Page 149 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Shallow marine volcaniclastic sandstone and
siltstone below the Lower Rhyolitic Tuff
Formation
Steeply dipping strata that underlie pyroclastic
deposits from initial eruptions of the Lower
Rhyolitic Tuff Formation comprise thinly bedded
volcaniclastic sandstone and siltstone (Unit 7 of the
Upper Cwm Eigiau Formation — Orton et al., 1990).
Sandstone sequences display swaley cross stratification,
amalgamated sets of hummocky cross stratification and
contain scours (S) that are draped by overlying beds.
These bedforms indicate deposition above storm wave
base, and storm processes have dominated sediment
transport, consistent with an inferred inner
submarine shelf setting (Orton et al., 1990).

Unit 7, Upper Cwm Eigiau Formation, Ordovician;
Cwm Cneifion, northern Wales, UK.
2. Lower Rhyolitic Tuff Formation: Subaqueously
emplaced, non-welded outflow ignimbrite
The outflow facies of the Lower Rhyolitic Tuff
Formation comprises a 55 m thick, non-welded,
pumiceous pyroclastic flow deposit (P) sourced from
a caldera ~6 km to the south (Orton et al., 1990).
Weakly developed bedding in the deposit is thought
to be due to intraflow shearing during transport, and
low angle cross stratification and planar bedding
towards the top indicate fractional reworking. The
overlying sequence (~100 m) consists predominantly
of volcaniclastic turbidites (T) and records
resedimentation of more proximal, primary
pyroclastic deposits.
Outflow facies of the Lower Rhyolitic Tuff Formation,

Ordovician; Idwal Slabs, northern Wales, UK.
3. Shallow submarine volcaniclastic mudstone and
sandstone above the outflow Lower Rhyolitic
Tuff Formation
This exposure is part of a 20 m thick section
generated by shallow submarine reworking of parts of
the Lower Rhyolitic Tuff Formation (Howells et al.,
1991). Thin- to medium-bedded, coarse,
volcaniclastic sandstone (S) is interbedded with
planar laminated, shard-rich mudstone (M).
Sedimentary structures in sandstone include
hummocky cross stratification, small-scale wave-
ripple cross lamination, and cut-and-fill structures
(arrow) that indicate deposition in a shallow, above-
wave-base setting. Mudstone beds comprise ash
deposited from suspension and evidently rapidly
lithified.
Reworked facies of the Lower Rhyolitic Tuff
Formation, Ordovician; Moel Siabod, northern Wales,
UK.

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