Foraminiferal biostratigraphy and glacioeustatic control on cyclic carbonate microfacies in the Visean-Serpukhovian boundary beds (Aladag Unit, Eastern Taurides, Turkey)

摘要

The Aladag Unit, one of the tectonostratigraphic units in the Tauride Belt (Turkey), comprises a nearly uninterrupted Upper Paleozoic succession including the Visean-Serpukhovian boundary beds. These boundary beds in the Eastern Taurides are made up of mainly carbonates with some intercalations of sandstone and shale. A detailed micropaleontological study has revealed three biozones based on foraminifera. These biozones are, in ascending order, Eostaffella ikensis-Vissarionovella tujmasensis Zone (Mikhailovsky; Late Visean), Endothyranopsis (Reitlingeropsis) cf. sphaerica-Biseriella parva Zone (Venevsky; Late Visean) and Eostaffella pseudostruvei Zone (Tarussky; Early Serpukhovian). Traditional Visean-Serpukhovian boundary lies between the Endothyranopsis (Reitlingeropsis) cf. sphaerica-Biseriella parva Zone and the Eostaffella pseudostruvei Zone. The presence of 'Millerella' sp. aff. 'M.' tortula specimens in the last levels of the Venevsky horizon also suggests that the measured section, has the potential for the redefinition of the Visean-Serpukhovian boundary, which would coincide with the datum marked by the evolutionary appearance of the conodont Lochriea ziegleri. Boundary beds were deposited in open-marine, shoal, or bank and tidal flat environments, which were interpreted based on the analysis of 12 microfacies and 11 sub-microfacies types. The main microfacies types are (1) bioclastic packstone; (2) bioclastic packstone to grainstone; (3) bioclastic grainstone; (4) bioclastic-intraclastic grainstone; (5) intraclastic grainstone; (6) sandy bioclastic grainstone; (7) peloidal packstone to wackestone (8) wackestone-mudstone; (9) shale; (10) peloidal grainstone or peloidal packstone to grainstone with dark micritic intraclasts; (11) peloidal packstone to grainstone with fenestral fabric; (12) quartz arenitic sandstone. Based on the stacking patterns and vertical evolution of microfacies, several meter-scale shallowing-upward cycles, three sequences, and two intervening sequence boundaries were recognized in the studied section. Sequence boundaries, lying within the Mikhailovsky and Venevsky horizons, are the records of global sea-level changes during the first episodes of the Late Paleozoic Ice Age. The Visean-Serpukhovian boundary falls within the transgressive systems tract of the third sequence and is correlatable with the boundary delineated in Russia. The duration of cycles is calculated as 117 ky and interpreted as orbitally induced (Milankovitch eccentricity) glacioeustatic cycles.