This location is at the western end of the South Pembrokeshire coalfield. The folding and faulting demonstrates the tremendous degree of folding caused by the Variscan Orogeny at the end of the Carboniferous period. The folding was fascinating, as we had seen the effect of the Variscan Orogeny on our last weekend trip in the autumn 2003 in North Cornwall with the folding of the Carboniferous turbidities around Bude.
The Lower Coal Measures in Pembrokeshire provide evidence of changing environments. They were laid down in warm non-seasonal conditions in the Upper Carboniferous when this part of Wales was near the equator. The cyclothems or cycles of sedimentation begin with the development of delta flats with dense swampy forests of plants such as Lepidodendron, a lycopod like our present Tree Ferns. This produced the highly organic material that became coal seams. Shales containing marine fossils due to inundation by the sea with a rise in sea level overlie these. As the sea retreated non marine shales were formed in brackish water where the freshwater rivers were flowing into the salty lagoons. Uplift led to the building of prograding river deltas. This produced sandy cross stratified sediments with finer material deposited on top. Swampy forest then developed on the soils or seat earth. This was later flooded again so the cycle was repeated.
Broadhaven is on the northern flank of the Pembrokeshire syncline with the axis to the south. There is an impressive westward plunging asymmetric anticline to the north of Broadhaven beach. We debated why the beds above were not folded in the same way as the anticline and came to no definite conclusion. This is a classic example of why you need to look at structures from a number of viewpoints. It became obvious from the northern side that the uppermost beds had been thrust over the top of the anticline. There was a clear thrust plane and evidence of flexural slip [also known as layer parallel slip]. Stronger iron rich nodules stood out from the surrounding sandstones that erode more easily. The natural juxtaposition of iron and coal led to the start of the 18th C iron industry, for example in Coalbrookdale in Shropshire. The proportion of iron was not economically viable in this area.
The structures in the coal measures further north along the beach was well worth the slithering and sliding across the slippery rocks (with no casualties)….a definite "wow" factor. The competent beds had folded tightly with further thrusting breaking up the line of an S fold. We could tell the way up of the beds because of quartz infilling of the cracks in the upper side of the fold with the crack wider at the top. Some upper sides showed ripple marks. In places on the underside of beds were flute marks. The incompetent beds in the middle were squashed and crumpled with no sedimentary structures preserved. The direction of thrusting appears to be approximately towards the NNE which suggests that the maximum pressure during the Variscan Orogeny was coming from a SSW direction.
Many thanks to John for an excellent trip and to Debbie for arranging it.
Sheila Alderman