ABSTRACT: Kashagan Field is an isolated Devonian-Carboniferous carbonate buildup located in the southern Pre-Caspian Basin of Kazakhstan. A stratigraphic framework interpreted from biostratigraphy demonstrates that the Carboniferous part of the Kashagan buildup formed in four stages, each corresponding to a change in the position or geometry of the buildup margin. Stages I (Tournaisian- lower Visean) and II (upper Visean) represent successively backstepped margins above a Devonian foundation. Stage III is an upper Viseanâ€“Serpukhovian prograding margin that filled in accommodation created above the Devonian by the retreat of Stages I and II. Stage IV is formed by an aggradational Bashkirian margin at the top of the buildup. Kashagan is characterized by a narrow structural elevation (rim) around the outer edge of the buildup 160-m higher on average than the buildup interior. The chronostratigraphic units comprising Stages III and IV reveal that the elevated rim was not present during deposition, therefore it is most likely the result of differential compaction during burial. The Stage III biozones are parallel in the buildup interior but then successively thicken and become younger basinward under the elevated rim, indicating margin progradation. Stage III is truncated by a buildup-wide unconformity capping Serpukhovian-aged strata. Stage IV contains horizontal biozones that maintain relatively uniform thickness between the interior and the rim, and have shallow marine facies in both regions. This arrangement precludes the existence of significant positive depositional relief in the rim area. The elevated rim most likely formed as a result of differential resistance to burial compaction between rigid, low-porosity facies in the rim and more compressible high-porosity facies in the buildup interior. The Stage III margin contain microbial facies, including massive microbial boundstones at least 180-m thick formed by early lithification on the prograding upper slope. The Stage III margin also includes a zone of mixed microbial and non-microbial facies overlying the microbial slope leading up to the Serpukhovian unconformity. The microbial boundstones and mixed facies form a laterally contiguous shallowing-upward facies succession distributed diachronously across upper Visean and Serpukhovian strata in the rim. The shallowing-upward succession contains nine distinct microfacies that indicate systematic paleobathymetric variations along the margin that may be applicable to other Late Paleozoic carbonate systems.
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