Page 178 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 41 — Syn-eruptive volcaniclastic deposits from shallow submarine explosive activity
1. Volcaniclastic breccia and stratified—graded
pumiceous sandstone
A, B. Part of this section (C; arrow; graded
pumiceous sandstone and breccia) is interpreted to be
water-settled, pyroclast-rich fallout related to nearby
shallow submarine explosive eruptions (Cashman
and Fiske 1991). The volcanic breccia at the base (A)
has been interpreted to be a debris-flow deposit
separated from the water-settled fallout by a
"transition zone" (B). Diffusely planar and cross-
stratified pumiceous sandstone (D) occurs at the top.
The grain size and sorting of the graded pumiceous
interval are consistent with fallout from suspension in
water. However, such a sedimentation process and
the overall context are also consistent with deposition
from a submarine, high particle concentration,
granular mass flow. Labels 2 and 3 show the
approximate positions of 41.2 and 41.3.
Shimhama Group, Miocene-Pliocene; Dogashima,
Izu Peninsula, Honshu, Japan.
2. Coarse, graded volcanic breccia
This photograph shows the gradational boundary
(arrow) between the dense lava clast-rich basal part
(A) and pale pumice-rich transition zone (B) (41.1
A). Juvenile blocks in the breccia have chilled glassy
margins and quench fractures, and were derived from
disintegration of a submarine lava dome or flow. The
matrix consists of coarse pumiceous and crystal-rich
sandstone, and is notably poor in finer components.

Shirahama Group, Miocene-Pliocene; Dogashima,
Izu Peninsula, Honshu, Japan.

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