Christian Blanpied

Sedimentology and Sequence Stratigraphy of the Late Precambrian Carbonates of the Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin (Democratic Republic of the Congo)

The late Mesoproterozoic–middle Neoproterozoic carbonate succession (1155 Ma–800 Ma) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) represents a classic late Precambrian carbonate sequence whose architecture is poorly known. Here we present new data and synthesis of microfacies analysis, sequence stratigraphy, Fischer plots coupled with C and O isotopes, to evaluate the paleoecology and sea level variations of the carbonate series of the Mbuji-Mayi Supergroup, and to establish hierarchical approach stratigraphic framework from which to resolve the evolution of the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Our microfacies and sequence stratigraphy analyses show that the carbonate succession consists of strata accumulated on a ramp, during cyclic sedimentation across the inner ramp. Here plurimetric ‘thin’ peritidal cycles (±4 m-thick on average) record a relative maximum sea level of ca. 4 m, with fluctuations in the range around 1–4 m. This shallow-water depth and the abundance of cyanobacteria suggest that water column was oxygenated. By contrast the subtidal cyclic facies at the outer/middle ramp, preserve ‘thick’ subtidal sequences characterized by an average thickness of ±17 m. Accurate relative sea level fluctuations are difficult to assess in this ‘deeper’ environment since the facies could have been deposited in a wide range of shallow water that did not completely fill the accomodation space or available space. A probable magnitude for sea-level fluctuations here is around 10–20 m. These data are the first to place a quantitative constraint on the late Mesoproterozoic to middle Neoproterozoic carbonate deposits that have lively covered much of the Congo Shield at the end of the Precambrian, and is therefore an important type section for Central Africa.

Structure and evolution of the Bukhara-Khiva region during the Mesozoic: the northern margin of the Amu-Darya Basin (southern Uzbekistan)

Abstract The Bukhara-Khiva region forms the northern margin of the Mesozoic Amu-Darya Basin. We reconstructed several cross-sections across this margin from subsurface data. The objectives included examining the structure of the Bukhara and Chardzhou steps and determining the tectonic–sedimentary evolution of the basin during the Jurassic. Subsequent to the Cimmerian collision in the Middle Triassic, an extensional event controlled the deposition of the Early–Middle Jurassic siliciclastic succession in the Bukhara-Khiva region. The main Late Palaeozoic inherited structures were reactivated as normal faults during this period. Continental coarse-grained siliciclastic sediments are mainly confined to the basal Lower Jurassic section, probably Pliensbachian–Toarcian in age, whereas marine siliciclastic sediments occur in the early Late Bajocian. In the Early–Middle Jurassic the Bukhara and Chardzhou steps were predominantly sourced by areas of relief, the remains of Late Palaeozoic orogens located to the north. The rate of extension significantly declined during the Middle Callovian–Kimmeridgian period. Deposition of the overlying Lower Cretaceous continental red-coloured clastic sediments was related to the interaction of basin subsidence, a fall in eustatic sea-level and sediment supply. Subsequent marine transgression in the Late Barremian, partially related to broad thermal subsidence in the Amu-Darya Basin, resulted in the deposition of an extensive Late Cretaceous clay–marl succession.