Page 177 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Diverse sedimentation processes: interbedded
volcaniclastic mudstone and sandstone
Three lithofacies are present (Smith et al., 1990): (1)
planar laminated beds of siltstone and mudstone (M)
that were probably deposited from suspension; (2)
intervals of coarse grained sandstone with abundant
mudstone intraclasts and massive to crudely stratified
bases (St) which pass up into planar to low-angle cross-
stratified sandstone (P); these probably record rapid
deposition from sandy turbidity currents; (3) trough
cross-stratified, very coarse sandstone and pebbly
sandstone (C), probably generated by tractional
reworking of the underlying sediments by currents.
Seathwaite Fell Formation, Ordovician; Lingmoor
Fell, English Lake District, UK.
6. Traction and suspension: interbedded
volcaniclastic mudstone and sandstone
Two interbedded lithofacies occur in this sequence
(Kokelaar et al., 1990): (1) mudstone (M) and sandstone
in laterally continuous, even laminae and very thin beds,
the tops of which are normally graded; the components
are crystal and lithic fragments in a fine-grained,
altered, originally vitriclastic matrix; this lithofacies
formed by deposition from suspension following fallout
of ash onto water; (2) siltstone and sandstone
characterised by shallow scours (S) and ripple cross
laminae (R) with thin mud drapes; this lithofacies
resulted from reworking of suspension sediments by
shallow water currents.
Whorneyside Bedded Tuff, Ordovician; Sourmilk

Gill, English Lake District, UK.
7. Tractional structures in volcaniclastic sandstone
This sequence has been interpreted as lacustrine, and
although deposited mostly below wave base, tractional
sedimentary structures are not uncommon (Smith et al.,
1990). Graded and diffusely stratified, tabular, thin
sandstone beds with sharp bases were probably
deposited from low-density turbidity currents. Load
structures and dish structures (D) occur in some of die
turbidites. The tops of the turbidites were reworked by
residual currents, generating tractional bedforms such as
ripple cross lamination and planar lamination. Many of
the ripples have thin mud drapes (M) which settled from
suspension during periods between the influx of
turbidity currents.
Seathwaite Fell Formation, Ordovician; North
Langdale, English Lake District, UK.
8. Tractional structures in pumiceous sand and
gravel
These unconsolidated pumiceous and crystal-rich sand
and gravel deposits have been generated by fluvial or
lacustrine reworking of subaerial, non-welded, rhyolitic
primary pyroclastic deposits. The deposits show delicate
ripples, planar lamination and cross bedding. Trains of
pumice pebbles occur together with sand-size crystal
and lithic particles in some laminae, reflecting hydraulic
sorting according to clast density rather than size. The
pumice clasts are well rounded, and although poorly
sorted, the deposit lacks mud-size components.
Post-AD 186 fluviolacustrine deposits; Highway 1,
Lake Taupo shore, New Zealand.

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