Early diagenesis in meteoric versus brackish environments: Example of the Late Oligocene-Early Miocene, littoral, mixed sedimentary succession of Carry-Le-Rouet (southeastern France)

Abstract

AbstractIn the Carry-Le-Rouet region (southeastern France), the well exposed Late Oligocene-Early Miocene succession allows studying the early diagenesis in meteoric and brackish environments. This area provides unique and continuous outcrops of a mixed sedimentary succession (carbonate-siliciclastic shallow facies) with five particular sedimentary discontinuities resulting from various processes such as subaerial exposure, submarine erosion or bioturbation.Results from the diagenetic characterization (petrography and isotope geochemistry) show that the succession may be divided into three parts. The base of the series is dominated by diagenetic processes associated with brackish water, reflecting a restricted lagoonal environment. Dissolution and mineral replacement occurred directly under the sediment-water interface and a subsequent reprecipitation of the dissolved calcium carbonate took place below this layer. The precipitated calcite is non-ferroan and dull luminescent, showing highly variable stable C- & O-isotopic signatures (−6.31‰ to −0.08‰ for δ13CV-PDB and −5,78‰ to −1,36‰ for δ18OV-PDB), organized in a covariant trend, typical of mixed fluids. The middle part of the series, show a progressive evolution to open marine conditions and records two potential emersion events, associated with the development of meteoric vadose and phreatic conditions. The postulated vadose zones are dominated by mineral replacement rather than leaching and characterized by two cements: a non-ferroan and non-luminescent calcite, changing downward to zoned cement. Isotopic signatures of these calcites are ranging from −6.45‰ to −5.10‰ for δ13CV-PDB and −5,44‰ to −4,17‰ for δ18OV-PDB. Below this zone, a ferroan, non-luminescent calcite (−3.25‰ to −1.18‰ for δ13CV-PDB and −4,93‰ to −3,52‰ for δ18OV-PDB) is observed, suggesting precipitation under reducing conditions associated to a confined phreatic zone, isolated from the overlying zones and the recharge area by a marly interval. The third part of the series (at the top), partly eroded, may have undergone at least one emersion event, with the initial development of a dull cement followed by a non-ferroan, non-luminescent cement. This is linked to a gradual change from downdip slightly reducing to updip oxidizing conditions.The early timing of the diagenetic modifications, the differences of diagenetic patterns observed in each interval and the absence of common trends in the series tend to indicate multiple episodes of early diagenesis, associated to sedimentary discontinuities (more specifically exposure surfaces). These discontinuities act as major controls on the repartition of the diagenetic phases. The excellent preservation of the early diagenetic transformations is linked to the shallow total burial depth, and the lack of mesogenetic and telogenetic imprint. This study also highlighted the influence of the Nerthe massif that acted as a principal meteoric fluid recharge area located less than 1 km away from the sedimentary system.