Limnogeologic studies on an intertrappean continental deposit from the northern Ethiopian Plateau (37°03′E, 12°25′N)

Abstract

The Chilga lacustrine deposit from a small graben in the heart of the Northwestern Ethiopian Highlands (37°E, 12°N) is one of the ubiquitous intertrappean continental sedimentations on the Plateau. The basalt layer which makes the bottom of the basin has been dated as 8 Ma. The lacustrine sedimentation occurred contemporaneously with active volcanic phases in the region. Silicic aggregates are common in the cements and as important mineralogic constitutents of these phases whereas periods of calm sedimentation are characterized by thick lignite seams and the presence of authigenic minerals such as pyrite and vivianite. The sequence shows a general upward fining and evolution from shallow fluviatile to reduced lacustrine basin. The palynoflora from the sequence has a unique palaeofloral assemblage where the abundance of Guineo-Congolian-like pollen taxa, pteridophytes and absence of conifers imply a regional palaeoaltitude much lower than at present. The uplift of the Plateau at a rate of 0.1 mm yr⁻¹ similarly suggests palaeotitudes of ca 900–1000 m.