Preliminary sedimentology and stratigraphy of the enigmatic middle lake beds succession (pleistocene?) in the Rukwa Rift Basin, Tanzania

This paper presents sedimentology and stratigraphic overview of a series of previously unrecognized lithostratigraphic unit in the Rukwa Rift Basin, Tanzania. Based on the cross-cutting relationship and pilot detrital zircon geochronology, these isolated, enigmatic strata are herein informally termed the middle Lake Beds (mLB). Facies analysis reveals that the mLB units were deposited within a complex array of depositional environments, ranging from alluvial to fluvial channels, floodplains and shallow lakes. The mLB are characterised herein as isolated outcrop exposures with uncertain age relationships, but which can be divided into five distinctive lithostratigraphic units herein identified as informal members (A-E). These are:(1)at least 36 m-thick lacustrine (limestone) unit (member A); (2) a thin (<15 m-thick) volcaniclastics succession of alluvial sandstones, siltstones, mudstones and thin conglomerates (member B); (3) well-sorted siliciclastic fluvial sandstones (member C); (4) tuffaceous/ash-rich siliciclastics fluvial sandstones and conglomerates (member D); and (5) thin lacustrine volcaniclastics siltstone unit, termed member E. Deposition of the mLB was most likely transpired during tectonically active periods, at times contemporaneously with explosive and effusive volcanism associated with the Rungwe Volcanic Province. The identification of the mLB unit provides an important insight into revising the Lake Beds stratigraphy, and establishing a formal nomenclature of this uppermost mega-sequence in the basin.