Dolomitization of the late Cretaceous–Paleocene platform carbonates, Gölköy (Ordu), eastern Pontides, NE Turkey

Abstract

Late Cretaceous–Paleocene Ağıllar Formation in the eastern Pontide zone, NE Turkey, is composed of shallow-marine platform carbonates that were buried until uplift in Late Paleocene time. The formation, which is represented by 100 m thick reefal limestones around Gölköy, is completely dolomitized. On the basis of petrographic and geochemical features, four different replacive and cement dolomite phases are present. The replacive dolomites, which are cut by low-amplitude stylolites, include 1) fine crystalline planar-s dolomite (Rd1) and 2) coarsely crystalline planar-s(e) dolomite (Rd2). Two types of dolomite cements were developed in minor amounts: 1) a coarse dolomite cement (Cd1) that mostly fills molds/vugs and fractures in Rd1 dolomites, and 2) rim dolomite cement (Cd2) that commonly develops on the space-facing surfaces of Rd2 dolomite. The replacive dolomites are nearly stoichiometric (Ca 49–51 Mg 48–50) and have δ 18O values of − 3.5 to − 1.6‰ VPDB, δ 13C values of − 1.1 to 0.8‰ VPDB, and 87Sr/ 86Sr ratios of 0.707104 to 707458. Sr, Na, Mn and Fe concentrations of replacive dolomites are 102–171 ppm, bdl–371 ppm, 244–9941 ppm and 2643–8951 ppm, respectively. Petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from Late Cretaceous–Paleocene seawater and/or seawater slightly modified by water–rock interaction process, and then recrystallized by meteoric waters at increasing burial depths and temperatures. Compared with replacement dolomites, Cd1 dolomites yield lower δ 18O values (− 4.9 to − 5.3‰ VPDB), similar δ 13C values (− 1.2 to − 0.5‰ VPDB) and higher 87Sr/ 86Sr ratios (0.707660–0.707609). The Cd1 dolomites overlie the low-amplitude stylolites and have homogenization temperatures between 94 and 112 °C, indicating that they were deposited from basinal brines. Although replacive dolomites and dolomite cements have similar Na (bdl–413 ppm), Mn (259–6990 ppm) and Fe (2184–6690 ppm) concentrations, the cements have lower Sr contents (63–114 ppm). Like replacive dolomites, Cd1 dolomite is nearly stoichiometric (Ca 49–50 Mg 47–49), but Cd2 dolomite is slightly Ca-rich (Ca 51–52 Mg 47–48). The Cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite.