19 May, 2010

Accretionary Wedge: The Geo-Image Bonanza

Chris and Anne from Highly Allochthonous have asked us to chose our favorite geo-images for the current edition of the Accretionary Wedge carnival. Similiar to Suvrat I chose a microphotograph of carbonates. The image below is from my on-going thesis about Jurassic carbonates. I have chosen this image because it beautifully unites simplicity and complexity and allows me to tell the story of these rocks - with just a single image.

 Microphotograph of Jurassic carbonate and diagenetic history, crossed nicols.

What you see in the image above are primarily tangential ooids that formed in a shallow-marine, highly energetic depositional setting like a ooid bar or tidal delta with lots of wave and/or current activity as can be observe nowadays on the Bahamas. The good preservation might indicate primary calcitic composition of the ooids. The partially visible peloid in the lower right is probably a micritized ooid. The laminae are still barely visible. Intensive flushing of the ooid sand probably caused rapid and early cementation by marine-phreatic isopachous rim cements. They are not so well preserved. I think these were primary aragonite that was later neomorphosed to calcite. The early cementation clearly pre-dates the compaction and formation of micro-stylolitic contacts between the ooids. White rim cement is trapped inbetween two ooids on the lower right corner. Also note the brownish stylolite there. Probably during burial the pore space was filled by poikilotopic cement.

The combination of a high-energy depositional environment with intense cementation makes these rocks a good material for construction or the production of lime. The well cemented grains will not disintegrate during the calcination process.