Page 98 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Intrusive hyaloclastite breccia (peperite)
Breccia at the margins of the rhyolitic cryptodome
consists of pale, quartz- and feldspar-phyric rhyolite
clasts, locally with jigsaw-fit texture, separated by
narrow bands and seams of mudstone. The breccia is
interpreted to be the product of auto- and quench-
brecciation of cryptodome rhyolite that came in
contact with wet, unconsolidated mud during shallow
intrusion (Cas et al., 1990).

Facies 1, Bunga Beds, Late Devonian; Aragunnu
Bay, New South Wales.

4. Rhyolite-siltstone megabreccia
Megabreccia above the intrusive hyaloclastite (18.3)
consists of angular rhyolite clasts (R) up to 4 m and
black siltstone clasts up to 2 m across, with a matrix
(M) of massive black siltstone and millimetre-
centimetre rhyolite fragments. The megabreccia is
considered to be a debris-flow deposit comprising a
mixture of rhyolite and sea-floor sediment dislodged
during growth of the subjacent rhyolite dome (Cas et
al., 1990).

Facies 2, Bunga Beds, Late Devonian, Aragunnu
Bay, New South Wales.

5. Rhyolitic pumice- and crystal-rich breccia
Gradationally above the megabreccia (18.4) is a 1 m
thick, massive to graded breccia bed comprising
ragged pale green rhyolitic pumice clasts (P), minor
black siltstone and abundant crystal fragments. The
breccia is a submarine mass-flow deposit and the
juvenile clasts are thought to be pyroclastic in origin,
indicating the onset of explosive activity (Cas et al.,
1990).

Facies 3, Bunga Beds, Late Devonian; Aragunnu
Bay, New South Wales.

6. Rhyolitic pumice- and crystal-rich breccia in
polished slab
The rhyolitic pumice clasts (pale green) are generally
<5 cm, evenly feldspar- and quartz-phyric and in
thin-section show relict fibrous tube vesicle textures.
The crystal-rich matrix comprises coarse sand-size,
angular, volcanic quartz and feldspar crystals and
crystal fragments, pumice granules and minor non-
volcanic lithic particles.

Facies 3, Bunga Beds, Late Devonian; Aragunnu
Bay, New South Wales.

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