Monday, 8 October 2012

Week 1 update from the field - a lot of logging


For the last week I have been out on fieldwork in the Scottish Borders. I have been sedimentary logging the section that is exposed along the coast in a small coastal village. The rock succession is over 400 metres thick, exposed in cliffs and in barnacle and seaweed encrusted rocks on the foreshore. There is a lot of work to do and I will be here for another two weeks. This week I have been working with TWeed team members Tim Kearsey and Mike Browne from the British Geological Survey (BGS) in Edinburgh.

The weather has been nice and sunny but a bit cold in the mornings and evenings. There have been a few rainy days (it is Scotland!), but luckily it has blown over quite quickly. The wonderful part about coastal fieldwork is amazing geology that is well exposed, the landscape, wildlife and being by the sea. The difficult part is that you have to take care of the tides and slippery seaweed. There have already been waves over the top of my welly once, falling over on seaweed a few times and Tim got his foot trapped in a rock pool. And its only week one!

 
The tide waits for no man (or woman). Me looking at rocks that are only exposed at low tide.

What and why is sedimentary logging?
What sort of fieldwork are we doing day to day? A sedimentary log is an illustrative representation of the sedimentary rocks in the sequence you are studying. It is in effect a column that is drawn from oldest rocks at the bottom to youngest at the top, with the rock units stacked on top of each other in sequence. The units vary in width on the column according to their grain size, and vary in thickness according to their actual thickness. So everything is drawn according to scale and you can rapidly see the differences between different types of sediments.

The great value of sedimentary logs is that you can show the nature of the rock succession easily and quickly. You can choose what scale to use, showing small details or large changes in sediment type such as big sandstone channels.  For the TWeed project this is why we are undertaking sedimentary logging:

1) to record the environment the rocks were deposited in, by noting what types of sediment are in the sequence and the fossils they contain.

2) to record the exact position of tetrapod and other fossil-rich rocks, relative to the entire sequence.

3) to correlate all the sections we are studying in the years to come, by reference to this and other sedimentary logs. This includes small field exposures of a few metres thick to the borehole that we hope will be 500 metres thick.

As I am logging the entire Ballagan Formation at this locality I am really getting to grips with how the sediments and their different depositional environments changed over time. I have found some amazing fossilised wood and rooted sediments, river sandstones, strange cemented dolomites and a range of different coloured siltstones packed with fossils.

An example of part of my log
In the photos below you can see part of the sequence I have been recording. The rock units have been tilted by mountain building events and are nearly vertical. On the sedimentary log these are represented in order of age, so the tilt of the rocks is not recorded, but the thickness and relative grain size of the units is recorded. 

The main feature here that I am pointing to is a large conglomerate, which is a sediment composed of lots of pieces of other rock. This can be seen on the log at 37 metres, and it has the largest grain size because it is composed of large pieces of rock, so it has a large width on the log. Below the conglomerate are siltstones, a rock type with very small grains, so these have a smaller width on the log. What is very interesting about this conglomerate is that it cuts into the units below (the beds labelled as ‘c’, which are cemented dolomites), and is actually made of pieces of these rocks and pieces of siltstone. This is an unconformity (as the conglomerate cuts into the underlying beds) and we can interpret it as having formed due to a river changing its direction and moving across this area or due to a flooding event in this region.

 
Me sitting on the rock units (left) which are drawn in the sedimentary log (right). The units get younger from right to left in the photo, which is represented vertically in the sedimentary log.


Future fieldwork
This week I will continue sedimentary logging, I have hit the 200 metre mark now so nearly half way there. To work around the tides (which are not as good as last week) I will also be making research visits to the BGS and National Museums Scotland in Edinburgh to examine the fossil collections there. I will study the collections of ostracods and spinicaudants; both small aquatic arthropod crustaceans found in these rocks. In week 3 the rest of the team are coming for a sampling week, so there could well be some more tetrapod discoveries coming up!

Until next time
Carys

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