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Saturday, 25 October 2014

Ketton Field Trip

I remember three things from my trip to Ketton in first year:

1) Being extremely hungover.
2) Thinking geologists are REALLY weird.
3) There was a lot of mud.

So, for those people out there who feel the same, here's a quick overview of what actually happened that day, and some tips for working in the field!


What we saw and some context

Lincolnshire Limestone

Warm shallow marine water
Ooids form by the precipitation of calcite around a
nucleus, such as a grain of sand.  They're round
because they roll around on the sea floor as they grow
This limestone unit was deposited in warm, shallow marine waters when Britain was at a similar latitude to the present day Mediterranean.  It's primarily formed of ooids; little round balls of calcite that form by the chemical nucleation of calcite (CaCO3) around a grain of sand or a shell fragment.  The oceans have high levels of calcite in them because lots of sea-dwelling creatures, such as clams make their shells out of it.  When these creatures die, their shells sink until they reach a depth (called the lysocline) where these shells easily dissolve.  

How do we know it was warm?  We can tell this from the latitude Britain was at during this time, paleoclimate studies and the fact that the little guys with calcite shells aren't a fan of cold water.

How do we know it's marine?  Again, it's all about our little sea-dwelling friends - by looking at the living equivalents of these fossils, we know where they like to live and where you're likely to find high numbers of them.  

Limestone forms between the lysocline, where calcitic
shells dissolve and the Carbon Compensation Depth
(CCD) where calcium dissolves faster than it can
precipitate.
How do we know what the water depth was? The limestone is just made up of ooids - they're round because they've been rolling around on the seafloor.  We need currents to roll them around, so we know we must be in relatively shallow water where surface currents are felt on the sea floor.  The fact we've got calcite also gives us a big clue - we must be below the lysocline, where calcite shells dissolve, but above the CCD (carbon compensation depth)  where calcite is no longer preserved.  These depths are well constrained by looking at the modern ocean.



Rutland Formation

An estuarine environment; tidal waters bring in sand and flood
the mud flats.



A Cetiosaurus skeleton was
found a few km from
Ketton


The Rutland is packed with all kinds of stuff.  Near the base of the formation there's loads of fossils, such as bivalve shells who like hanging out in very shallow estuarine environments.  These guys are found in sandy sediment, and then covered with mud.  You then get a cycle of sandy and muddy layers, getting progressively more muddy each time.  People have found dinosaurs in this formation too, and in the muddy layers you can also see plant rootlets.



Why do you get cycles of sand and mud? You get a cyclical sequence because we're in a tidal environment - what gets deposited depends on how deep the water flowing in is and how quickly it flows.  As the water flows in brings in sand, whereas when it flows out it leaves behind finer, muddy layers.  This is controlled by how 'energetic' the water is i.e. how big the particles are it can carry.

How have we got bivalves, dinosaurs and plants?  The presence of plant rootlets and dinosaur skeletons is a pretty good sign we're now on land, so it does seem strange to also have bivalves which live in water.  This is because we're in an environment which changes between the two; the bivalves live in the estuarine channel, whilst plants can happily grow and dinosaurs roam on the flood plains around the estuary.


Blisworth Limestone

A quiet, deep water environment where nothing on the
seafloor gets disturbed.

This unit is made up of fine-grained limestone - there's no ooids this time.  There's also lots of big bivalves and oysters.  

So what environment are we in now?  Well, the fact this limestone is fine-grained and not made up of ooids suggests it's from a deeper marine environment.  Fine grained usually means it's a quiet environment where tiny particles can settle out - if there was any flow in the water at all it could easily pick them up and move them.  Ooids provide further evidence for this, since they need a current to roll around in.  The big bivalves and oysters are another clue - you need space to get big, and that means more water.


Blisworth Clay

A bay - an enclosed, tidal environment.

There's lots going on here - dark clays full of organic material, lenses of limestone, sandy layers and some oyster shells near the top.  This suggests the environment is changing.
Lenticular bedding - when lenses of one sedimentary rock
appear in another it's characteristic of a tidal
environment.


I'm confused, what's going on? So the dark clays are similar to the previous environment - we need a deep, quiet marine setting where lots of dead things can settle out and rot.  Clay suggests we've gone below the CCD so all the calcite has dissolved.  We then get lenses of limestone appearing though, and this suggests we're moving into shallower water and sitting around the CCD - some calcite is now managing to precipitate and form limestone.  Then the sandy layers come in, so we've moved into a less quiet environment - there's enough flow to move the mud and sand around here, so we must be starting to feel currents from the surface.  We've got oysters too - they like hanging around in bays, so we're near the beach now. 


Cornbrash

Out in the open sea.

This is basically just limestone and ammonites - simple.

Is it simple though? Yes, it is really.  Ammonites like swimming in open water and we know the depths limestone forms at, so right now we're in slightly deeper water out to sea, a bit further away from our beach environment.


Boulder Clay

When glaciers retreat, they leave behind a big mish-mash
of all the rocks and mud they've been carrying along.

