From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, September 14, 2007
The Search for Burial Dissolution in Carbonate Rocks
Bureau of Economic Geology, The University of Texas at Austin
Carbonate rocks are very susceptible to dissolution, a conclusion that can be attested to by the numerous caves and sinkholes found in the Cretaceous carbonates of Central Texas. Therefore, it is not surprising that dissolution is a favorite method for forming porosity in carbonates. Indeed, carbonate fabrics show ample evidence of dissolution. Some prominent geologists go so far as to suggest that all carbonate porosity is formed by dissolution in the burial environment. Evidence includes (1) a belief that carbonates rapidly lose their porosity and, therefore, that any porosity found in ancient rocks must be from dissolution; (2) stylolites are known to be a burial phenomenon, and porosity associated with stylolites must be formed by burial dissolution; and (3) carbonates often contain high-temperature minerals, such as saddle dolomite, fluorite, and sphalerite, suggesting that high-temperature brines capable of dissolving carbonates are present in the burial environment.
Modern carbonate sediments are highly porous, and data show that they do not lose their porosity rapidly. Porosity-depth plots suggest a variety of porosity-depth curves, probably related to early diagenetic history, thermal gradients, and geopressures. Therefore, under the right conditions, high values of primary porosity can be found at depths of over 15,000 ft (4.5 km). The relationship of pore space to stylolites is a key observation. Whereas pore space is commonly associated with stylolites, I have not seen clear evidence of pore space that formed by dissolution after stylolitization. In my experience, the most common pores associated with stylolites are short fractures that emanate from stylolite peaks and valleys. This pore space is formed by stresses and not by dissolution. To suggest that the presence of high-temperature minerals implies that the pore space is produced by the water that precipitated the minerals neglects to explain how water can precipitate and dissolve carbonate at the same time.
Some evidence suggests that aragonite is dissolved and reprecipitated as calcite at depth. However, the most common depth-related diagenetic events are cementation and compaction, events that may rearrange pore space but generally reduce porosity. Thus, dissolution is not a common porosity maker in carbonate rocks, as generally thought. Instead, porosity is formed during deposition and is lost with burial as a result of cementation, compaction, and recrystallization.