Friday Field Foto #60: Chunk of microbialite in mudstone
One of coolest things about studying ancient sedimentary deposits is thinking about the material that makes up the sediment. Where did it come from? What can it tell us? More often than not, this means taking a look at the composition and/or age of sand grains or the bulk geochemistry of the mudstone. If you’re lucky enough to have material bigger than sand-sized grains then it gets a bit more fun because of the potential to see internal features.
The photo below shows a chunk of carbonate within mudstone (note toe of boot for scale at bottom). Click on it for a higher-resolution version.
The photo below zooms in a bit so you can see the laminated structure. A lamina is simply a very thin sedimentary bed (less than a couple centimeters thick). The laminae are not very distinct, but if you squint your eyes you should be able to see a crude layering (slightly tilted to the right).
So, what is this rock all about?
Well … to be perfectly honest, I did not spend any time researching this specfic rock. Everything I say here is from the researchers that were showing us around (if someone is truly interested in these specific rocks I will track down some real references … let me know in the comments).
The basic interpretation is that the crude lamination originates from microbial mats. The ‘living’ mats trap and bind sediment, successive layers stack over time, and when the organic matter decays this laminated structure remains. What’s cool is that these chunks that were transported off the shelf into the marine basin are all that’s left of it. That is, in this area, only the slope and basinal deposits have been preserved … the shelfal environments were eroded away long ago.
One thing to note is that although the age of the sedimentary succession is ~600 Ma (give or take several million years), the age of the clast itself is not very well known. In this case, we were told that it’s been interpreted to be generally equivalent in age.