Page 155 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Lithic-rich and pumiceous breccia from about 26
m above the base of the lowest subunit consists of
green, feldspar- and quartz-phyric pumice clasts (P)
and feldspar crystal fragments (Fl) together with
abundant pale, volcanic lithic fragments.
Mount Read Volcanics, Cambrian; DDH HP2 (309.7
m), High Point, western Tasmania.
6. Closer to the base of the lowest subunit, the
megaturbidite is dominated by lithic fragments.
Dacite lava (D) and black mudstone (M) clasts are
abundant in this sample from 13m above the base.
Pumice wisps are subordinate to the lithic fragments.
Mount Read Volcanics, Cambrian; DDH HP2 (322.6
m), High Point, western Tasmania.
7. Poorly sorted, polymict, volcaniclastic lithic
breccia at the base of the megaturbidite is composed
of ragged and angular feldspar-phyric, pale, banded
dacite clasts (D), sparse basalt lava and pumice
fragments, large black mudstone intraclasts (M) and
dark grey mud-dominated matrix.
Mount Read Volcanics, Cambrian; DDH HP2 (331.1
m), High Point, western Tasmania.
Plate 31 — Volcaniclastic megaturbidites

1. Mudstone intraclast-rich, normally graded,
volcaniclastic turbidite
A, B. The graphic log shows a 40 m thick, normally
graded unit overlying laminated black mudstone.
Poorly sorted, polymict lithic breccia (Bx) at the base
of the unit includes abundant mudstone intraclasts
(M), volcanic lithic clasts, relict pumice and rare
massive sulfide clasts in crystal-rich sandstone
matrix. The finer top of the unit is moderately sorted,
diffusely bedded sandstone (St), rich in feldspar and
quartz crystal fragments (about 30%), relict shards
and minor pumice clasts. The internal organisation
indicates deposition from a high-density turbidity
current which locally eroded the underlying poorly
consolidated mud substrate. The grain size range and
particle types suggest that the volcanic components
were generated by an explosive, silicic eruption and
the unit is probably a syn-eruptive deposit.
Mount Read Volcanics, Cambrian; G10 Pit, Hercules
mine, western Tasmania.

146