Page 176 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Water-settled accretionary lapilli-bearing fallout
deposit

These accretionary lapilli were generated by a large,
rhyolitic, phreatomagmatic eruption and fell into a small
lake contained in the crater of a scoria cone. They
settled through the lake water and have been effectively
sorted according to size. The accretionary lapilli rest on
an erosion surface (E) in the underlying lacustrine
sediments but have not been reworked.

Oruanui Formation, 26.5 ka; Pukeonake, New
Zealand.

2. Suspension-settled mudstone associated with
volcaniclastic turbidites
Thick beds of massive to weakly graded, shard- and
crystal-rich sandstone (St) alternate with laminated
black mudstone and very thinly bedded, grey mudstone
(M). Stratification in the grey mudstone is largely
planar, laterally continuous and even in thickness,
although flame structures (F) occur locally. The
sandstone beds are interpreted as deposits from low-
density turbidity currents. The grey mudstone intervals
were formed by sedimentation from dilute suspensions
trailing the turbidity currents.

Mount Read Volcanics, Cambrian; Anthony Road,
western Tasmania.
3. Flotation: pumiceous rhyolite block in lacustrine
sediments
Several metres of thinly bedded, volcaniclastic
lacustrine sediments enclose a block of pumiceous
rhyolite. Along the base and right hand side of the block
are prismatic joints (J) attributable to contraction as
those surfaces were rapidly cooled on contact with
water. Several similar blocks are exposed in the
lacustrine sediments nearby. The blocks were probably
spalled from the surface of a sub-lacustrine lava dome,
and floated to the edge of the lake where they grounded
in near-shore sediments (Wilson and Walker, 1985).
Some blocks may simply have settled gently onto the
lake floor after becoming water-logged.
Taupo eruption, AD 186; Highway 1, Lake Taupo
shore, New Zealand.
4. Suspension and flotation: relict pumiceous clasts
in laminated volcaniclastic siltstone
Grey, crystal-rich sandstone (St) is sharply overlain by
white, laminated siltstone (Sl). The siltstone occurs at
the top of a very thick, volcaniclastic megaturbidite and
probably formed by sedimentation from suspension. It
contains dark green, evenly porphyritic, blocky and
ragged, chloritic (formerly glassy) clasts. The large
clasts truncate (arrow) or are mantled by laminae in the
siltstone. Although vesicular microstructures are not
preserved in the large chloritic clasts, they may be relict
pumiceous vitriclasts and have also settled from
suspension after flotation.
Mount Read Volcanics, Cambrian; Comstock Valley,
western Tasmania.

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