This layer is basically just one big mess - lots of surprisingly sticky mud you can't scrape off your shoes, and bits of every kind of rock imaginable - sedimentary rocks packed full of shells, igneous rocks, flint nodules, there's even metamorphics.  The perfect thing to baffle you when you've just started learning about geology.

How is this even a rock? It's less of a rock unit, and more of a deposit.  This is all the crap scraped along by glaciers which pick up and incorporate everything as they move along.  As the glacier melts, all this stuff gets left where the glacier ended in a great big muddy mess.

So hang on, we've had land, deep sea, shallow sea and now ice - but this is all in the same place?!  Yup, it's confusing and there's lots of things we need to think about to understand how Ketton seems to have had every environment under the sun.  There are three things to consider:

1) Britain's changing latitude through time.
Due to plate tectonic motions, the positions of the continents have varied throughout geological time.  Since the Late Permian, Britain has moved from a latitude similar to that of modern-day Northern Africa, through the tropics and up to its present day location.

2) Changes to sea level
Variation in sea level since the Jurassic
(note the small scale fluctuations as well
as the over-arching trend)

3) When ice ages happened
The Earth's global climate oscillates in and out of ice ages.


(The last two are interlinked and we have a good record of them due to clever studies by paleoclimate scientists).  

As Britain has trekked away from the equator due to plate tectonic motions, its climate has changed - it's gone from being hot and arid, like North Africa, to hot and tropical like the Mediterranean, to cold and drizzly like..well, Britain.  The position of the continents and oceans around Britain have also changed, meaning that sea level is dependent on what's going on around Britain, as well as changes in global sea level.  We also need to consider the global climate which cycles in and out of ice ages.  So essentially, we need to think on several different timescales and lengthscales to understand what's going on.  Also remember, these changes are happening very gradually over millions of years. 
These rocks were lain down over millions of years, with big gaps in the middle - it helps to think about this when
trying to understand how the paleoenvironment at Ketton has changed so much!



Important things to do with mapping

These are some useful things to actually be able to visualise, because they're terms that will come up a lot during mapping.  You saw plenty of good examples at Ketton!


Bedding

Bedding is the term used to describe the layers in which sedimentary rocks were originally deposited. It is usually assumed that sediments were deposited flat, such as on the sea floor, and then buried and squashed.  This process is then repeated, building up a series of layers, which are different beds. Over geological time, these layers can become tilted or folded due to tectonic activity.

When we map, we want to figure out what happened to an area over geological time.  By mapping bedding planes, we can figure out what tectonic activity has occurred in an area, and the relationship between different rock units.

Here's an example of some bedding seen at Ketton.  The bedding planes are drawn on in red.  Note how they've
been tilted from their original horizontal position.
An easy way to spot bedding can sometimes be to look for joints.  These are planar fractures that form perpendicular to the bedding planes.
Joints in the Lincolnshire Limestone at Ketton.  Note how I've only drawn on
joints and bedding where I can actually see them.  Joints are perpendicular
to the beds.

In the field you can take a strike and dip of the bedding, this tells you what orientation it's at.
The dip is the angle the bedding is dipping at relative to horizontal, and strike is perpendicular to dip and tells
the orientation of the bedding as a bearing


Which way up?

Sometimes, under extreme tectonic motions, bedding can get overturned so it's the wrong way up.  There are lots of ways of telling which way up a bed is.

Plant rootlets - we know that roots grow down, so you can tell from the root orientation which the top surface is.

Fining up - finer sediment is usually deposited on coarser sediment, so you know the coarse sediment must be at the bottom of the bed.

The way up is marked by a 'younging' direction, which is an upside down Y.
The bottom of the Y always points to the top of the bed.


Faults

Faults are caused by tectonic movements which cut through the rock layers and offset them.  There are several different types of fault, but the most common are normal, thrust and strike-slip faults.
Different types of faulting.
There's a really great example of a fault at Ketton, complete with slickensides, which are scrapes in the rock surface caused by sliding along the fault.  By looking at the orientation of these you can say something about the motion along the fault plane. You can also work out the motion along the fault plane by looking at its angle and the relative motion of the stratigraphy.

Normal fault brings down the younger Rutland Formation so it
sits next to the older Lincolnshire Limestone.


Unconformity

An unconformity is when two rock types are found next to each other, but there's a gap in their age.  Faults are an example of an unconformity, and these are easy to spot, however some unconformities are not so obvious.  There is an unconformity between the Lincolnshire Limestone and the Rutland Formation for example; they were deposited 7 million years apart, but they lie on top of each other.  You need to look for clues such as differences in strike and dip of the bedding of the two units to identify an unconformity.

An example of an unconformity - the lower beds are dipping
and get truncated by the upper horizontal beds.  The lower layers
must have been deposited, then tilted, and then some more time
may have passed before the upper layers were deposited.

What to write in your field notebook

Keeping a field notebook is really important.  There's lots of different styles and techniques people use to log their data in the field and it doesn't really matter which you follow, as long as you've got all the important points written down, and you can look back at your field notebook a few weeks later and still understand what was going on!

This is the way I like to keep my field notebook, but as long as all the important points are there - the style is up to you!


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