New tephrostratigraphic data from Lake Emakat (northern Tanzania): Implications for the eruptive history of the Oldoinyo Lengai volcano

摘要

The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from C-14 datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.