A penecontemporaneous intrusion into marine sediments of the lower Shoalhaven Group and significance to the volcanic history of the southern Sydney Basin.

Dr Glen Bann1, Dr Brian Jones1, Dr Ian Graham2

1University of Wollongong, Wollongong, Australia, 2University of NSW, Kensington, Australia

This paper describes a dolerite intrusion and stratigraphy located at Kinghorn Point, just to the north of Jervis Bay, within the upper Wandrawandian Siltstone of the lower Shoalhaven Group of the southeastern Sydney Basin. Approximately half of the intrusion is a bifurcating dyke <1m wide whilst the other half is a sill <1.2m thick. Bedding dips gently to the east and comprises grey siltstone beds up to 3m thick (mainly <25cm) with thin interbedded buff coloured, fine-grained sandstone beds up to 30cm thick (mainly 2-5cm). Geochemical and mineralogical analyses performed indicate that the dyke/sill complex is an olivine micro-dolerite which can be massive to highly vesiculated,

Evidence for penecontemporaneous emplacement into wet unconsolidated shallow marine sediments includes; destruction of primary sedimentary structures with extensive interaction and intermingling between the sedimentary and igneous materials, including the formation of peperite with the injection of the magma into the sediments forming brecciated contacts and with angular and rounded basalt and trachyte clasts, from microscopic spherical ‘droplets’ to decimeters in size, fluidization and entrainment of the sediments into the magma; the presence of tube-like flow features in the sill causing loading and deformation of the underlying sediments, hyaloclastite and baked sediments along the contact margins, cooling and flow fractures in the magma filled with fluidized sediment and sliding, slumping and load structures in the sediments, fumaroles, and vesicles within both the sediment and magma. High vesiculation is characteristic of intrusion near the surface, possibly extrusion at the sediment water interface.

Sediments were deposited in the deeper part of a coastal seaway characterised by northward-directed palaeocurrents and a Cruziana ichnofacies. Abundant glendonites, crinoid stem fossils and cross-bedding in a few sandstone beds exhibit a preferred palaeocurrent direction to the north. Periodic storm deposits or tempestites, thin (generally 2-5cm thick) fine-grained sandstone beds comprising widely scattered unaltered euhedral prismatic plagioclase and strongly perthitic K-feldspar grains, predominantly volcanic quartz although metamorphic quartz is also present, uncommon quartz shards, with detrital muscovite and biotite. Zircon crystals are also present. These sandier, lighter coloured beds are tuffaceous and were derived from volcanic edifices, probably island volcanoes to the southeast, indicating that regional volcanism was pervasive during this period.

The mineralogy, especially key element ratios of the HFSE and major and trace element geochemistry of the intrusion are very different to the shoshonites of the Gerringong Volcanics, suggesting they are unrelated, rather, they are similar to the Karuah dykes in the far north of the Sydney Basin, which have been dated as Mid Permian, suggesting that the Kinghorn Point intrusion may have formed part of an early intermediate phase of the Gerringong Volcanics, or it may be distinctive and unrelated.

Evidence therefore indicates that regional mafic volcanism was occurring and well evolved in the southern Sydney Basin during sediment deposition by the mid-Permian.


Biography

I discovered and worked on this intrusion in the late 90’s when undertaking an Hons thesis, presented it in NZ at their annual conf in 2001, wrote a paper, had it reviewed, then shelved it until last year due to other diversions, very pleased to have finally finished it

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