Hydrothermal Carbonate Mineralization, Calcretization, and Microbial Diagenesis Associated with Multiple Sedimentary Phases in the Upper Cretaceous Bekhme Formation, Kurdistan Region-Iraq

Abstract

Hydrothermal diagenesis during the Zagros Orogeny produced three phases of saddle dolomites (SD1, SD2, and SD3) and two phases of blocky calcites (CI and CII) in the studied sections of Bekhme Formation (Fm) (Campanian–Maastrichtian). Field observations, as well as petrographic, cathodoluminescence (CL), Scanning Elecron Microscope (SEM), and oxygen–carbon isotope analyses, indicated that the unit went through multiple submergence–emergence phases after or during hydrothermal diagenesis. These phases resulted in a characteristic calcretized 2–6-m-thick layer within the Bekhme Fm. Several pedogenic textures (e.g., alveolar, pisolite, and laminar fabric microfeatures) were observed. Strong evidence of microbial alteration and diagenesis in this formation brings new insights into its depositional history. The microbial activities developed on the original mineral surface were associated with a great variety of processes including dissolution, re-precipitation, replacement, open-space fillings, microporosity development, grain bridging, and micritization. Probable oxalate pseudomorphs embedded in these fabrics and regular filaments preserved along crystal boundaries suggest the activity of fungi, while frequent coccoidal, rod-like, and chain-like forms attached to the surfaces of dolomitic and calcitic crystals point to bacterial colonization. Extracellular polymeric substance (EPS) was often visible with fungal and bacterial forms. These features, together with stable isotope data, invoke that near-surface conditions occurred sporadically in the Bekhme Fm after the first generation of hydrothermal dolomitization. These new findings allow recognition of unreported sedimentological phases based on new evidence in the Spelek–Sulauk area during the Upper Cretaceous.

Geochemical and Dynamic Model of Repeated Hydrothermal Injections in Two Mesozoic Successions, Provençal Domain, Maritime Alps, SE-France

“…This phase attributed to the late stage of calcite cementation and was consistent with the very depleted δ 18 O VPDB values and the lowest δ 13 C VPDB values (cf. [50]). The meteoric water has the lowest δ 18 O VPDB values and highest 7 Sr/ 86 Sr ratios (0.709630 to 0.70948, [51]).…”

Origin of Drusy Dolomite Cement in Permo-Triassic Dolostones, Northern United Arab Emirates

“…Constraining the depths at which the flux of hot dolomitizing basinal brines occurred is difficult without radiometric dating of dolomite [54,55]. However, the cross-cutting relationship between the stylolites and dolomite cement suggests that dolomitization took place at shallow depths [36].…”

Tracking the Origin and Evolution of Diagenetic Fluids of Upper Jurassic Carbonate Rocks in the Zagros Thrust Fold Belt, NE-Iraq

“…This could be due to the long-term interaction between the host rock and the diagenetic fluids, promoted by fluids heated up to 80 • C or more (e.g., [31]). Therefore, the considerable and significant variation of δ 18 O-δ 13 C from Group II could be linked to the evolution of fluids (e.g., [31]), with slow precipitation rates under subsurface conditions (e.g., [32]), or rapid precipitation of dolomite and calcite due to abrupt change in pressure and temperature [8,33]. The extremely negative δ 18 O values in the coarse-sized dolomite and calcite crystals (Figure 8) highlight the interaction of the host limestones with diagenetic fluids (e.g., [8]) as indicated in Group II.…”

“…However, the fluids can be traced by geochemistry, and particularly by analyses of stable (carbon and oxygen) and radiogenic (strontium) isotopes. Oxygen isotope compositions can provide insights into the origin of mixed fluids or cross-formation water flows, and also to characterize paleo-temperatures and rainfall properties, such as the amount, seasonally, and moisture sources (e.g., [6][7][8]). Carbon-isotope compositions can provide information on effective rainfall and weathering processes [8].…”

“…Oxygen isotope compositions can provide insights into the origin of mixed fluids or cross-formation water flows, and also to characterize paleo-temperatures and rainfall properties, such as the amount, seasonally, and moisture sources (e.g., [6][7][8]). Carbon-isotope compositions can provide information on effective rainfall and weathering processes [8]. The conventional 87 Sr/ 86 Sr isotope composition is classically used to infer the main sources of radiogenic strontium in sedimentary basins related to the pulses of midoceanic hydrothermal fluxes [9] or to infer overprinting by radiogenic dolomitizing fluids (e.g., [10]).…”