Page 163 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Texture of a subaqueous volcaniclastic debris-flow
deposit

Poorly sorted, millimetre- to centimetre-sized scoriaceous
basalt and white lithic clasts are dispersed in mud-rich
matrix. The breccia occurs in a thick, massive unit within a
submarine volcaniclastic and mudstone sequence and has
textural and lithofacies features consistent with deposition
from a submarine debris flow.

Bunga Beds, Late Devonian; Bunga Head, New
South Wales.

5. Submarine syn-eruptive debris-flow deposit: very
poorly sorted, mud matrix, volcaniclastic breccia
The graphic log shows part of a texturally complex,
160 m thick, volcaniclastic interval within which the
relative proportions and size of the principal
components change considerably. The interval occurs
within a thick sequence of black, laminated, trilobite-
bearing mudstone and was deposited in a submarine,
below-wave-base setting. The juvenile volcanic
components are feldspar and quartz crystals,
feldspar- and quartz-phyric relict pumice clasts, and
relict shards. Other components are black mudstone
intraclasts, black mud matrix and sparse volcanic
lithic clasts. The mudstone intraclasts were clearly
unconsolidated when incorporated and their
disintegration contributed to the mud matrix. Some
mudstone intraclasts are highly contorted and have
sharp boundaries. Others contain scattered quartz and
feldspar crystals and merge with the mud matrix. The
composition and organisation suggest that the
interval was deposited from high-concentration,
volcaniclastic mass flows, probably debris flows, that
incorporated a significant volume of the largely
unconsolidated mud substrate during flowage. The
volcanic components are juvenile pyroclasts and
were generated by an explosive, rhyolitic eruption
from a shallow submarine or subaerial vent nearby.
Photographs 34.5A to 34.5D show samples from the
top half of the graphic log section.
Mount Read Volcanics, Cambrian; DDH HP2, High
Point, western Tasmania.
A. Poorly sorted, crystal-rich, coarse sandstone
(right) grades into diffusely laminated, crystal-rich,
fine sandstone (left). The topmost part of the main
debris flow was probably more dilute that the rest,
and formed deposits similar to sandy high-density
turbidites. The arrow shows the younging direction.
Mount Read Volcanics, Cambrian; DDH HP2 (112
m), High Point, western Tasmania.
B. The pale juvenile volcanic components (crystals,
relict pumice wisps) in this poorly-sorted crystal-rich
sandstone are thoroughly but inhomogeneously
mixed with the black and dark grey mud matrix.
Mount Read Volcanics, Cambrian; DDH HP2 (114
m), High Point, western Tasmania.

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