Poster by Andrew Evans, of the University of Leeds, School of Geography.
Please feel free to email any comments.
The micromorphological slides shown would unfortunately take up too much memory space to present here at the present time at a resolution that would do them justice.
The broad structure of the poster is...
Introduction
The glacial history of the Lleyn Peninsular, North Wales, is
still a matter of some controversy. It is unclear whether there are
sediments from one or two glaciations preserved in the area. Saunders (1968) suggested that two glaciers over-rode the area depositing two tills. Boulton (1977) has suggested in the East of the peninsula that the upper of the two diamicts is a meltout-then-flow till associated with the ice depositing the lower till. In the West of the area diamicts and gravels / sands have been variously interpreted as glaciomarine (Eyles and McCabe, 1989) and flow tills (McCarroll and Harris, 1992). A small number of
preliminary samples were taken to assess thin sectioning techniques, and
these were selected from interesting features of several deposits on the
peninsula so as to serve a dual purpose. This study looks at a deposit
close to that studied by Boulton near Criccieth, in particular looking at
one section of the boundary between the 'flow' till and the lodgement till
beneath. It is suggested that the unusual feature sampled at this boundary
was formed in the late stages of clast lodgement into the lower till and
that it may throw some light on both the history of the area and the
mechanism of large clast deposition.
The sediment bodies were deposited in consistent relationships. The sands
were deposited in bands with little or no silt in the pore spaces, suggesting the flow of clean water. They were also weakly aligned
horizontally. In the one place the sands were overridden by silts they were
mixed together in a 4.3mm zone. In places in this zone slugs of silt were
armoured with sand grains suggesting the edge of flow units.
While it was impossible to determine the true flow direction of these
features it was apparent from associated extensional features and folding
that they did not flow West - East.
The quartz grains were found throughout all the other bodies except the
sands, suggesting they may have winnowed out of these in a water body. The
grains were prevalent at body boundaries suggesting fallout deposition
between events. However, they also formed units of just
quartz which moved with the surrounding flows and filled veins.
The silts were found in three types of bodies...
It can be suggested that free-water movement through the coarse beds away
from or to the particle reorientation zones of the listric faults allows a
single fault to form. This is in contrast to the post-faulting situation in
which flow is reduced along the coarse layer and stress leads to increased
pore water pressure / reduced effective pressure close to shearing areas
and, therefore, more pervasive movement / a wider shear zone. This provides
the pervasive movement necessary to give the 'graded' appearance of the
bed. The alternative is that these represent flow noses, with the larger
grains being extruded to the surface.
Clast gouge and lodgement induced meltout.
Pressure pumping of sediments.
In reference to the fluvial nature of the sediments it should be noted that
it is difficult to see how the lower lodgement till would be found in such
a prominence in the feature if it was formed simply from eroded layers of
the (overconsolidated) lower till combined with water born sands and
sediments in the early stages of flow slumping. One must also remember that
this feature is inextricably included in the lower lodgement till.
This interpretation of the deposit could be strengthened or refuted through
an examination of the blue till above, below and within the resistant
layer, as the expected sediment changes in each area are easily estimated.
For one glaciation...
The fact that the feature's silt and clay fabrics have not been altered
since deposition suggests that the feature has not been subjected to
glacial loading. However, the alignment of the sands (which might be
expected to dewater first under stress) may hint at loading. The sand
alignment would then be the equivalent of the small number of long shear
zones which cross sediment body boundaries in the thin sections. This could
be studied in a triaxial loading rig, comparing loading reorientation with
the observed fabrics.
For two glaciations...
Boulton's model for the area relies on a vertical change in debris type up
through the ice to explain the difference in sediment between the lodgement
till and overlying flow till. The later was claimed to have partly melted
out of stagnant debris rich ice and partly formed of reworked lodgement
till. If it can be shown that the yellow material in the interbedded area
is the same as that above (and on the basis of the presently available thin
sections of both, there is nothing to suggest that it is not) there must
have been a horizontal change in the sediment at the very base of the ice
from the blue lodgement till to the yellow till above. In this structure it
can be seen that the ice was active when this sediment was deposited.
However, this is wild speculation and simple 'colour' is hardly a reliable
indicator of sediment type. It would be reckless to go any further than
this at present until the nature of the upper till and the yellow silts in
the mixed area can be examined in more accurate mineralogical detail.
Eyles, N. and A.M.McCabe. 1989. The late Devensian Irish Sea Basin: the
sedimentary record of a collapsed ice sheet margin. Quaternary Science
Reviews, 8, 307-351.
McCarroll, D. and C.Harris. 1992. The glacigenic deposits of Western Lleyn,
North Wales: terrestrial or marine? Journal of Quaternary Science, 7,
19-29.
Saunders, G.E. 1968. A reappraisal of glacial drainage phenomena in the
Lleyn peninsula. Proceedings of the Geologists Association, 79, 305-324.
Talbot, C.J. and V.von Brunn. 1987. Intrusive and extrusive (micro)melange
couplets as distal effects of tidal pumping by a marine ice sheet.
Geological Magazine, 124, 513-525.
Jump to...
Analysis
Two thin sections were prepared from the zone, unfortunately these were
from only one (West-East) plane. The sections showed that the alternating
banding was the result of interbedding of three sedimentary types -
There were no boundaries found between the different bodies which were
shear zones and in places the boundaries were erosional. This suggests
deposition was by flow. In places there was simple shear where boundary
irregularities produced friction and in other places fabrics could be seen
moving into the topography of lower sediment bodies.
The latter category is the most interesting as it was unexpected in this
environment (either two overriding tills or a till and massive flow above).
The fact that the larger grain sizes are above the fine silts in each 'bed'
suggests these are not water formed features. While it is possible to
suggest that the units with this architecture were inverted in transport, a
progression can be found which supports a non-fluvial origin for these
features. Some areas show low strain fracturing of coarse units (fine
grained quartz bed?) into lenses along listric faults. In the more
developed features remnants of these can be seen
to bound the lens like external structure. This process causes the observed
juxtaposition of fine and coarse material.Synthesis
There are a fixed number of possible environments for the formation of this
interbedded feature. These are combinations of...
Plainly there are a large number of possible processes that could form the
feature, however, on the basis of the sediments and the macroscale form
they have been whittled down to two...
Problems - How do the aligned, clean, sand bodies form? I am loathed to
resort to ice frozen blocks of channel sand.
Macroscale fabrics make it unlikely that simple meltout without reworking
was a significant factor in the deposition of the whole of the upper till
and similar fabrics are not found anywhere else in the deposit. Therefore
my inclination is to the former of the two explanations though local
meltout may have been significant (see Boulton, 1977). The amount of
meltout material in this case would be expected to increase as the clast became
lodged and, if we take the resistant layer seen in the field extending East from under the clast as a
consolidation trace of the path of the lower part of the clast, we see that
this is what happened. The ice pushing the clast East to West and lodged it
against a massive till lump, with meltout producing the mixed zone only in
the very last stages.The Deposition Controversy
With further study this feature, and the other structures at the unusual
boundary between the two diamicts, should throw some light on the
controversial nature of the upper till. The inferences that can be drawn
from the work so far and avenues of further research are outlined below.References
Boulton, G.S. 1977. A multiple till sequence formed by a late Devensian
Welsh ice-cap: Glanllynnau, Gwynedd. Cambria, 4, 10-31.