Page 162 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

8. Mixed provenance submarine mass-flow deposits
Trilobite-bearing limestone clasts (Lm) have been rounded
in a shallow submarine environment prior to
resedimentation. Rhyolite clasts (R) came from a
submarine lava, and have not been reworked. Mudstone
fragments (M) are intraclasts.
Mount Read Volcanics, Cambrian; DDH MAC20
(354 m) , Hellyer mine, western Tasmania.
Plate 34 — Deposits from subaqueous volcaniclastic debris flows
1. Subaqueous debris-flow deposits: volcaniclastic
breccia and sandstone
Tabular medium and thick beds in this outcrop consist
predominantly of angular porphyritic andesite clasts
supported in a mud- to sand-sized matrix. Some beds show
reverse, coarse-tail grading and pass gradationally or
sharply upward into massive or diffusely laminated, poorly
sorted sandstone (S t ). The breccia beds are interpreted to be
deposits from submarine debris flows, and the sandstone
intervals may comprise sediment settled rapidly from
accompanying, more dilute turbulent suspensions
(Kokelaar et al., 1990).

Whorneyside Bedded Tuff, Ordovician; Sourmilk Gill
area, Borrowdale, English Lake District, UK.

2. Submarine mass-flow deposits: volcanic lithic
breccia
The lower 1.5 m thick, tabular bed of volcaniclastic lithic
breccia is reversely graded from a matrix-rich base to a
coarse clast-rich top. The clasts are dacitic (pale) and
andesitic (dark) lava. The lithofacies characteristics suggest
deposition from a density-modified grain flow in which
matrix strength, clast buoyancy and dispersive pressure
probably all contributed to the support of coarse dense
clasts (cf. Lowe, 1982). The upper part consists of a 1 m
thick bed of a very poorly sorted, matrix-supported,
volcaniclastic lithic breccia interpreted to be the deposit
from a cohesive debris flow. These beds were deposited in
a submarine, below-wave-base setting. Notebook (arrow)
for scale.
Green Tuff Belt, Miocene; Suttsu Peninsula,
Hokkaido, Japan.
3. Submarine mass-flow deposits: reversely graded
volcaniclastic lithic breccia
The two mass-flow depositional units in this outcrop show
weak reverse grading of angular, andesitic lava clasts. They
were deposited in a below-wave-base, submarine setting.
The lower clast-rich, clast-supported unit may be a density-
modified grain-flow deposit, whereas the upper, matrix-
rich unit is interpreted to be a debris-flow deposit.
Rucksack for scale.

Green Tuff Belt, Miocene; Kabuto, Shakotan
Peninsula, Hokkaido, Japan.

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