Author Topic: Field trip to Freshwater West and West Angle Bay 12/05/18  (Read 19 times)

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johnd

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Field trip to Freshwater West and West Angle Bay 12/05/18
« on: May 17, 2018, 02:14:25 PM »
FIELD TRIP TO FRESHWATER WEST & WEST ANGLE BAY

We assembled at Freshwater West car park at 10.30am. 14 members were present. A short walk brought us to Little Furzenip headland close to the B4139 road.  From here we had an excellent view of the rock platform exposed at low tide. The steeply dipping Old Red Sandstone sequence youngs to the south forming the southern limb of the Castlemartin-Corse anticline. The foreshore is bisected by the Flimston Bay Fault, a Variscan dextral wrench fault with a displacement of about 120 metres. This fault is a major tectonic feature extending southwards through the Pembroke peninsula, across the Bristol Channel and through Devon where it is known as the Sticklepath Fault.
From Little Furzenip we followed the track down to the beach where the ORS strata was exposed in a series of isolated outcrops to the east of the Flimston fault. Here we looked at conglomerates, sandstones and calcrete bearing mudstones usually arranged in fining upward sequences each with an erosion surface at the base of the unit followed by lag gravel, coarse sandstone and then grading upwards into fine sand and mudstone. This graded bedding indicated the way up of the rock sequence. We also examined some beautifully displayed ripple drift bedding sequences. These are typical of highly sinuous meandering streams that must have flowed across the arid alluvial plains in Devonian times.  On the eastern side of the fault gap at Little Furzenip there are thich layers of calcretes. These represent fossil soils produced where alternating wet and dry seasons result in leaching followed by evaporation and the precipitation of calcareous minerals within the soil.  The multiple calcrete  profiles at Freshwater West are of regional importance and are known as the Chapel Point Calcretes from the type area on Caldey Island.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW–ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW–ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
Next we clambered over the rocks into Cove 3 where there are several interesting geological structures on the foreshore including two periclinal folds that appear as shallow elongated domes rather like the upturned hull of a boat. The axes of the folds are orientated parallel to the main synclinal axis of West Angle Bay. The rocks are also cut by a series of en echelon veins that are where the rock has been sheared and the resulting tension gashes have later been filled with the mineral calcite. You may also notice some small circular holes with radiating fractures about 30 cms long. These are not natural features, rather they result from blasting operations during quarrying many years ago. Many of the limestone faces contain fragmented brachiopods, corals and crinoids and some calcareous mudstones are bioturbated showing infilled burrows. Some of the latter were preserved in chert that had been precipitated from silica rich waters. The burrows are referred to as trace fossils since the original organism that produced them has long since been destroyed. Finally we reached the far northern side of the cove where the Avon Group mudstones were faulted against the limestone. This is a normal fault with a downthrow to the south. The mudstones are orange brown due to downwash from the overlying iron rich glacial drift. Note that the rock fragments (clasts) in the drift are very angular since they have been shattered under periglacial conditions.                  John Downes

johnd

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Re: Field trip to Freshwater West and West Angle Bay 12/05/18
« Reply #1 on: May 17, 2018, 02:19:52 PM »
Here are a few images of Freshwater West.

johnd

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Re: Field trip to Freshwater West and West Angle Bay 12/05/18
« Reply #2 on: May 17, 2018, 02:44:31 PM »
Here are some images of West Angle Bay.

welshcol

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Re: Field trip to Freshwater West and West Angle Bay 12/05/18
« Reply #3 on: May 18, 2018, 06:06:30 PM »
Looks like you had a great day not only geology but the weather as well.😁😁
Sorry to have missed it 😞😞