Geology: Like Cluedo, but with less death and more rocks

The job of a geologist is to study the rocks around them (their clues) and then come up with a hypothesis as to how they all ended up in their current state and condition.The photo below shows a megaclast (larger than a boulder >4.1m) of Cambrian Gonessa Limestone within younger Ordovician Mt Argentu Schist that was observed in southwest Sardinia. But how did a huge isolated clast of limestone end up standing proud within a large outcrop of schist?

It is unlikely the clast was deposited by fluvial processes due to its anomalous size. Faulting was also disregarded due to the lack of field evidence (the contact is non-linear and not at a consistent orientation). After careful consideration of the tectonic history and depositional environments it was proposed that the megaclast represents an olistolith ( ‘a pebble to several kilometre sized clast foreign to the host rock in which it is emplaced’ Cieszkowki, et al., 2012) emplaced shortly after the Sardic Phase (Cambrian age collisional event) of deformation.

A likely hypothesis is that this outcrop originated as a Limestone cliff overhanging an alluvial fan. Due to weathering, erosion and an increase in pore fluid pressure, a large section of the cliff cleaved off and fell into the muddy sediment below. This piece of cliff was originally emplaced horizontally within the sediment, however it is now standing vertical (stromatolite fossil evidence confirms this).

So how did it go from horizontal to vertical? Well Sardinia has had a rough history, having undergone several orogenic (mountain building) events as well as its fair share of volcanism. During this time it is proposed that folding led to the clast being returned to vertical and that subsequent erosion of the more easily weathered schist left the clast standing tall (see my rather crude sketch below).

However, as with most things in geology this conclusion is not set in stone (badum dum tss). The evidence we have so far indicates that the most likely emplacement of this clast was an olistolith but if new evidence comes to light this could easily change.

- Watson

Refrences: 
Paper Citation - Cieszkowki, M., Golonka, J., Ślączka, A. & Waśkowska, A., 2012. Role of the olistostromes and olistoliths in tectonostratigraphic evolution of the Silesian Basin in the Outer West Carpathians. Tectonophysics, pp. 248-265.

Further Reading:
Paper Citation - Ślączka, A. et al., 2012. Sedimentary basins evolution and olistoliths formation: The case of Carpathian and Sicilian regions. Tectonophysics, pp. 306-319.
http://www.sciencedirect.com/science/article/pii/0037073870900229

Image Credit: Watson
Horrific Sketch Credit: Watson