Page 151 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 29 — Volcaniclastic turbidites
1. Volcaniclastic sandstone turbidites

A. Thin beds of massive to diffusely laminated, coarse
sandstone are interpreted to be deposits from low-density
turbidity currents and are interbedded with mudstone
probably deposited from suspension. The sandstone
consists of black, chlorite-altered, non-vesicular to poorly
vesicular, cuspate and blocky particles in much finer
grained, dark grey mudstone matrix. The sample comes
from a Volcaniclastic interval below basaltic pillow lava
and fossiliferous black mudstone and was deposited below
wave base, in a relatively deep submarine setting.

Mount Read Volcanics, Cambrian; specimen HVS1,
Hellyer mine, western Tasmania

B. The chlorite-altered particles have a trachytic texture
(arrows) and were originally basaltic. They have both sharp
and ragged outlines and may have been produced by
quench fragmentation of basaltic lava. Mixing with mud
and sparse, rounded, polycrystalline, non-volcanic quartz
grains (Q) evidently occurred during mass-flow
resedimentation. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen HVS1,
Hellyer mine, western Tasmania.

2. Volcaniclastic turbidites and water-settled fall
deposits
Laterally continuous, planar thin bedding dominates this
exposure of interbedded mudstone, fine sandstone and
pebbly sandstone. The main components are crystal and
lithic fragments and recrystallised, formerly vitriclastic
matrix. The thin beds of fine sandstone and mudstone have
been interpreted as water-settled pyroclastic fallout,
deposited in water not much deeper than wave base
(Kokelaar et al., 1990). Medium to thick, coarse sandstone
beds display faint internal lamination, dewatering
structures, evidence of substrate erosion and loading, and
rare low-angle cross stratification, suggesting rapid
depositon from Volcaniclastic turbidities.
Whorneyside Bedded Tuff, Ordovician; Sourmilk
Gill, English Lake District, UK

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