmicroliths, and primary, phreatomagmatic quartz crystals. Detrital zircons from the sandstones
are mostly Early Jurassic (Toarcian) in age (182+4 Ma) with minor Triassic (226 Ma), Devonian (380 –
360 Ma) and Proterozoic populations. Basaltic andesite, hereafter referred to as andesite, caps the
volcanilithic units and displays similar ratios of fluid-immobile trace elements (e.g. Zr/Nb, Ti/V), to the
Jurassic dolerite found in Tasmania, indicative of a common source. The andesites are correlated with the
Jurassic Kirkpatrick Basalts (Trans-Antarctic Mountains, Antarctica) based on their field relationships with
bounding strata, age, and distinctive similarities in major-element composition and fluid-immobile traceelement
ratios. The andesite is interpreted as an extrusive equivalent of the Tasmanian dolerite.
Importantly, drillcore from Lune River contains stoped clasts of andesite in fine-grained dolerite, indicating
that the andesite pre-dates the dolerite. Thermal alteration index of microfossils (3 – 3.3) and reflectance of
organic material within the sediments (0.54 – 0.77 Ro) resulted from contact metamorphism associated
with the emplacement of this basalt. The sedimentology and stratigraphy of the depositional environment,
plus the presence of hydrophilic pteridophytes and gymnosperms, indicates that the Toarcian
environment was temperate to warm and humid, with an abundant supply of water.

Item Type: Article
Authors/Creators:Bromfield, KE and Burrett, CF and Leslie, RA and Meffre, S
Keywords: andesite, Antarctica, dolerite, Jurassic, plants, Pteridophyta, Tasmania.
Journal or Publication Title: Australian Journal of Earth Sciences
ISSN: 0812-0099
DOI / ID Number: 10.1080/08120090701488297
Additional Information:

The definitive published version is available online at: http://www.tandf.co.uk/journals

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