Comparison calcite, whereas the rest of the rock

 

Comparison of dip of several areas of shale on St. David’s Head shows large variations, which points to folding and faulting at one point. This is a result of the collision between Scotland, south and north Britain during the late Silurian and the early Devonian. The next period to consider is the Silurian. This was studied at Marloes Sands. The coast here was studied at five main sites, A to E. Along the coast here the strata has been turned on it’s side, so the bedding planes are almost perpendicular to the ground surface.

However this stretch of coast has been faulted to an extent during the late carboniferous, through the collision of Europe and Britain, so the strata do not systematically from north to south. However there is only minor faulting from location B to E. Location B is of an igneous rock type. There are several pieces of evidence for this. Firstly small dark spheres in the rock here indicate gas bubbles that were trapped when then rock solidified, and then were infilled by other minerals. The second piece of evidence is the colouration of some of the rock.

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Although the majority of the rock is dark green, some patches were red/brown. The green colour originates from the mineral epidote, however the red results from the oxidation of the iron bearing minerals to haematite mineral. This weathering and the fine grain of the rock indicate that the rock is basaltic, and therefore volcanic. The next younger rock in the Silurian sequence can be found at location C. This is also volcanic, but of a different nature. Here the sediment particles are poorly sorted, however with some well graded strata.

This can be interpreted as pyroclastic tuff, where falling ash and pumice has no mechanism whereby it can be sorted. The graded strata are the result of periodic water flow through the ash where layers of pyroclastic material can be sorted. The green colouration of the rock also points to pyroclastic origin. This rock is described as the Skomer Volcanic Group sediments. Further west of location C are the three chimneys. These are mudstone and limestone strata, with a massive degree of fossil preservation, especially brachiopods and some corals.

However further west of here at site D, coralliferous rocks can be found. These are predominantly dark rocks with white patches. These are fossils of coral and are composed of calcite, whereas the rest of the rock consists of very fine grained mudstone. This indicates a low energy environment, which contrasted with the three chimney area indicates an increase in sea level. The corals consisted of two main types, tabulated corals (hexagonal shaped) which tend to mass together and rugose (elongated shape) which are solitary. Location E consists of alternating beds of sand, silt and mudstones.

These alternating layers exhibit different colours, with the sandstone being light brown, the siltstones being a light purple/red colour and the mudstones being a deeper red/purple colour. This is in relation to grain size, where sandstone has the largest and mudstone the smallest. The greater surface area of the mudstone and to a lesser extent the siltstone allows a greater rate of oxidation, and so an increase in haematite minerals. Iron (III) is therefore responsible for the red colouration. Slightly further north of here calcrete minerals become more and more common.

These are formed when CaCO3 is deposited in fractures in a rock. Therefore frequent alterations of wet and arid are required for calcrete formation. The sand, silt and mudstones however display strong evidence for marine or even a lake conditions for their formation. Firstly ripples on the surface of some of the exposed rock beds indicate flow and cross bedding indicated fluvial activity. Due to the faulting in the area, location A even though west of B is the same age as the rocks at E. There is also a selection of evidence here for the environment in which the sediments were layered down.

A fine sand sediment size points to a medium energy environment. Ripples are also present, and here they are bi-directional. Brachiopods can also be found with other evidence of marine life being mottled texture to the rock bed surface. This is bioturbation, being the disturbance of sediments by burrowing animals. Hence the sediments were deposited in a marine environment. These rocks along with those at E are classed as Old Grey Sandstones. The Devonian did not feature during this field trip, although the volcanic island of Skomer could be seen from Settling Nose.

The youngest rocks studied on the field trip were the soft shales at Broadhaven. These consisted of cyclic strata of graded sediment, where cycles of large to medium to small sediments in a younging direction (vertically upwards) are repeated. Asymmetrical ripples could also be spotted on the surface of some rock beds. These two features indicate the presence of a migrating river channel, where coarse sediment is deposited in and in the close vicinity of the river, and as one moves progressively away from the channel the sediment deposited becomes finer.

These will be swamp environments. Then when the river channel has migrated a large enough distance from an area, then large forests will form. At this site there was a thin deposit of coal at the top of the strata, which reflects the deposition of organic matter from a forest. The most recent geological activity that was studied in the area was at settling nose. Here faulting has resulted in slumping of several blocks of thinly bedded mudstones.

Much of the movement here has occurred along the bedding planes in the mudstone, which is due to the inherent weakness along bedding planes. The faulting here is described as neo-tectonic, and therefore occurred within the last 30 million years or so. Evidence indicating the youth of the movement of these blocks along the faults can be seen at Settling Nose. Firstly there is a lack of vegetation and weathering on the debris. Also an overhanging soil implies only a short period of weathering. The blocks here will eventually fall into the sea and be removed through marine erosion.

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