Jurassic physiography of southeastern Australia: Evidence from detrital zircon from the Nambour Basin

Henderson, Robert 1, Foley, Elliot1, Roberts, Eric1

1James Cook University, Townsville, QLD 4811

Jurassic fluviatile infill of the Nambour Basin consists largely of quartzose sandstone of the Myrtle Creek and Landsborough formations and succeeding heterolithic sandstone, shale and coals of the Tiaro Coal Measures. Detrital zircon age spectra for representative sandstone samples constrain the ages and sediment sources of these units. Maximum depositional ages from zircon, broadly consistent with published biostratigraphic age determinations, assign this succession as Early to Middle Jurassic (195-163 Ma). With a coastal location in southern Queensland, these units represent the easternmost record of Jurassic sedimentation for Australia.

Zircon age spectra from Early Jurassic samples of the Myrtle Creek Formation are dominated by a 650 – 500 Ma (Pacific-Gondwana) age cluster, with a Grenville age cluster (1300-950) also showing prominence. Detrital zircon of these ages is characteristic of Cambro-Ordovician metasediments of the Tasmanides as widely represented in southeastern Australia. Rocks of this Tasmanide assemblage stood as epeirogenic Jurassic upland, shedding sediment northwards and eastwards across southeast Queensland. Transport vectors obtained for the Myrtle Creek Formation fluviatile sandstone horizons support this conclusion.   Devonian – Triassic ages of detrital and igneous zircon characteristic of the New England Orogen, which abuts the Nambour Basin and forms a broad crustal tract to its west, are sparingly represented in these samples.

Relief across the orogen in southeastern Australia, as generated by the Permo-Triassic (260-230 Ma) Hunter Bowen Orogeny, had therefore been reduced towards base level, with little contribution to ongoing erosion and sediment production, by the Early Jurassic (~190 Ma). By implication, the base level surface forming the floor to the Great Artesian Basin, marking a unconformity of remarkable extent, continued eastwards as a surface of low relief across much of the New England Orogen.  Detrital zircon from sandstone samples of the Middle Jurassic Tiaro Coal Measures indicate a continuing provenance contribution from Cambro-Ordovician Tasmanide metasediments but also from a more pronounced New England Orogen source, suggesting some physiographic rejuvenation of this crustal sector subsequent to the Early Jurassic.

Jurassic aged zircon is scarce in most samples. However, it is well represented in a sample from the Myrtle Creek Formation and dominates the detrital zircon age spectrum of an arkosic sample from the Tiaro Coal Measures. As no source terrain for Jurassic zircon is known for the crustal fabric of eastern Australia, it must have been derived from igneous assemblages on continental crust to the east, now represented by submerged northern Zealandia, which rifted from the Australian continent during Late Cretaceous – Paleocene. An eastern source is supported by westerly paleocurrent directions measured for the sandstone intervals from which these samples were obtained.


Biography

Researcher with long term interests in the Tasmanide orogenic system, especially the Mossman and New England Orogens. Research interests also in Paleozoic and Mesozoic sedimentary basins located in Queensland inclusive of stratigraphy, sedimentology, paleontology and tectonic setting with contemporary utilisation of detrital zircon LAICPMS geochronolgy in basin studies

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