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    Genesis of Late Miocene-Pliocene lacustrine palygorskite and calcretes from Kirsehir, central Anatolia, Turkey
    (MINERALOGICAL SOC, 2014) Kadir, S.; Eren, M.; Kulah, T.; Onalgil, N.; Cesur, M.; Gurel, A.
    This paper addresses the mineralogy, geochemistry, and genesis of palygorskite and associated calcretes in the Kirsehir region. In this area, the Quaternary calcretes are present within and/or on lacustrine mudstones of the Kizilirmak Formation (Late Miocene-Pliocene). The calcretes occur in the form of nodules, tubes, fracture-infills, and hard laminated crusts (hardpans). Calcrete samples are predominantly composed of calcite associated with smectite, quartz, feldspar, and minor palygorskite. The lacustrine mudstone samples consist mainly of smectite and palygorskite with minor calcite, quartz, feldspar, illite, kaolinite and amorphous material. Petrographic studies revealed that the calcrete samples are predominantly micrites that were partially converted to microsparites by recrystallization. Scanning and transmission electron microscopy images indicate that palygorskite fibres formed authigenically on and between calcite rhombs in the calcrete samples and at the edges of smectite flakes of host rocks adjacent to the calcretes. The oxygen and hydrogen stable isotope values of the calcite suggest that it precipitated from meteoric water under arid or seasonally arid climatic conditions. Calcite precipitation occurred in the vadose zone, as evidenced by desiccation cracks and vadose pisolites. The enrichment of Rb+Ba +/- Sr and Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE) with a distinct negative Eu anomaly are responses to the fractionation of volcanogenic feldspar and hornblende under the influence of meteoric water. The palygorskite associated with calcretes was precipitated from alkaline water rich in Si, Al and Fe and low in Mg. The palygorskite precipitation occurred at increased temperatures, characterising the advanced stage of calcrete formation which lowers the Ca content in the soil water. The required chemical elements for palygorskite and calcite were most likely derived from the dissolution of smectitic clays, ignimbrites and carbonate minerals.
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    Palaeoenvironmental approach to the geology, mineralogy and geochemistry of an Early Miocene alluvial-fan to cyclic shallow-lacustrine depositional system in the Aktoprak Basin (central Anatolia), Turkey
    (MINERALOGICAL SOC, 2010) Gurel, A.; Kadir, S.
    The Aktoprak Basin, Turkey, provides important clues regarding regional palaeoclimatic changes which have not so far been elucidated. The purpose of the present study is to fill this gap using mineralogical, geochemical and isotopic characterization of cyclic sedimentation within Neogene sedimentary units. Early Miocene alluvial-fan to cyclic shallow-lacustrine sediments of the Aktoprak Basin consist of conglomerate, sandstone, mudstone, marl, limestone and palaeosol. The increasingly fine upward grain size and bed thicknesses suggest that the alluvial fan was fed by an ephemeral braided river. The cyclic sandflat mudflat shallow-lake succession is predominantly composed of small-scale sandstone-mudstone beds. Sandstone intervals in the sedimentary cycles are interpreted to be a result of sedimentation from shallow, ephemeral braided-river channels, the deposits having evolved under water-saturated conditions in response to climate changes. The mudstone intervals are interpreted as having been laid down via sedimentation during distal sheet floods which developed either under sub-aerial exposure or in a water-saturated environment. Palaeocurrent measurements indicate that the general transport direction in the Early Miocene was from east to west. The lacustrine sediments of the Aktoprak Basin consist of calcite + quartz + feldspar + serpentine coexisting mainly with smectite palygorskite illite chlorite. Illite and chlorite are either detrital, having originated from basement units, or the illite formed authigenically from smectite. Calcite and clay appear as cement-building conglomerate, sandstone and mudstone. The clay content decreases with increasing calcite and the presence of an inverse relationship between CaO and other oxides may correspond to an inverse relationship between calcite and other minerals. Furthermore, Al2O3, Fe2O3, TiO2, MgO, Ni, Co, Cu, Sr, Ba and Zr values in the smectite-bearing mudstone and sandstone are consistent with the chemical compositions of rocks in the surrounding area, mainly ophiolitic and volcanogenic associations. Micromorphologically, the development of smectite as a cement on leached feldspar, the occurrence of palygorskite fibres as bridges between crystals and rock fragments, as well as the presence of 'edging' smectite flakes suggest an authigenic origin. Field observations, mineralogy, geochemistiy and the results of delta O-18 and delta C-13 isotopic determinations reveal that periodic palaeoclimatic changes were controlled by alternating wet and dry periods that resulted in a relative upward decrease in precipitation of authigenic smectite +/- palygorskite within the sections, under the influence of geochemical and pH fluctuations in meteoric water within the shallow-lake environment.