A Miocene Gilbert-type fan-delta from a volcanically influenced lacustrine basin, Rusinga Island, Lake Victoria, Kenya

Abstract

A lacustrine, volcanically influenced fan-delta is recognized in the Kulu Formation of the Miocene Rusinga Group in western Kenya. In the Nyamsingula Gully area on Rusinga Island in Lake Victoria, mapped lithologic units include the classic topset, foreset (mega-foreset) and bottomset structure of a Gilbert-type delta. A distinctive mega-breccia unit of colluvial origin interfingers laterally with the delta deposits and consists of pyroclastic and sedimentary rock fragments derived from underlying and laterally adjacent strata. The breccia formed from the bevelling of a fault escarpment, from which the fan-delta also originated. Away from the fan-delta axis, slumping of fault-scarp colluvium into the lake produced matrix-supported debris flows that are interbedded with lacustrine shales. In the topset beds of the delta, clasts of a distinctive melilitite–nephelinite tuff of airfall origin can be identified and traced down a palaeoslope through laterally adjacent units from the original bedded tuff, into the mega-breccia unit, and finally into debris-flow deposits in the foreset and bottomset beds. During resedimentation the tuff behaved in a brittle fashion due to quick cementation as a result of its water soluble, ultra-alkaline composition. The Kulu Formation is a distinctive facies that interfingers with the lowermost Hiwegi Formation. Both formations are part of the Rusinga Group. The bulk of the Rusinga Group, excluding the Kulu Formation, represents braidplains and alluvial aprons that flanked the Miocene volcano of Kisingiri. The braidplain and alluvial apron deposits have primary dips away from the volcano and were derived from material erupted and eroded from the volcano. The Kulu Formation, in contrast, was deposited in two small, elongate lacustrine basins separated by a small horst (approximately 5 km long and 2–3 km wide), from which the deposits were largely derived. The fan-delta systems prograded symmetrically away from the horst margins, into the adjoining lacustrine basins. The long dimensions of the horst and lake basins roughly parallel the local slope of the Kisingiri volcano, and therefore deposition in the Kulu Formation was perpendicular to deposition of the other Rusinga Group formations in the vicinity