Page 153 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

C. The base of the thick graded unit (2 m above the
base) consists of massive, feldspar (Fl) crystal-rich,
coarse volcaniclastic sandstone. Other components
are formerly glassy and vesicular, trachytic-textured
volcanic lithic particles (L), quartzofeldspathic wisps
with ragged terminations that may originally have
been pumice, and cuspate and platy relict shards (S)
in much finer matrix. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 92-4C,
Anthony Road, western Tasmania.

Plate 30 — Syn-eruptive submarine volcaniclastic megaturbidite
The graphic log shows the grain size variations and
sample positions in a 120 m thick, volcaniclastic unit
that is inter-bedded with black, laminated mudstone.
Subtle changes in grain size and relative proportions
of components probably indicate amalgamated
subunit boundaries. The principal components are
crystal fragments (quartz, feldspar), relict shards,
relict tube pumice, volcanic lithic fragments,
mudstone intraclasts and fine, unresolvable matrix.
The dominance of juvenile pyroclasts, internal
organisation and great thickness suggest that the
deposit is syn-eruptive, sourced from an explosive,
rhyolitic eruption at a subaerial or shallow marine,
basin-margin or extrabasinal vent, and was emplaced
by large volume, submarine, high-density turbidity
currents.

Mount Read Volcanics, Cambrian; DDH HP2, High
Point, western Tasmania.

1A. In hand specimen, fine-medium grained
volcaniclastic sandstone from the top of the unit (107
m above the base) shows scattered feldspar crystal
fragments in finer grey matrix that contains a
bedding-parallel, wavy stylolitic foliation (S). The
arrow gives the younging direction.
Mount Read Volcanics, Cambrian; DDH HP2 (228.6
m), High Point, western Tasmania.

144