Paleosols and Carbonate Sequence Stratigraphy, Carboniferous, South Kazakstan

P. J. Lehmann, H. E. Cook, W. G. Zempolich, V. G. Zhemchuzhnikov, V. YA. Zhaimina, A. Ye. Zorin, M. Bowman, Giovannelli, M. Viaggi, D. Hunt, N. Fretwell, and D. Craik

Carbonate platform facies in the Karatau Mountains of southern Kazakhstan are analogs for coeval oil and gas fields in the North Caspian Basin, West Kazakhstan. Understanding the sequence stratigraphy of these analogs is enhanced by the recognition and interpretation of paleosols. Thirty four paleosols subdivide 620 m of Visean-Bashkirian carbonates that span ~30 my. M. and U Visean strata consists of slope apron to platform margin skeletal/oolitic grainstone subdivided by 15 paleosols, that stack into 5 upward-thinning sequence sets. The upper Visean marks a significant increase in accommodation where shelf lagoon burrowed, skeletal wackestone/packstone shoal upward into skeletal/oncolitic grainstone/packstone. Here depositional units thicken and are capped by thin perit dal laminites, not paleosols. The Serpukovian has 13 paleosols developed in biostromal open shelf grainstone/packstone and skeletal/oolitic grain shoal complexes. The lower Bashkirian records a major flooding event where lower slope laminites lie only 7 m above three stacked, deeply-rooted paleosols. This section shoals upward into upper slope ooid-rich turbidites. Sequence boundaries are marked by 4 burrowed firmgrounds in the lower slope facies and by 3 paleosols in the upper slope facies.

Paleosols are characterized by the following: (1) laminated micrite crusts (i. e. Multer crusts); (2) rhizoliths; (3) alveolar texture; (4) brown isopachous and pendant spar; (5) desiccation cracks; (6) glaebules; (7) micritized grains; and (8) tangential needle fibers of calcite. Most rhizoliths and micrite crusts penetrate less than 1 m of strata. The shallow penetration, dark color of the micrite crusts and rhizoliths, and common isopachous cements may indicate significant paleosol formation in a humid climate. The-repeated occurrence of paleosols on subtidal upper slope, platform margin and platform interior facies strongly suggests a eustatic or glacio-eustatic origin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California