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Plymouth Field Course 2019
Group 7
Methodology
Strike and dip of bedding planes were recorded with a Suunto MC-2D compass clinometer around the primary antiform fold in 3 locations, on each side of the crest. The plunge of the fold was also recorded with the clinometer, before a visual inspection for the presence and direction of faults was performed.
Lastly, visual inspection of the bedding layers was carried out to determine the possible geological history of the area.
Results
The primary antiform fold observed was a recumbent fold with a maximum dip recorded of 60°; dips of 34° and 35° were observed on the south-east side of the crest. The strikes recorded on the south-east of the crest were 050° and 052°, with a strike of 034° on the opposite side. The plunge of the fold was recorded to be 4°, with the direction of plunge being 210°, facing Renney Rocks.
A dextral (right lateral) fault was observed with around 5m of slip between the rocks on either side of the fault.
Introduction
The survey took place on the Renney rocks formation separating Heybrook Bay and Westlake Bay in Eastern Plymouth Sound. This area lays on the border between the South Devonian and the Looe Basins. The sediment present in the Renney Rocks was laid down in a distal alluvial fan setting, which typically occurs when the flow of a river which runs through a steep valley into a flat plain is slowed down. The mudstones and sandstones found here were likely deposited by a river, remains of freshwater fish can be found every so often. The closing of the Rheic Ocean caused a multi-phase deformation of the sedimentary rocks, which lead to faulting, folding and the creation of cleavage.
A Geological SSSI that may penetrate into the Plymouth Sound Benthos, Renney Rocks is a site with several geological features of interest.
Inspection of the beach cliff revealed several bedding layers likely deposited after the last ice age when the periglacial environment warmed up and slopes began to collapse.
The lowest layer (Layer 1, Figure 1) contained rounded, clast supported cobbles and boulders, suggesting river transport. Above this was a very fine and rounded silt layer (Layer 2), very well supported and strongly sorted.
Layer 3 (Figure 1) is a poorly sorted, red coloured and very angular sand layer with mid-sized grains. This matrix supported portion was formed by mudflow deposition. The Ice shattering after the last Ice Age (about 16.000 years ago) broke down the rock into this poorly sorted sediment transect.
The layer above that (Layer 4, Figure 1) presents grey coloured and well sorted sand, silt and gravel. This highly angular smaller sediment constitutes a lower energy environment.
The top portion of the sediment is a thin vegetated layer which stabilises the soil.
Aims
The aim of this excursion was to expand our knowledge of the geology of the Plymouth area and to complement the geophysics side scan survey taken offshore on the Echo Explorer. The Renney rocks outcrop extend out from Heybrook Bay and into Plymouth Sound, adjacent to the area covered by the sidescan habitat mapping survey.
Date: 03/07/2019
Time: 12:30- 16:00 UTC
Eastings: 249225
Northings: 48810
Weather: 20°C, sunny. Moderate winds (6mph westerly). 1 okta cloud coverage.
Tide: Low water at 12:11 UTC, 0.91m
High water at 18:12 UTC, 5.57m
Staff: Alan Kemp & Morgan
Mood: Positive
Figure 1 - Image of the bedding in the cliff, annotated with the observed distinct layers