Origin of dolomite in the Late Cretaceous–Paleocene limestone turbidites, Eastern Pontides, Turkey

Abstract

The Late Cretaceous–Paleocene Tonya Formation of the Hacımehmet region in Trabzon (Eastern Pontides, Turkey) is composed of limestone turbidites. These limestone turbidites were deposited at water depths of around 500 to 600 m. and remained buried until uplifted in Late Paleocene time. The lower part of this formation, which is made up of clast-supported calcirudites and pebbly calcarenites, is partially to completely dolomitized. Two morphological types of dolomite are identified: (1) fine to medium crystalline dolomite (RD1), (2) coarse to very coarse crystalline dolomite (RD2). Each one of the dolomite types is non-stoichiometric, Ca-rich (Ca 57–61Mg 43–39), and has similar geochemical features and uniform dull red/brown luminescence. These dolomites have δ 18O PDB values from − 2.6‰ to − 3.3‰ (average = − 3.0‰), δ 13C PDB values from + 2.3‰ to + 3.1‰ (average = + 2.7‰) and 87Sr / 86Sr ratios from 0.707719 to 0.707772. Fluid inclusion data of RD2 dolomites yield homogenization temperatures ( T h) of 48–58 °C and salinities of 20.4 to 21.7 wt.% NaCl equivalent. The Sr concentration in dolomites, varying from below a detection limit of 20 to 360 ppm, is not homogeneous within each dolomite crystal. The Na concentrations of dolomites are similar, varying from 600 to 760 ppm (average = 680 ppm), and are higher than those for dolomites formed in normal marine environments. The Fe and Mn concentrations range from 2500 to 7000 ppm (average = 3800 ppm) and from 50 to 850 ppm (average = 330 ppm), respectively. Considering the petrography, paragenesis, trace element and stable isotope data, the dolomites in the Tonya Formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths. Dolomitization in the Tonya Formation was most probably driven by seawater that was slightly modified through rock–water interaction and, in part, at elevated temperatures.