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.
Highlights
Hydrothermal mineralization and calcretization diagenetic processes involve specific paleo-fluid conditions, e.g., chemical composition of solution temperature and salinity, leading to altered rocks with particular petrographic and fabric textures [1]
Fracture-filled dolomite was absent at the lower part of the flanks along Harir and Sulauk blocks
In the Harir–Safin area, a large volume of the basal parts of Bekhme Formation outcrops were invaded by hydrothermal fluids that produced brecciated dolomitic matrix, which was eventually
Summary
Hydrothermal mineralization and calcretization diagenetic processes involve specific paleo-fluid conditions, e.g., chemical composition of solution temperature and salinity, leading to altered rocks with particular petrographic and fabric textures [1]. Geosciences 2019, 9, 459 dolomites) and calcretization are reported as two separate processes. Hydrothermal minerals (saddle dolomite and/or calcite) were documented in many localities [1,2]. Hydrothermal dolomitization has a positive effect on oil migration pathways [4], such as enhancing migration and increasing porosities in the host carbonate rocks. Reference [4] asserted that many reservoirs associated with hydrothermal dolomitization are related to fault systems and, to the tectonic evolution of the series concerned. In the field, HT fluids appear globally restricted to the HT dolomite distribution; their origins need detailed geochemical analyses [5]
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