Page 169 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 37 — Massive sulfide clasts in submarine volcaniclastic mass-flow deposits
1. Normally graded, volcaniclastic lithic breccia—
sandstone
This photograph shows part of a 13m thick, graded,
polymict, lithic breccia—diffusely bedded, crystal-rich
sandstone unit. Clasts in the breccia comprise abundant
dacite, and less abundant rhyolite and andesite fragments,
together with minor chert, massive sulfide and mudstone
clasts. The matrix of the breccia consists of volcanic lithic
granules and crystal fragments. The breccia has a sharp
basal contact, and the lower parts are clast- to matrix-
supported, and reversely graded. Upper parts are
dominantly matrix-supported and normally graded. The
lithofacies organisation suggests deposition from a high-
density, gravelly and sandy turbidity current.
Mount Read Volcanics, Cambrian; Newton Dam
Spillway, western Tasmania.
2. Crystal-rich volcaniclastic sandstone
There are scattered dacite clasts (D) up to a metre across in
the lower part of the diffusely bedded crystal-rich
sandstone shown in 37.1. The sandstone probably
accumulated by rapid aggradation at the base of a residual,
coarse clast-depleted, sandy high-density turbidity current
following deposition of the lithic breccia. The isolated
coarse lithic clasts are lags that would have been
transported as bedload at the base of the mass flow.

Mount Read Volcanics, Cambrian; Newton Dam
Spillway, western Tasmania.

3. Jigsaw-fit texture in clasts in volcaniclastic lithic
breccia
Within the lower polymict, volcaniclastic breccia shown in
37.1, groups of dacite clasts locally show jigsaw-fit texture
(arrow). Many of the clasts have curviplanar margins. The
clasts may have been hot when incorporated in the mass
flow and broken in situ by quench fragmentation after
deposition, or else were entrained cold but with prepared
fractures that progressively opened during transport. In
either case, a dacite lava dome or flow contributed most of
the clasts, and gravitational collapse, hot or cold, probably
initiated the high-density turbidity current.

Mount Read Volcanics, Cambrian; Newton Dam
Spillway, western Tasmania.

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