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- MakaleYalnız künye2012Petrography and geochemistry of Eocene sandstones from eastern Pontides (NE TURKEY): Implications for source area weathering, provenance and tectonic setting
Eker, Cigdem Saydam
Subaerial weathering level, source area and tectonic environments were interpreted by using petrographic and geochemical characteristics of Eocene age sandstones found in the eastern Pontides. The thickness of Eocene age clastic rocks in the eastern Pontides ranges from 195 to 400 m. Mineralogical components of sandstones were mainly quartz, feldspar, rock fragments, and opaque and accessory minerals. Depending on their matrix and mineralogical content, Eocene age sandstones are identified as arkosic arenite-lithic arenite and feldspathic wacke-lithic wacke. CIA (Chemical Index of Alteration) values observed in the Eocene age sandstones (43-55) suggest that the source terrain of the sandstones was not affected by intense chemical weathering. Low CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values of the sandstones studied here suggest only slightly decomposed material and having undergone little transport until final deposition. Zr/Hf, Th/Sc, La/Sc and CIA ratios are low and demonstrate a mafic source; on the other hand, high LREE/HREE ratios and a slightly negative Eu anomaly indicate a subordinate fclsic source. Modal mineralogical and SiO 2/Al 2O 3 and K 2O/Na 2O and Th, Zr, Co, Sc of Eocene age sandstone contents indicate that they are probably magmatic arc originated and deposited in the back arc basin.
- MakaleYalnız künye2011Mineralogy and whole rock geochemistry of lzate Cretaceous sandstones from the eastern Pontides (NE Turkey)
Eker, Cigdem Saydam; Korkmaz, Sadettin
This study focuses on Upper Cretaceous sandstones of the Eastern Pontides to identify the source area, subaerial weathering level, and tectonic environment. Thirty four sandstone samples from six stratigraphic sections were analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MC) and by petrographic investigation. The late Cretaceous sandstones are defined as lithic arenites and feldspathic wackes, and lithic wackes depending on mineralogical and matrix content. The sandstones have low to moderate SiO2 contents (25-59%; on average -56%). There are high positive correlations between SiO2 Al2O3, and TiO2 and negative correlations between SiO2 and K2O. Geochemically, the sandstones are classified mainly as grey wackes and lithic arenites. Chemical index of alteration (CIA) and chemical index of weathering (CIW) values for Upper Cretaceous sandstones vary from 19 to 48 % and 20 to 49 % respectively. The combination of CIW/CIA values suggests little transportation until final deposition. The ratios of Th/Sc (on average -0.42), La/Sc (-1.81), Co/Th (-5.05) and Zr/Hf (-39.1) of sandstone samples in the study area indicate a mafic source. Furthermore, high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (on average -19.72) and negative Eu anomalies (except in the Dagbasi section) suggest a contribution from a felsic source. The lower trace element contents of the Upper Cretaceous sandstones in relation to the Post-Archean Australian Shale (PAAS), which are considered to be representative of the upper continental crust, show that the recycling processes in their source areas were probably less intense than those of the PAAS. SiGyAlA (3.2-7.9) and K2O/Na20 (0.3-1) ratios, together with the Th (on average -3.3 ppm), Zr (on average -91.9 ppm), Co (on average -15 ppm), and Sc (on average -12.6 ppm) contents of the late Cretaceous sandstones of the eastern Pontides indicate a magmatic arc origin. Sandstones of the Trabzon region were probably deposited in fore-arc basins, and those from the Gumushane and Giresun regions in back-arc basins.