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Rhyolite-Basalt-Soft Sediment Relationships at Bunga Beds, Aragunnu, Mimosa Rocks, Southern NSW, Australia



Roberto Weinberg
Monash University, Australia



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I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.


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This page covers a number of volcanological features of the Bunga Beds at Aragunnu in the Mimosa Rocks National Park, NSW, Australia. Here, basalt and rhyolites intruded soft sediments more or less simultaneously. The volcanic rocks caused mobilization of coarse sandstone (with angular terrigenous quartz clasts) which intrudes other soft sediments. The features are divided into the sections below, and roughly presented from North to South in Fig. 1a:

  • a) Columnar jointing
  • b) Cryptodome margins and intrusive relationships (peperites)
  • c) Disaggregation of rhyolite and basalt
  • d) Breccia dike features
  • e) Ductile-brittle behaviour

    Figure 1a) Map indicating key localities referred to on the text.


    a) Columnar jointing

    Columnar jointing Columnar jointing
    Figure 2a) Hexagonal pattern of columnar jointing. This dominates the northern part of the map in Fig. 1 above. Figure 2b) Columns.


    b) Cryptodome margins and surrounding peperites

    Cryptodome and peperites Cryptodome and peperites
    Figure 3a) Vertical wall showing contact between banded rhyolite forming the margins of a cryptodome, and remobilized black shale. Figure 3b) Same.
    sandstone dyke into soft sediment sandstone dyke into soft sediment
    Figure 3c) In the surroundings of the rhyolite cryptodome in Fig. 3a-b, the shore platform exposes another cryptodome, but this is a basaltic one, intruding and disrupting a sequence of black shales, with grey sand layers, intruded by irregular dykelets of basalt giving rise to a peperite. There is also evidence for mobilization of coarse terrigenous sandstone layers, with continental quartz clasts to form intrusive dykes. This photograph shows two main type of intrusive dykes: one a sandstone dyke, the other a massive beige basalt (box indicates approximate positions of figures d and e). Figure 3d) Coarse sandstone dyke (see Fig. 3a for position).
    basalt intrusion and brecciated margins basalt intrusion into shale
    Figure 3e) Massive beige basalt intrusion brecciated at contact with surroundings (see Fig. 3a for position). Figure 3f) Massive beige basalt intrusion cross-cutting bedding in shale.
    sandstone intrusion basalt dikelets into shale
    Figure 3g) Coarse sandstonte intrusion. Figure 3h) Intrusive dykelets of basalt into shale. Notice at top left, the in situ brecciation of the rhyolite, demonstrating multiple or evolving responses to contact with shale.
    sandstone intrusion
    Figure 3i) Coarse sandstone intrusion wedging layering in turbidites and eroding it. Notice that on the top part the sandstone has narrow dark flakes of sedimentary rock, 2 cm long.


    c) Disaggregation of basalt in peperites

    break up of basalt in peperite break up of basalt in peperite
    Figure 4a) Intimate ductile relationship between intruding basalt and soft sediment. Figure 4b) Same relationship in a block in the intrusive basalt-sediment breccia in Fig. 6 in Cas et al. 1990.
    break up of rhyolite in peperite break up of rhyolite in peperite
    Figure 4c) The break up and effective mixing of rhyolite with sediments. Note gradients in density of rhyolite clasts away from the lower right to the upper left of the photograph. Figure 4d) Clast of rhyolite in shale, with two phases of break-up. One that create isolated angular clasts, and the other that created a mixture with sediments that gives rise to fuzzy boundaries.
    break up of rhyolite in peperite break up of rhyolite in peperite
    Figure 4e) Two types of behaviour apparent here: ductile and brittle. Figure 4f) Again two type of break up of rhyolite: one giving rise to angular clasts, the other to fuzzy boundaries with mixture with sediments at grain scale.


    d) Rhyolite-sediment breccia dyke features


    Intrusive breccia Intrusive breccia
    Figure 5a) From Cras et al. 1990 Figure 6. Massive sediment-rhyolite mass intruding the stratified tuff-cone succession north of Aragunnu Beach, and associated discordant slide surfaces. 1, slide surfaces; 2, intrusive sediment-rhyolite breccia; 3, pumice breccia with crystal-rich matrix; 4, diffusely bedded crystal-rich sandstone (= crystal tuff); 5, bedded fine crystal-poor sandstone (= vitric tuff); 6, diffusely bedded pumice. The black core in the intrusive sediment rhyolite breccia (2) is an up-dragged antiformal mass of bedded black shale. The exposure is orientated east-northeast. Figure 5b) Photograph of the outcrop in a.
    multiple intrusions multiple intrusions
    Figure 5c) Intrusive margin of dyke into diffusively bedded pumice (6 in Fig. a above). Notice composite nature of the dyke with an internal band rich in elongated clasts absent in the more external band. Figure 5d) Intrusive relationships between basalt (beige color) and sediment-rhyolite breccia within the dike. Notice clasts of black organic shale clasts and light yellow rhyolite clasts in a grey mixed sediment-rhyolite matrix. Interpretation: basalt was still liquid and present in the core of the breccia dike.
    multiple intrusions multiple intrusions
    Figure 5e) Block of black shale with intrusive bands of rhyolite, within dark grey matrix including an gular blocks of rhyolite on the right. Figure 5f) Irregular clast of rhyolite within the dike suggesting it was still ductile during dike intrusion.