Abstract

Pliocene volcaniclastic lacustrine rocks of the Doğanbey formation in the Beyşehir region (central Anatolia) are composed of organic-matter-bearing claystone, clastic units and dolomite interbeds, suggesting an anaerobic, shallow swampy, lacustrine depositional environment. The depositional environment was subjected to periodic climatic change during which diagenesis occurred, and smectite, and locally palygorskite, were precipitated. Smectite flakes formed authigenically on feldspar and palygorskite fibers between dolomite rhombs and at the edges of smectite flakes. Increases in leaching of Na, K, Sr, Ba and Rb, increasing Al/Si ratios, and Fe with increasing degree of alteration reveal that hydration of volcaniclastic grains (feldspar, glass) by meteoric water – determined from O and H isotopic values – was the main cause of precipitation of beidellite and montmorillonite based on the tetrahedral charge/octahedral charge ratio, with average structural formulae of (Si7.72Al0.28)(Al3.20Fe0.53Mg0.25 Mn0.02Ti0.04)(Ca0.11Na0.09K0.11), and (Si7.88Al0.13)(Al3.18Fe0.53Mg0.18Mn0.02Ti0.05)(Ca0.11Na0.11K0.09), respectively. Therefore, the Doğanbey-area smectite is presumed to have formed by chemical weathering and dissolution-precipitation from feldspar and glass during diagenesis; palygorskite formed by direct precipitation from Mg-rich solutions during dolomitization, and by transformation from smectite in an alkaline lacustrine environment.

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