PALEOCLIMATIC SIGNIFICANCE OF THE NEOPROTEROZOIC MANYOVU REDBEDS ALONG EASTERN LAKE TANGANYIKA, TANZANIA

The East African Rift system is a continental rift that began in the Early Miocene and comprises half-graben basins with normal-fault blocks exposing Neoproterozoic volcanic and sedimentary rocks along Lake Tanganyika. The Manyovu redbeds, a poorly dated and little studied unit exposed in one of the fault blocks, have been described as a monotonous succession of red, micaceous siliciclastics deposited in a shallow marine setting. We logged a detailed section and conducted thin section, geochemical, and grain size analyses, and U-Pb detrital zircon geochronology to reassess the depositional setting and possible paleoclimatic significance of these units.

We measured 285 m east of Mkalinzi, Tanzania. The basal ~70 m consist of structureless, medium-grained white and pink sandstone with sheet-like tabular bedding. In thin section, this facies is well-sorted and predominantly quartz with pristine feldspars, and metamorphic and volcanic lithic fragments. The remaining section (~215 m) consists of structureless, red mudstone (~70%; size modes ~30–50 μm) with blocky fractures and subordinate (~30%) interbedded fine- to medium-grained tabular sandstone. Some horizons of the mudstone contain large (>15 cm diameter) concretions and in thin section display porphyroskelic and clinobimasepic microfabrics with predominantly quartz, feldspar, muscovite, lithic fragments, and dispersed gypsum. U-Pb ages (n=1537) reveal primary peaks at 636–697 Ma (Pan African orogeny), and secondary peaks at ~1000 Ma (Kibaran orogenic belt), as well as ~1870 Ma, and ~1970 Ma (Ubendian orogeny), with maximum depositional ages between 608–668 Ma, corresponding approximately with the Marinoan (~630–650 Ma) Snowball Earth event.

These preliminary data, including apparent pedogenic features, are inconsistent with exclusive marine deposition, and the silt modes and massive character of the mudstone facies record possible loess deposition. The immature composition of the units further indicates a possible derivation from cold-climate (physical) weathering. Future work will include additional laser particle size analysis (LPSA) to determine up-section grain size modes and distributions, further pedogenic characterization, and provenance interpretations.