Structural Analysis of the Late Paleozoic Carbonate Platform of the Northwestern Bolshoi Karatau: Geological Map and Cross-Sections

Abstract

This thesis presents a detailed structural analysis of the Devonian - Carboniferous carbonate platform in the Bolshoi Karatau, southern Kazakhstan, addressing a gap in outcrop-scale mapping and integration with existing lithostratigraphic models. Previous studies focused largely on regional facies architecture, depositional environments, and platform evolution, but lacked systematic mapping of folds, small faults, thrusts, and bedding orientations at the outcrop-to-valley scale. To overcome this, extensive fieldwork was conducted across multiple valleys, including Akuyuk, Besaryk, Zhertansai, Locomotive, Ushozen, Aktobe, Zhankorgan, and Shert, resulting in high-resolution geological maps, cross-sections, and stereographic analyses of structural measurements. A key methodological consideration was the choice between lithostratigraphic and chronostratigraphic boundaries. The results demonstrate that a lithology-based approach, which emphasizes lateral continuity and facies relationships, provides a more coherent framework for mapping carbonate platforms than strictly age-based divisions. This approach preserves depositional architecture across time-transgressive boundaries, enabling more accurate reconstructions of three-dimensional platform evolution. Structural analyses reveal generally NW-SE trending fold axes across most valleys, with local deviations indicating polyphase deformation. A critical observation is that the Maidantal Thrust does not cut across folds but is consistently folded with adjacent anticlines and synclines, reflecting syn-kinematic thrusting. Cross-sections AK1 to AK4 illustrate the geometry of the Akuyuk Nappe and underlying Carboniferous units, showing steeply plunging folds in the west gradually flattening eastwards. In the Aktobe valley, the graben structure records multiple deformation stages, including early thrusting, folding, Cretaceous peneplanation, subsequent tilting, and Upper Pliocene subsidence, providing a detailed temporal and geometric reconstruction of deformation. The integration of geological mapping, cross-sectional analysis, and stereographic data enables a refined understanding of platform-scale structural evolution, bridging the gap between sedimentological interpretations and structural frameworks. The findings highlight the importance of outcrop-scale observations in studying polyphase deformation, fold-thrust interactions, and fold-axis variability, while also informing lateral facies transitions and potential reservoir distribution. Overall, this study demonstrates that quantitative structural data provides a robust framework for reconstructing the complex evolution of the Bolshoi Karatau carbonate platform, offering insights relevant for regional tectonic synthesis, platform evolution models, and applied geological studies.