From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004

Hydrothermal Overprint on a Complex, Coalesced, Collapsed Paleocave System

Robert G. Loucks

Abstract:

Some pore networks in collapsed paleocave systems are commonly filled with hydrothermal baroque dolomite. The hydrothermal fluids flowed through previously developed paleocave pore networks and passively precipitated baroque dolomite and, in some cases, developed an intricate boxwork structure along fractures.

A well-developed, coalesced, collapsed-paleocave system is exposed in Central Texas. Two distinct periods of brecciation interpreted to be associated with collapsed caverns are recognized. The older breccia system (Middle Ordovician to Mississippian in age) was overprinted with baroque dolomite during burial. It occurs as a replacement product and cement. The fluids responsible for this phase of hydrothermal alteration are thought to be associated with the Ouathica orogeny. Another feature associated with this period of alteration is the thermobaric boxwork structure that developed along fractures as a result of high-temperature fluids. The open pores in the boxwork are as fine as 1 mm, and the walls are dolomite that replaced host rock along fractures.

A second period of brecciation occurred during Pennsylvania time. Clasts are composed of older lithified breccia, and some clasts contain older boxwork structure. On top of each clast is a layer of geopetal vadose pink carbonate silt. An isopachous layer of very steep, rhombic, baroque-dolomite cement surrounds both clasts and vadose-silt deposits. The dolomite cement is interpreted as having precipitated by warm subsurface fluids during shallow burial in Cretaceous time. Upon renewed uplift, the larger interclast pores were filled with surface-derived detrital carbonate. This case study demonstrates an intricate interaction between karst brecciation and thermobaric processes.