Insights into a million-year-scale Rhenohercynian carbonate platform evolution through a multi-disciplinary approach: example of a Givetian carbonate record from Belgium

摘要

In this study we formulate answers to three important questions related to Givetian carbonate records and their use for reconstructing million-year past palaeoenvironmental changes. First, we provide detailed illustrations of the fascinating depositional diversity that shaped a shallow reefal platform during the early- to late-Givetian in the Rhenohercynian Ocean; secondly we improve the sedimentological model of the Givetian carbonate platform in the Dinant Basin (Belgium) and thirdly we evaluate the application of magnetic susceptibility (MS) as a tool for long-term trends, correlation, and palaeoenvironmental reconstruction. These three goals are reached by making a detailed sedimentological, geophysical and geochemical study of the La Thure quarry. Petrographic analyses revealed eighteen microfacies which represent a carbonate platform evolution through early-late Givetian time, ranging from a homoclinal ramp to a discontinuously rimmed shelf and then a drowning shelf. These results allowed us to illustrate the fascinating environmental diversity that shaped one the largest carbonate platforms of Europe and provide an up to date model of the vertical and lateral development of the Belgian Givetian platform. Early-late Givetian La Thure sequence reveals five main depositional intervals, which could be correlated with the southern margin of the platform. These correlations allowed us to define the facies belts distribution, the major depositional changes that affected this platform and to highlight the Taghanic Event. Palaeo-redox proxies reveal a significant change in the oxygenation level, from oxygen-depleted to more oxic condition, between middle and late Givetian. This change is well-correlated with an increase in global temperature in the late Mid Devonian. Combination of MS and geochemistry demonstrates the inherent-parallel link existing between variation in MS values and proxies for siliciclastic input (such as Si, Al). Collected geochemical and MS data confirm that the La Thure can be considered as a key section for the internal shelf setting in the Rhenohercynian Ocean bordering Laurussia’s south-eastern margin.