Page 172 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Megaripples in shallow submarine, volcanogenic
granule conglomerate

The poorly sorted, medium and thick, granule
conglomerate beds (M) are mass-flow deposits, the tops of
which have been reworked by storm waves resulting in
megaripple bedforms (R) on upper surfaces. The
megaripples are draped by normally graded fine sandstone
and siltstone deposited from suspension and/or from dilute
turbidity currents following storms. The lithofacies
association indicates a shallow submarine depositional
setting, most probably between fair weather and storm
wave base (Kokelaar, 1990). Pen for scale (arrow).

Bedded Pyroclastic Formation, Ordovician;
Llanberis Pass, northern Wales, UK.
4. Angle-of-repose cross stratification in shallow
submarine pumiceous sandstone
The large scale, angle-of-repose cross stratification in this
exposure was formed by a migrating sandwave in a trans-
gressive, tide-dominated, innermost shelf environment. The
depositional setting was above wave base and probably in
water from 5 to 20 m deep (Abbott and Carter, in press).
Thin mud drapes (arrow) separate poorly sorted, uneven to
lenticular pebble conglomerate beds. The coarser beds are
composed of rounded pumice and greywacke pebbles,
transported shelly fossils and crystals (quartz, feldspar).
The mud drapes are dominated by glass shards.
Kaimatira Pumice Sand, mid-Pleistocene; junction of
State Highway 4 and Kaimatira Road, Wanganui,
New Zealand.
5. Cross stratification in pyroclastic surge deposits

The conspicuous tractional structures in this cutting
comprise low-angle cross stratification, planar diffuse
very thin bedding and low-angle truncations (arrow).
The deposit consists predominantly of poorly sorted
basaltic lapilli and ash. The tractional structures were
produced by primary pyroclastic surges (base surges) in
which the interstitial fluid was hot volcanic gas and
steam. The uniform dip of the foresets in the deposits
indicate transport from right to left. The sequence is part
of the rim of a maar volcano and probably accumulated
very rapidly from surges generated by numerous
successive explosions over a period of hours or days.
Rim beds of Purrumbete maar, Tertiary; western
Victoria.
6. Cross lamination in pyroclastic surge deposits
This photograph shows in detail the low-angle cross-
stratified basaltic pyroclastic surge deposits illustrated
in 38.5. Traction currents of volcanic gas (and steam)
generated the good bedding and cross stratification.
Some very thin beds are very poorly sorted mixtures of
fine ash and lapilli. The presence of steam strongly
influences the deposition of fine ash. Moisture makes
fine ash pyroclasts clump together and adhere to other
larger pyroclasts. Deposition of fine ash together with
coarser particles results in the relatively poor sorting
that characterises surge deposits and helps distinguish
them from primary fallout deposits.
Rim beds of Purrumbete maar, Tertiary; western
Victoria.

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