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Geology

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(Above) Mouse-over for way-up criteria

Introduction

06/07/18

Aims

To make a small structural map of the folded rocks complementing the side scan sonar survey of Plymouth Sound.

Method

Using compass clinometers, bearings were taken of the dip and strikes of the bedding planes. These were then located and drawn onto the map to help create an accurate depiction of the geology at Heybrook Bay. Using the bearings, areas of folds were identified and classified. Faults were identified by comparing key features, such as colour changes between layers and types of folding, then matching these features on the opposite side of the suspected fault. Finally, we identified the different layers of rocks and the causes in the small cliffs at Renney point.

Renney Point history  

The rocks in this area of Plymouth mainly comprise the continental river plain and shallow marine sediments. These were intruded by granites and deformed in the Hercynian orogeny that was caused by the closure of the Rheic Ocean. When this occurred, the sedimentary rocks underwent multi-phase deformation involving folding, faulting and the development of cleavage.

Faults + Folding

Results - MAP

The bedding planes showed anti-forms which had started to fold over each themselves and started plunging towards land with a bearing of 8°. The bearings change as we move across from East to West, however the dip stayed in the same direction.

The faults

It was found that there were 2 faults present at Renney Point, and that secondary folding had occurred further down the bed creating the rounded shape.  The faults were identified as a dextral fault and a sinistral fault as shown on the map (Above).

Way-up criteria

The way-up criteria made it possible to determine that the rocks were no longer the same way up as when originally deposited. The rocks had been overturned by subsequent deformation, also known as inverted beds.

Bedding

The layers of rocks identified in the cliffs were bedload at the bottom, suspended matter and then frost shattered debris. This top layer was created at the last glacial maximum when Renney point would have been Periglacial, therefore there would have been a permafrost at the top. This permafrost then melted and shattered the surrounding rocks creating debris that was unstable and moved down the slope creating a layer of frost shattered debris.

(Above) Renney Point

Summary

These results gave us an insight into the conditions of the seabed of The Sound. When using this information in combination with the side scan sonar we were able to see different features in the seabed and identify the type of substrate and therefore where to carry out the video transects.  

(Above) The bedding structure we studied