Microstructures and Environmental Reconstruction.

A Use of Microstructures for Environmental Reconstruction

The Upper Glaciogenic Sediment at Criccieth, North Wales

Poster by Andrew Evans, of the University of Leeds, School of Geography.

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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 area, the sedimentry controversy, the samples.
Analysis - the microstructure of the samples.
Synthesis - the timeline suggested by the samples.
The depositional controversy
References and Original Diagrams List

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 the second glaciation till / proposed meltout-then-flow till.

Location of Samples

Samples were taken from a hillock of till on the East side of Criccieth, between the town and the sea caves (SH 508382). The mound has been described elsewhere as a 'drumlin' purely on the basis of its form. A set of orientated samples were taken from the upper of two proposed glaciogenic sediments: a clast rich light yellow Dmm. Poor cutting on one section led to a second thin section being cut despite the adequate quality of the former section. Thus, three thin sections were examined, one orientated North West to South East and two orientated North East to South West, to see if they could cast any light on the controversial nature of the glaciogenic sediments in the area.

Analysis

At hand specimen level the sections appear to be of a yellow fine silt with no internal structure.

The micromorphology of the samples is outlined in the table below.

North West to South East sample North East to South West samples

Strong primary fabric with an apparent dip 45° down to the North West. The upper sixty percent of the slides are of blocks with a single primary fabric or random fabric material separated by shears of varying orientations. The lower forty percent of the slides consist of areas below a strong iron stained boundary which have strong primary fabrics of an apparent dip 45° South West. This is cut by two shear zone sets.

Strong, discrete, shear fabrics exists with an apparent dip orthogonal to the primary fabric. The fabric is reoriented so mineral grains are approximately vertical. This suggests sinistral displacement and this interpretation is backed up by the movement of slight hetrogeneities in the primary fabric. The first shear zones have an apparent dip orthogonal to the primary fabric with sinistral and dextral displacement. Internally grains are aligned along the shears.

Zones of stronger primary fabric alignment displace the above shears suggesting later shear in the primary fabric direction. The displacement is dextral suggesting the two shear zones were the result of a single compressive deformation. However, there is no evidence for the second shears being truncated by the first, therefore their simultaneous development is speculation based on the likely environments of formation (two opposing stresses causing simple shear are less likely than compression). A second, horizontally orientated, shear set displace dextrally those mentioned above. The internal fabric is random. There is no evidence of the second shears being truncated by the first, however, there is a variation in the displacements of the first shears along one of the second shears which may suggest the sets formed simultaneously. The later shears of the first set formed more vertically suggesting a rotation of the primary stress direction. Upper area blocks contain shear zones of both sets suggesting the blocks were moved after they formed.

The section is iron stained and shows an intensity gradient building up to a boundary, below which there is no staining. The boundary often has gaps, after which the boundary gradient is reversed. Iron staining exists other than at the boundary between the two areas on the slides. Often it bounds the blocks of the upper area where it has strengthened the material.

Dewatering deformation has occurred, particularly at the boundary between the upper and lower areas. Shears from the first set can be seen crossing the dewatering areas suggesting that dewatering occurred before they formed.

Two explanations are possible for the distribution of iron stains in the sample:

The iron solution did not flow from the top to bottom of the material, but moved by capillary action and high fluid head against gravity, such that the staining records barriers to flow from below as well as above. Evidence for such anomalous movement exists in the form of iron staining around (apparent) pores. That these are pores and not sampling and preparation cracks is suggested by their location in the hardest part of the sediment; the iron stains.
That the boundaries were implaced and subsequently deformed. Primary fabric inside the bound areas is largely disturbed, however, horizontal bands of original fabric, broken by the first shear set, are present within this. Such fabric could be indicative of folding.

Synthesis

To summarise the analysis given above a timeline may be constructed, thus,

Primary Fabric Deformational Event forms a fabric with a true dip of 35° down to the West. This is either due to compression orthogonal to the fabric (suggesting flow rather than subglacial consolidation) or pervasive shear (which may be subglacial).
The iron stains were deposited.
Fold Event One distorts the iron stain boundaries?
The sediment dewaters. This may explain the change from the pervasive deformation of (A), if it occurred, to the discrete shear of the two following events.
Shear Event One produces a shear plane with a true dip 35° down to the East. Note that the ice in the area moved East to West.
Shear Event Two produces a shear plane with a true dip 45° down to the North West. This may be the later development of Shear Event One and therefore represent the Primary Displacement Zone of this event.
Blocks move by pervasive deformation, however, the large number of differently orientated shear zones in the upper area may suggest movement by discrete shears initially. These shears may be later developments of the Shear Events; vertical shears were late formed in the lower areas of the North East to South West samples (protected from block breakup by the iron boundary?). The return to pervasive shear may imply a lower effective pressure than in the shear events.

This timeline is by no means certain, features seem to be formed diachronously as well. There are instances of Shear Event One shears stopping at dewatering fabrics, however, these are small and this may be attributed to the extra strain needed to orientate the more random fabric. Dewatering fabrics exist which appear to have been formed after block emplacement. This suggests either events (D) to (G) occurred simultaneously or that there was a second dewatering event. The second interpretation it would certainly match a lower effective pressure in event (G) due to high pore pressures.

The Deposition Controversy

The suggestion of some workers that iron staining at the boundary between the upper and lower diamicts is indicative of an interglacial or interstadial period is thrown into doubt by the timeline. The samples suggest that the iron staining occurred prior to the last deformation, though possibly after the primary fabric formation. At the longest estimation this is sometime between the start of the last glaciation and the end of periglacial activity (under cold climate conditions). There is no reason to suppose the iron staining at the boundary between the two tills is not from the same event, concentrated by the permeability change to the heavy, consolidated, clayey, lower till. Such an interpretation is backed up by macroscale deformation in the deposit.

