Tuesday, February 3, 2009

Stylolites

Reactions:  
During field work one may encounter a lot of fascinating things (i.e. flying deer - but that's a weird anecdote for later). More of a geological context can be observed in the below foto. Wonderful, wonderful stylolites! They are plentiful where I work but these where the best ones to capture right there in the wild. Dirty fingers for scale...



There has been some debate over how exactly stylolites form which I would recommend you to read in the available literature. I think we can all agree that they represent some form of pressure solution related to diagenesis of limestone rock. They are usually absent in limestones containing more than 5 to 10% clay. So plenty of stylolites are a good first indication that you are looking at a pretty pure limestone. They will cross-cut any kind of rock fabric without discrimination. Nevertheless when working in limestones you may find horizons that are preferably transected by stylolites. There is sufficient indication that early cementation is respondsible for this phenomena. Early cementation protected grains from mechanical and chemical compaction. So it may also be a hint for episodic cementation. It is hard to tell how much rock has undergone solution though. Dispite that I think it is safe to say that it was at least the same amount as the amplitude of the stylolites. Probably a lot more.

The dark color of most stylolites is caused by the accumulation of insoluble substances. Clay and iron oxides from the dissolved limestones will be concentrated. Stylolites often represent barriers to permeability. Something you should keep in mind especially when interested in petroleum production. Nevertheless there are examples where stylolites can act as important porosity and conduits for fluid migration.

Having said that: Perhaps I should scrap-off some of the dirt of some stylolites and have it analysed?

If you want to know more: Carbonate Sedimentology by Tucker & Wright (1990) has a short but good summary.

No comments :