Page 97 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 18 — Products of silicic lava dome eruptions in shallow water: Bunga Beds, New South Wales
1. Associated sedimentary facies: mudstone and
turbidites
The rhyolitic coherent and volcaniclastic deposits of
the Bunga Beds are associated with interbedded
black, pyritic mudstone and coarse, mass-flow
emplaced sandstone and breccia. The dominant
lithofacies characteristics (planar, even, continuous,
internally massive or graded bedding) are consistent
with deposition from suspension and turbidity
currents, and constrain the initial setting of the
rhyolitic volcanism as subaqueous, below wave base
and relatively quiet (Cas et al., 1990).

Basin centre facies sandstone turbidites and black
mudstone, Bunga Beds, Late Devonian; Aragunnu
Bay, New South Wales.

2. Rhyolite-black mudstone contact relationships

Rhyolitic magmatism in the Bunga Beds included
partly extrusive cryptodomes and isolated intrusions
(Cas et al., 1990). Contact relationships such as that
illustrated here suggest that the enclosing sediments
were poorly consolidated at the time of intrusion. The
black mudstone is highly disturbed adjacent to the
rhyolite and penetrates along fractures. The rhyolite
shows well-developed polyhedral jointing as a result
of rapid cooling and contraction.

Rhyolite type 2, Bunga Beds, Late Devonian;
Aragunnu Bay, New South Wales.

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