Page 150 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Pitts Head Tuff: shallow, subaqueous welded
ignimbrite
Siliceous nodules are common in ignimbrites
emplaced on wet ground or in shallow water. In the
Pitts Head Tuff, they form a distinct zone 2-3 m
above the base. Slight deformation of the now steeply
dipping pumice foliation (F) adjacent to the nodules
(N) suggests that they developed during the later
stages of welding compaction. Some contain a central
vug (arrow), and superficially resemble lithophysae.
The nodules may have originally been steam bubbles
that formed in hot ignimbrite deposited on a wet
substrate and have subsequently been infilled with
quartz (Wright and Coward, 1977).
Outflow fades of the Pitts Head Tuff, Ordovician;
Llanberis Pass, northern Wales, UK.
5. Pitts Head Tuff: shallow, subaqueous welded
ignimbrite
A. Weathered surfaces exhibit dark grey lenses with
ragged terminations that define a well-developed
foliation. They are interpreted to be recrystallised,
originally welded, pumice clasts (fiamme) although
vesicular microstructures are not preserved. Other
components evident in hand specimen are scattered
angular lithic lapilli, crystal fragments and very fine
matrix.

Outflow fades of the Pitts Head Tuff, Ordovician;
Idwal Syndine,

B. Original vitriclastic textures are only locally
preserved in the Pitts Head Tuff. The sample shown
in 28.5A is thoroughly recrystallised. Relict
pumice(?) lenses consist of interlocking quartz and
feldspar. The matrix is a finer felsic mosaic and
contains scattered feldspar crystal fragments. Crossed
nicols.

Outflow fades of the Pitts Head Tuff, Ordovidan;
Idwal Syndine, northern Wales, UK.
6. Shallow submarine sandstone above the Pitts
Head Tuff

Scours, low-angle cross stratification and possible
hummocky cross stratification in well bedded, well
sorted, brachiopod-bearing sandstone above the Pitts
Head Tuff indicate deposition in a high energy
environment above storm wave base (Orton et al.,
1990). This sequence overlies distal storm beds at the
base of the Upper Cwm Eigiau Formation and
records the depositional environment which existed
following the emplacement of primary and reworked
facies of the Pitts Head Tuff Formation.

Upper Cwm Eigiau Formation, Ordovidan; Idwal
Syndine, northern Wales, UK
141