Project Introductory Fieldtrip

Project Introductory Fieldtrip
Thursday, 6 November 2008
9.00am – 2.00pm

As planned in our 1st meeting, Richard and I met in Barry Island’s train station on Wednesday the 12th at 9.00am. The objective of this fieldtrip is to observe features of the outcrops that we’re going to get our data from. It was a perfect day to engage into fieldwork that day. The sun was up and the weather was pleasant.

We started off by walking towards the beach westwards onto Barry Island. Facing the beach, there is a solid 5-metre wall of layered Triassic sandstone outcrop. A fault trending North-South was identified and we recognised a few small-scale (less than 1 metre) beds’ displacements. About 3 metres W-SW-wards from the ramp, we were able to see the unconformity layer which distinctly separates the Carboniferous limestone beneath the Triassic sandstone formation. The Triassic sandstone was  brownish-reddish, multi-layered beds with bed thickness of approximately 30cm-50cm each. Walk 10 metres W-SW-wards further and you’ll see the late-Carboniferous-Permian unconformity. The unconformity layer was a mess! Chuncks of brecciated rocks were accumulated and piled on top of the erosional unconformity surface. The unconformity layer is an important feature of the overall structure, of which I will explain later.

As we walked down along the eastern side of the island, Richard informed that the rocks we stepped onto from that point onwards were the Carboniferous limestone. The limestone beds were tilted and dipped southwards. It is on the bedding planes of this formation that the stresses occurred. This formation extended from that spot onwards until the tip of the island.

Chronologically, shelly limestone was deposited in the Bristol Channel basin throughout the Carboniferous period (about 350 million years ago). Then there was a series of tectonic activities (the Variscan mountain building event) which tilted and uplifted the carboniferous beds, exposing it to erosion. A significant amount of rocks were eroded since mid-Carboniferous and Permian, and these rocks were then deposited as unsorted breccias. Then came the Triassic period (about 250 million years ago) and sandstones and other formations were deposited.

It was pointed out in a few papers, of which I am unable to cite for now, that there has been several extensional events which formed normal faults features in several localities across the Bristol Channel basin. Satisfied with our observations in Barry Island, we later moved on to Sully Island by car, Richard was driving of course. It took us about 20 minutes driving to reach the site.

The most interesting part of the fieldtrip that day was that we identified a feature that could pin-point possible locations of faulted bedding planes! Outcrop structures in Barry and Sully Islands offer an ideal example to identify the structure mentioned.

Firstly, a fault is recognised in the Triassic sandstone. That fault has to cross through the Triassic sandstone, the brecciated unconformity and the underlying Carboniferous limestone. Secondly, orientation of the tilted Carboniferous limestone is notified. And thirdly, the correlation of fault growth and the orientation of the underlying bedding planes of the limestone is determined.

As a result of this observation, we were able to see that the bedding planes which presumably experiencing stresses were more prominent and well-defined than the other bedding planes. I suggested that the two beds of different shear directions might ground the surface between them. This may cause for incompetent materials to be eroded and washed away, resulting for that bedding plane to be clearly defined than the others.

This suggestion is justified by the presence of lineations on the hanging wall of the extensional outcrop.

To-do

I had to plan for my next fieldtrip to collect a significant amount of datasets. The datasets have to contain  location of records, strike/dip/dip directions, characteristics of bedding planes and faults, measurement of lineations and some remarks on individual recording.