Some Structural and Mineralogical Problems in the Corris Slate Belt By Prof. David James

At the April meeting Prof. David James gave a talk entitled: Some Structural and Mineralogical Problems in the Corris Slate Belt

The talk centred on the area known as the Corris Slate Belt. This is an area between Dinas Mawddwy and Towyn which is on the slope of the Harlech Dome. The rocks are Upper Ordovician /Lower Silurian in age and are formed from deep water turbidites. The rocks form a homocline with a NE-SW trend. Two of the formations i.e. the Narrow Vein and the Broad Vein contained sufficient pure mud that workable slate was produced when metamorphosed. The narrow vein being of a higher quality of slate. At the base of the Broad vein lies the Nod Glas formation which is a characteristic black hemipelagic mudstone deposited in deep oxygen-poor conditions by quiet settling of suspended mud particles from the overlying column of seawater.

Prof. James then went on to point out that the anticlines in mid-Wales peter-out into this inclined belt. He then posed the question : “were the folds formed with the belt roughly horizontal and then tilted down to the south or did the folds develop on a homocline that already existed?”
He went on to suggest that during the deposition of these rocks the Bala - Tal y Llyn Fault (Bala Lineament) was active and downthrowing to the south.

by Liz Dawson wiki Commons https://www.geograph.org.uk/photo/1167043

Results from various studies actually suggest this to be the case. Metamorphic studies do suggest that there was movement of the fault during sedimentation. Measurements show that with increasing burial there was an increasing low grade metamorphism from the Dolgau Mudstones to the Nod Glas. But as the Bala fault is crossed the degree of metamorphism is reduced. This shows that by the time these rocks had metamorphosed the Bala fault had downthrown the sequence to its south against the sequence to its’ north before the final phase of Acadian heat.

Paleo-current studies also support this idea. If you observe the palaeocurrents i.e the distribution of the sandstones, their transport direction, the deep water passages and thickness in the sequence you can work out when the fault was quiescent and when it was moving.
Sediment was found to flow towards the fault and flow along the fault thus showing that the fault was moving.
Further it can be found that the area contains two basins. The main turbidite basin in central Wales and a smaller basin which is the Denbigh Trough and the dividing line being the Bala lineament.

He went on to discuss the 1923 paper by WJ Pugh. Here he pointed out that the axis of the Corris anticline was anomalous for which Pugh had suggested torsion had occurred in the axis.. This area became known as the “Corris Appendix”. Prof. James went on to show that unlike the surrounding area where the strike of the cleavage is axial planar the cleavage goes straight across the “appendix”. This suggests that ,if correct, the Corris anticline must be compound as it has an axial planar cleavage in one part but predates the cleavage in another. But this is irrational.

The dip on the anticline is shallow on one side but steep on the opposite limb. This has suggested the presence of faulting. The problem with the “appendix” can be resolved by the presence of a low angle reverse fault on the eastern portion of the anticline. Others had suggested the presence of a fault but had suggested that the fault was vertical but by going underground into the Garnedd Wen mine prof. James viewed the fault which did prove to be of a low angle. This fault can be traced approximately parallel to the bedding for some distance when it then merges with a second fault.
Before any folding occurred there was a syn-sedimentary slide off the Bala lineament which produced thickening in the hanging wall. So the Corris anticline was developed when the hanging wall sequence was backthrust over the footwall sequence during the Acadian deformation

By b3tarev3 wiki commons https://www.flickr.com/photos/b3tarev3/8155267763/in/album-72157631929108476/

Prof. James went on to explain what he described as a geometrical puzzle and mapping problem at Aberllefenni where there is a repetition of the Narrow Vein.
The geological structure at Aberllefenni is again affected by faulting, which causes repetition of the Narrow Vein outcrop on the south of the valley. Therefore on the northern side by Foel Grochen there is only one outcrop of the Narrow vein but on the southern side of the valley the vein is repeated. Both Pugh 1923 and the BGS 1995 have presented ideas of what happens to cause the repetition but Prof. James suggests that they are both wrong. They suggested that the faulting is vertical and the fault displaces the rocks to the north as a downthrow but Prof James argues against this idea. He suggests the apparent downthrow to the north is the result of the bedding dipping more steeply than the fault plane.
He then made the prediction that the fault should come down into the valley and then climb out. So he went on an excursion into level one at Ceunant Ddu to find the fault. The fault was in the location that he predicted, showing Prof. James to be correct.

He ended with discussing scams that had occurred in the area when coal and gold where suggested to be present, shares bought and sold on speculation only for neither to be present.

A very interesting and informative talk.

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