Page 154 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1B. In thin-section, it is evident that undeformed,
bubble-wall and bubble-junction relict shards (S) are
the dominant component of the fine sandstone at the
top of the unit. The formerly glassy shards are
recrystallised to a fine quartz-rich mosaic. Other less
abundant components are relict pumice (replaced by
sericite), black mudstone clasts, feldspar and quartz
crystal fragments, and sericite-rich fine matrix.
Stylolites (arrow) are defined by concentrations of
fine, opaque grains. Plane polarised light.

Mount Read Volcanics, Cambrian; DDH HP2 (228.6
m), High Point, western Tasmania.
2. This sample from about 79 m above the base is
weakly stylolitic, volcaniclastic sandstone composed
of scattered feldspar and quartz crystal fragments,
sericite-altered pumice wisps (P) and abundant shard-
rich matrix.
Mount Read Volcanics, Cambrian; DDH HP2
(256.6 m), High Point, western Tasmania.
3A. A 50 cm thick interval about 73 m above the
base is crowded with pumice wisps. Although no
sharp bedding plane is present, the pumice wisp
concentration may mark the top of a 70 m thick
subunit. The foliation defined by alignment of the
pumice wisps parallel to bedding superficially
resembles eutaxitic texture in welded pyroclastic
deposits.

Mount Read Volcanics, Cambrian; DDH HP2 (268.3
m), High Point, western Tasmania.

3B. In thin-section, some of the pumice wisps (P)
show relict tube vesicle texture in random
orientations (arrows) and are clearly non-welded.
Other components are sparse feldspar (Fl) and quartz
(Q) crystal fragments, and undeformed bubble-wall
shards. Plane polarised light.

Mount Read Volcanics, Cambrian; DDH HP2 (268.3
m), High Point, western Tasmania.

4. About 44 m above the base, the megaturbidite
comprises coarse volcaniclastic sandstone-granule
breccia mainly composed of feldspar crystal
fragments, dacite clasts and black mudstone
intraclasts, together with sparse, ragged, feldspar-
and quartz-phyric relict pumice wisps (P).
Mount Read Volcanics, Cambrian; DDH HP2 (291.6
m), High Point, western Tasmania.

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