Posted by: Rob Viens | March 20, 2013

Falkland Geology Part I: Ancient Shells and Glacial Remains

Strangely enough, Darwin seems to find himself bored in the Falkland Islands.  After a week of no diary entries, all he really has to say on March 17th is:

“This is one of the quietest places we have ever been to.— Nearly all the Ships are gone; & no one event has happened during the whole week: The boats are employed in surveying.—

I walked one day to the town, which consists in half a dozen houses pitched at random in different places.— In the time of the old Spaniards when it was a Botany Bay for Buenos Ayres, it was in a much more flourishing condition.— The whole aspect of the Falkland Islands, were however changed to my eyes from that walk; for I found a rock abounding with shells; & these of the most interesting geological area.” (March 10-17)

Even his Zoological Notebook is rather quiet – mainly commenting on some coastal critters. I find this to be quite unusual, given that Darwin had an entirely new island ecosystem at his fingertips, yet didn’t really get out and explore. It makes me wonder if he was not feeling well, shaken by the death of a crewmate who wandered off on his own, or  just stuck on the ship with no easy way to explore.  To be fair, the Beagle does return to the Falklands about a year later (in 1834), so he may have a lot more to say about the biology at that time.

I can’t help but notice, however, that little glimmer of excitement in that last line – where he notes his view of the islands changed when he “found a rock abounding with shells”.  Nothing like geology to inspire a new appreciation of the landscape :). So what do we know about the geology of the Falklands?

Interestingly, it appears that the first real geologic map of the islands was only published about 15 years ago (in 1998) by the British Geologic Survey, and that Charles Darwin made the first real geologic observations of the region. So, when our friend picked up his shell-filled rock 180 years ago, we can note the event as the beginning of the geological exploration of the islands. Below is one of those very first rocks (maybe even the one he mentions above) that Darwin himself collected (image from the British Geological Survey). How cool is that!

fossil sample from Falklands collected by Darwin

The shells in this rock are brachiopods – a completely different phylum of animal life than the shellfish (such as clams, mussels and oysters) that we are familiar with today.  Brachiopods are still alive today, but are far outnumbered by Phylum Mollusca, which includes bivalves (clams, etc.), gastropods (snails) and cephalopods (octopi, squid, etc.).

Back to the rocks… The following geologic map (also from the British Geological Survey) shows the basic layout of the island.

Geologic Map of the Falkland Islands

Click for a larger image

A couple of things jump out just by looking at the colors:

  1. The geology appears to be dominated by two basic regions (the purples and the greens), and
  2. The ‘purple” regions cover West Falkland and the northern part of East Falkland, while the “greens” (and orange) covers the central and southern parts of East Falkland. (Incidentally, this corresponds with the topography – the “purple” regions are more mountainous while the “green” areas tend to be flat rolling hills.)

Geologic speaking, these two regions have different origins.  The “purple” area is older – consisting of rocks from the Silurian/Devonian “West Falkland Group”. In geology, a basic “unit” of rock – one with a similar makeup and origin – is called a formation.  A “group” is made up of several formations that are in some way related.  If you think of the  layers of rock being like the pages of the book, you can think of formations as chapters in that book, and the groups as several chapters that have been lumped together into a “section”. Like with books – we almost always define formations/chapters, but not all formations/chapters are lumped into groups/sections.

Anyway, as you can see from the map, the West Falkland Group consists of several formations – One of these is the Fox Bay Formation from which Darwin’s fossils originated. All of the rocks found here (mudstones and sandstones) represent the shallow seas that covered the region about 400 million years ago. Some of them have been metamorphosed into quartzite – a rock that forms from the metamorphism of sandstone.

In the south, you find similar types of rocks (e.g., sandstone and mudstone of the Lafonia Group), however the rocks are about 100 million years younger (forming during the Carboniferous and Permian time periods). There is some speculation that this particular group, which is relatively carbon-rich, may be a potential source of oil.  This economic resource does not bode well for the modern political situation in the islands (…another story for another day).

Anyone reading this blog, can’t help but notice that one of the main formations in the “younger” rock layers is the Fitzroy TIllite – of course, named for the Beagle‘s captain.  This geologic unit, at least in my mind, is one of the most interesting, because tillite is a glacial deposit. Effectively it represents time (about 300 million years ago) when the region was covered in a sheet of ice. As it turns out, a significant portion of the Earth’s landmass was covered with ice at that time, placing this formation in one of the five great Ice Ages in Earth’s history.

FitzRoy Tillite (from British Geologic Survey)

FitzRoy Tillite

The southern tip of West Falkland also has some older rocks from the Precambrian Eon.  These metamorphic rocks, which are similar in age to some of the crystalline rocks Darwin observed earlier in Brazil and Argentina, date back to over 1 billion years ago.

More on the importance of these geologic observations in the coming days… (RJV)

PS – By far, the best resource on the Falkland Geology can be found on the Falkland Island Department of Mineral Resources Page (where I have also acquired many of the images used today). Check it out for more details.

Advertisements

Responses

  1. What a good discussion. It does seem a bit odd, considering these islands have been the objects of such contentiousness, that geological studies took so long to complete.

  2. […] – For more on the geology of the Falklands see Falkland Geology Part I: Ancient Shells and Glacial Remains and The Slow Boat from Africa (Falkland Geology Part […]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: