Tuesday, February 12, 2013

Gorgeous Paper On Carbonate Diagenesis In Journal Sedimentary Research

The January 2013 issue of the Journal of Sedimentary Research is open access. There is a long and beautifully illustrated study on the diagenesis of Permian carbonates from the Guadalupe mountains of New Mexico U.S.A. by David A.Budd and colleagues.

These carbonates were fractured very early during their depositional history, in fact the fractures are syndepositional i.e. they formed as the sediments were accumulating. Sometimes calcium carbonate sediments undergo cementation and hardening by sea water just a few centimeters to meters below the sediment water interface and they then are rigid enough to fracture. Budd and co-workers painstakingly analysed the materials filling these fractures using a variety of sedimentary petrology and geochemical techniques and found out that these fractures acted as conduits for the movements of fluids not just early on but through the entire geologic history of these rocks. So the geometry of fluid flow networks may be established very early in the rock history with implications for the distribution of porosity and localization of economic deposits.

I love such detailed petrologic studies. I was consumed by this kind of research during my PhD days, thinking about marine and meteoric cements and porosity formation and the movement of fluids and its interaction with rock from its deposition to deep burial. It was a real pleasure to read this long and exhaustive work.

I said beautifully illustrated so let me post below an example from the paper. The image shows the cementation and interpreted geologic history of a fracture using plain light and cathodoluminescent microscopy.

Explanation from the paper:

A) Paired plane light and B) cathodoluminescent photomosaics through fracture fill B. The wall of the large fracture is lined with bladed dolomite cement (black arrows). Overlying the dolomite is a first generation of luminescently zoned (non to dull to bright orange) calcite cement (CC1). With subsequent refracturing, those first generation cements were mechanically rotated, brecciated, and lightly etched (white box), and then the combined fracture opening was encased in a bright orange luminescent calcite (CC2).

Just to show off, let me put up a similar kind of image from my PhD work in the Upper Ordovician strata of the southern Appalachians U.S.A.  Again a pore space illuminated by cathodoluminescence shows different cement generations.

Long live Sedimentary Geology!

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