Two features similar to those West of Castle rock interpreted as frost wedges by Saunders (1968) display a morphology unlikely in such structures. One feature has been folded into an open parallel form, while the other has the morphology of a flow deposit moving into a fluid sediment. Internally, the folded feature has horizontally orientated clasts which continue outside the feature with a more random fabric in an arborescent structure. The features are closer to clastic dykes than wedges, but even this classification is dubious. Both these and other features, for example 'flames' of material intruding up into the upper diamict, in the deposit suggest the till adjacent to the boundary between the units was fluid at some time. Whatever, it seems unlikely that enough stress could have been imposed under the simple proglacial flow of meltout tills alone proposed by Boulton (1977) for the yellow diamict to produce such a feature .

Permeability change alone, is not a sufficient reason for the chemical precipitation of iron as subglacial or flow tills would be expected to be saturated throughout. From the amount of 'primary' iron minerals in the thin sections, it seems that the origin of the iron was the sediment itself or its source rock. Any account of iron precipitation should, therefore, take the sediment to be a closed system as far as the iron is concerned and not, for example, look to an influx of iron containing water from outside of the sediment. This view is backed up by the depth the sample was taken from. It is unlikely that iron containing fluid would precipitate so widely throughout such a thick sediment if such a precipitation was caused by fluid being altered by the diamict's chemistry.

How is it then that between the formation of the primary fabric and the shear deformation of the deposit the conditions changed and Fe2+ in solution was converted to solid Fe3+? The usual conditions under which this happens is a conversion from acidic to alkali environments, for example, in soils from organic acid rich upper layers to anoxic, waterlogged horizons.

I would suggest that the upper diamict was deposited in a saturated state and deformed, the deformation being pervasive so as to remove the structures associated with deposition and form the primary fabric (A). This occurred subglacially or in a proglacial moraine. The deposit was then subjected to an environment change, for example a saline event (B) which caused the iron deposition. This may have been in a proglacial lacustrine / glaciomarine setting caused by the trapping of water against the hills to the North or by isostatic depression in front of the ice respectively. This throughflow was either heterogeneous or the iron stains were deformed after deposition (C). The sediment then dewatered as the external water level fell (D) and was subsequently deformed by shear (E,F) and pervasive movement (G). The change from discrete to pervasive shear may have been due to a change in water phase (for example, the change from permafrost to solifluxion) between two environments or in one, seasonally variable, environment.

References

Boulton, G.S. 1977. A multiple till sequence formed by a late Devensian Welsh ice-cap: Glanllynnau, Gwynedd. Cambria, 4, 10-31.

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.

Jump to...

Introduction - the area, the sedimentry controversy, the samples.
Analysis - the microstructure of the samples.
Synthesis - the timeline suggested by the samples.
The depositional controversy

Diagams in the Original Poster

Figure 1. Location map of North Western Wales.

Figure 2. Profile of the Sampled deposit.

Figure 3. Primary fabric and Shear Event One shears.

Figure 4. Showing the relationship between primary fabric and shears.

Figure 5. Example of the reversal of iron stained boundaries.

Figure 6. A section of the iron stained boundary between the two areas of the North East to South West samples. Dewatering has disrupted areas of the boundary.

Figure 7. Example of iron stain bounded block.

Figure 8a. Diagram of iron stained boundary variations.

Figure 8b. Diagram of possible fold structure accounting for variations in iron stains.

Figure 9. Showing an iron stained block. The deformation geometry can only have formed by dewatering, not simple shear strain.

Figure 10. Possible pore with surrounding iron stain.

Figure 11. Macroscale deformation structures.



North West to South East sample	North East to South West samples
Strong primary fabric with an apparent dip 45° down to the North West.	The upper sixty percent of the slides are of blocks with a single primary fabric or random fabric material separated by shears of varying orientations. The lower forty percent of the slides consist of areas below a strong iron stained boundary which have strong primary fabrics of an apparent dip 45° South West. This is cut by two shear zone sets.
Strong, discrete, shear fabrics exists with an apparent dip orthogonal to the primary fabric. The fabric is reoriented so mineral grains are approximately vertical. This suggests sinistral displacement and this interpretation is backed up by the movement of slight hetrogeneities in the primary fabric.	The first shear zones have an apparent dip orthogonal to the primary fabric with sinistral and dextral displacement. Internally grains are aligned along the shears.
Zones of stronger primary fabric alignment displace the above shears suggesting later shear in the primary fabric direction. The displacement is dextral suggesting the two shear zones were the result of a single compressive deformation. However, there is no evidence for the second shears being truncated by the first, therefore their simultaneous development is speculation based on the likely environments of formation (two opposing stresses causing simple shear are less likely than compression).	A second, horizontally orientated, shear set displace dextrally those mentioned above. The internal fabric is random. There is no evidence of the second shears being truncated by the first, however, there is a variation in the displacements of the first shears along one of the second shears which may suggest the sets formed simultaneously. The later shears of the first set formed more vertically suggesting a rotation of the primary stress direction. Upper area blocks contain shear zones of both sets suggesting the blocks were moved after they formed.
The section is iron stained and shows an intensity gradient building up to a boundary, below which there is no staining. The boundary often has gaps, after which the boundary gradient is reversed.	Iron staining exists other than at the boundary between the two areas on the slides. Often it bounds the blocks of the upper area where it has strengthened the material.
	Dewatering deformation has occurred, particularly at the boundary between the upper and lower areas. Shears from the first set can be seen crossing the dewatering areas suggesting that dewatering occurred before they formed.