Page 152 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Sandy high-density volcaniclastic turbidites
Some beds in this sequence of diffusely stratified and
massive bedded, coarse sandstone display normal to
reverse grading (cf. S 2 and S 3 turbidite divisions — Lowe,
1982). Siltstone intraclasts (I) form lenticular bands
within the sandstone beds and lag deposits above
erosional basal contacts. Low-angle cross stratification and
scour structures also occur and are characteristic of the
lower parts of deposits from sandy high-density turbidity
currents (cf. S 1 turbidite division — Lowe, 1982; Smith et
al., 1990).

Seathwaite Fell Formation, Ordovician; North Langdale,
English Lake District, UK

4. Volcaniclastic megaturbidite
These three samples come from within one very thick
(~50 m), graded mass-flow unit comprising crystal-
rich sandstone (C) overlain by diffusely laminated
shard-rich fine sandstone and mudstone (B and A),
interbedded with black laminated mudstone. The
components, organisation and context are consistent
with deposition from a large volume, syn-eruptive,
high-density, volcaniclastic turbidity current. The
framework components are largely pyroclasts
(shards, relict pumice wisps, crystals).

Mount Read Volcanics, Cambrian; specimens 92-4A,
92-4B, 92-4C, Anthony Road, western Tasmania.

A. In thin-section, the mudstone in the top part of the
unit (40 m above the base) is dominated by
devitrified shards (S) with rod and spike shapes in a
very fine, sericitic matrix, accompanied by
subordinate feldspar and quartz crystal fragments
(C). Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 92-4A,
Anthony Road, western Tasmania.
B. In thin-section, the middle part of the graded unit
(20 m above the base) is composed of undeformed,
platy and cuspate relict shards (S), minor blocky
shards and sparse angular crystal fragments (C).
Diffuse planar lamination in the hand specimen
reflects alternating shard-rich and shard-poor, crystal-
rich layers. Plane polarised light.

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

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