Abstract: Evaporite Collapse Breccia versus Karst Breccia: The Upper Devonian-Lower Carboniferous Balaturlan Unit, Bolshoi Karatau Mountains, Southern Kazakhstan

LAPOINTE Ph. A., BERNET-ROLLANDE M. C., V. G. ZHEMCHUZHNIKOV, JSC "Eezdenees"; H. E. COOK, USGS; W. G. ZEMPOLICH, P. J. LEHMANN

Famennian carbonates of the Bolshoi Karatau, span a stratigraphic interval of over 1,300 meters. The platform margin consists of reef-rimmed algal-stromatoporoid boundstone whereas platform interiors are skeletal-rich mud mounds and shallow water cryptalgal laminites and stromatolites. A major diachronous (?) breccia occurs at or near the Famennian - Tournaisian boundary in the upper Balaturlan Unit. This breccia is up to 90 m-thick and extends regionally parallel to the platform margin, discontinuously, for at least 50 km. Most of the outcrops evidence evaporite collapse, followed by karstification, whereas some may have a dominantly karstic origin.

Platform-interior facies consists of black laminae interbedded with whitish dolomitic laminae, interpreted to represent former evaporites. These laminae show enterolitic features, plastic deformations at scales ranging from centimeters to ten meters. Two major phases of evaporite diagenesis are interpreted to have occurred in these facies: -1) water expulsion during the gypsum - semi hydrate - anhydrite transformation; -2) development of collapse breccia due to evaporite removal and sulfate reduction processes. Dolomitization was probably coeval with the sulfate reduction as there is no clear evidence for an initial calcite that was later dolomitized.

The Balaturlan Breccia was initiated when a sufficient thickness of evaporite was deposited. This collapse breccia resulted in a mechanically weak stratigraphic interval that was overprinted by a minimum of four karsting phases, each one identified by a specific internal sediment filling. This superposition of multiple karsting phases on the evaporite collapse breccia enhances the potential reservoir characteristics of the Balaturlan breccia.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria