Page 147 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. Recrystallised welded shards are well preserved in
the matrix of the ignimbrite shown in 27.1A. The
shards are deformed, stretched and compacted at the
margins of a large quartz crystal fragment and
indicate hot, primary emplacement of the
volcaniclastic unit even though the depositional
setting was below wave base and submarine (27.1C).
Plane polarised light.

Tyndall Group, Mount Read Volcanics, Cambrian;
specimen CT52, Comstock Valley, western Tasmania.

C. The section containing the welded ignimbrite in
27.1A and 27.1B is presented here as a graphic log.
The occurrence of graded bedded, conglomerate and
sandstone turbidites and the absence of tractional
sedimentary structures strongly suggest that the
ignimbrite was emplaced under water, below wave
base. The source vent, however, was probably
subaerial and the locality is inferred to have been
near the Cambrian basin margin.

Tyndall Group, Mount Read Volcanics, Cambrian;
Comstock Valley, western Tasmania.
2. Densely welded, moderately crystal-rich
ignimbrite
A. The graphic log shows the context of another unit
of welded ignimbrite in the Mount Read Volcanics.
The section is disrupted by faults and the neither the
base nor the top are clearly exposed. The ignimbrite
has irregular contacts with crystal-rich sandstone and
occurs within a sequence of sandstone and siltstone
turbidites and trilobite-bearing mudstone that was
clearly deposited below wave base. One possible
interpretation is that the ignimbrite is actually a large
allochthonous block included in deposits from a giant
submarine mass flow. It may have been redeposited
from a subaerial, basin-margin site.
Mount Cripps Subgroup, Mount Read Volcanics,
Cambrian; Cradle Mountain Link Road, western
Tasmania.
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