Page 173 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

7. Dune bedform in pyroclastic surge deposit
A dune bedform is preserved in this pyroclastic surge
(base surge) deposit. The laminae show upward
thickening and migration from initially nearly flat beds.
A flow direction from left to right is indicated by the
asymmetry of the dune. The components are lithic and
basaltic ash pyroclasts, and the dune bedform occurs
within other more poorly sorted surge deposits in the
rim of a small maar volcano.

Ubehebe maar, Holocene; Death Valley, California,
USA.

8. Block sag in pyroclastic surge and fallout deposits
The most conspicuous feature is the sag structure caused
by impact of a ballistic block (B). The block actually
belongs with the dark grey, moderately sorted, planar
stratified lapilli fall deposits (F) at the top of the
exposure. It has caused deformation of bedding in
cream, much finer, stratified ash deposits below. In the
lower parts of the cream ash deposits there are subtle
unidirectional traction current structures, such as low-
angle cross stratification (arrow), attributable to
pyroclastic surges. Planar bedded units (P) higher up
may be either pyroclastic surge or fall deposits. Dark
grey, planar bedded, poorly sorted, lapilli and ash at the
base of the exposure are probably pyroclastic surge
deposits.
Rim beds of Tower Hill maar, Tertiary; western
Victoria.
Plate 39 — Subaerial pyroclastic fall deposits
1. Mantle bedding in subaerial pyroclastic fall
deposits
This exposure shows mantle bedding in fallout deposits
from several explosive eruptions over a period of 10,000
years. The locality is about 13.5 km from the source. Each
fallout layer faithfully mimics the underlying surface. The
dark brown intervals are palaeosols that developed in
relatively long repose periods between explosive eruptions.
The thickness and grain size variations shown by the layers
reflect the influence of the size of the eruption, the distance
of this locality from the source vent, and prevailing wind
directions during fallout. The dashed line marks the base of
the pyroclastic fall deposit sequence.
Post-1 Oka fallout deposits from Taupo Volcanic
Centre; Rotopuha Road, Taupo, New Zealand.

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