Characteristics, genesis and parameters controlling the development of Cretaceous-Tertiary hydrothermal dolomitization (SE-France and NE Iraq) associated with a newly discovered calcretization phase (NE Iraq): timing of the sedimentary and diagenetic events

Triassic-Jurassic outcrops in Provençal Domain-SE France and Upper Cretaceous Bekhme Formation in Harir-Safin anticlines-NE Iraq are extensively fractured and dolomitized along open spaces in carbonate rocks. Extensive fieldwork, enhanced petrography and geochemistry (trace, REE, major elements, 87Sr/86Sr, δ18OVPDB, δ13CVPDB), and U-Pb datings demonstrate the multi-phase generation of saddle dolomites and blocky calcites formed by the action of deep hot brine fluids, which migrated along fault zones.Petrography and geochemistry revealed three main diagenetic stages in the French Triassic (T)-Jurassic (J) studied outcrops. The first stage is characterized by crystalline replacive dolomites (D1T/ DJ) and medium-sized dolospars (D2T) precipitated in the eogenetic realm from normal seawater and meteoric fluids. The second stage with medium- to coarse-grained saddle dolomites (SD1T/J and SD2T/J) formed under shallow diagenetic realm during Early Cretaceous times, and very coarse-sized zoned and unzoned saddle dolomites (SD3T/J, SD4T/J, and SD5J, SD6J, DrJ) precipitated under deep diagenetic realm (Th between 120 °C and 278 °C) during a Late Cretaceous tectonic activity. Two types of stylolites, extensive fracturing of the carbonates, and breccia/zebra structures, were also formed as a result of the activities of two recognized sub-generations of hydrothermal fluid influxes associated to the second stages. Therefore, they are characterized by a pervasive polyphasic hydrothermal dolomitization that occurred along fractured zones with a wide range of δ18OVPDB and 87Sr/86Sr values. The transition from high (Th between 81 °C and 278 °C; av. = 207 °C) to low (Th between 44 °C and 77 °C; av. = 61 °C) fluid temperatures identifies the third stage of diagenesis. This stage produced the late calcitic cements C1T and C1J with extra-negative oxygen and carbon isotope compositions, and this is related to two different fluids during the uplifting of the studied area in Late Cretaceous-Eocene times. The Triassic dolomites mostly show depleted 87Sr/86Sr values compared to the Jurassic dolomites that have striking higher 87Sr/86Sr values with respect to the marine sea facies. The same lowered radiogenic compositions are measured in the Jurassic calcites (C1J) while the one of the Triassic calcite is higher (C1T). These are probably linked to the pulses of the seafloor hydrothermal activity that lowered the 87Sr/86Sr ratios and to an increase of the continental riverine input during Late Cretaceous and Early Cenozoic.In NE-Iraq, the Bekhme Formation along the Harir-Safin anticlines experienced extensive hot brine fluids that produced several phases of saddle dolomites (SD1, SD2, SD3) and blocky calcite cements (CI, CII). Detailed petrography and geochemical analysis showed that the saddle dolomites and blocky calcites precipitated from deep hydrothermal fluxes (83 °C - 190 °C) and from very saline fluids (up to 25 eq. wt.% NaCl; i.e. 7 times the seawater salinity) that interacted with the crystalline basement rocks during their circulation before invading the Bekhme Formation. Fluid inclusion petrography, fluorescence microscopy and microthermometry revealed two entrapment episodes of oil FIs hosted in the HT cements, i.e. early and late episodes. The early entrapment episode of FIs is linked to the fault-related fractures in the Bekhme Formation and was contemporaneous with the precipitation of the HT cements. The late entrapment episode of FIs is consistent with low saline fluids (0.18 and 2.57 eq. wt.% NaCl) formed under near-surface conditions (13 °C).Shortly after the HT emplacement, an alteration and in situ brecciation of the host limestone and HT saddle dolomites/blocky calcites by alveolar texture led to the formation of two calcrete levels in the dolomitized Bekhme Formation. Extensive fieldwork and geochemistry show repeated occurrence of 2-6 m thick pedogenic levels within the Bekhme carbonates. These levels resulted from a complex interplay between sea level fluctuations and/or tectonic events that produced multiple phases of submergence and emergence during the depositional age of the Bekhme Formation. Consequently, sea level fluctuations and tidal signals are strongly implied.The LA U-Pb dating analysis using small scale isochrones (SSI) method defines the first generation of major HT diagenesis occurring at ~73.8 Ma and predates the calcrete formation (~ 70 Ma) and postdates the early matrix dolomite (~74.8 Ma). Therefore, this diagenetic generation was emplaced in the lowermost part of the Bekhme Formation (75.1 Ma) and was synchronous with the formation of depositional age (Campanian-Early Maastrichtian). The second generation of major HT diagenesis during which a new phase of saddle dolomites/blocky calcites precipitated, spans ages between 8.6 Ma and 30.3 Ma. Within this phase minor phases of HT fluids precipitated similar products (CI = 30.3 Ma - Early Oligocene; CII = 18.7 Ma - Late Miocene; SD2/SD3 = 14.5/8.6 Ma - Late Miocene). Tectonically, the numerical age data (~ 73.8 Ma to 8.6 Ma) is in an acceptable agreement with the two major generations of orogenic folding-faulting systems during the Late Cretaceous and Tertiary interval times and caused by Arabian-Eurasian plates convergence.Samples from the lower calcrete level returned two LA U-Pb ages ~70 Ma and 3.8 Ma corresponding to two horizons within the calcrete, and strongly suggesting that the same calcrete level was twice exposed to subaerial conditions. The earlier exposure was associated with alveolar and other microbial diagenetic features such as dissolution, micritization, cementation…etc. while the second calcrete exposure is associated with laminae, pisolitic, and microstromatolite features during regional uplifting of the area within the Pliocene. In conclusion, a tectonic model is developed for Harir-Safin anticlines, combines fieldwork observations, petrography, geochemistry, and U-Pb numerical age data. The latter method brings new insight into the dating of the fractures/geodes formation and the generation of the HT fluids controlled by tectonics.Doctorat en Sciences