Cycles of Doming and Eruption of the Miocene Kisingiri Volcano, Southwest Kenya

Abstract

Volcanic cycles of doming and eruption of the Miocene Kisingiri volcano produced three sedimentary cycles recorded in the volcaniclastic strata of Rusinga and Mfangano Islands, Lake Victoria, Kenya. Each of the three cycles began with the deposition of cobble and boulder conglomerates shed from the volcanically domed Precambrian basement, followed by deposition of pyroclastic and volcaniclastic strata, representing nephelinite-carbonatite eruption of the Kisingiri volcano. Volcanogenic strata produced by the first two cycles (lower two-thirds of the Rusinga Group) are predominantly fine-grained tuffs and medium-grained volcaniclastic deposits, indicating alluvial deposition from a low-relief volcanic edifice. The third cycle is dominated by boulder debris flows, lava flows, and minor tuffaceous beds (upper part of the Rusinga Group and overlying Kisingiri Group). This last cycle records the formation of the high-relief Kisingiri stratocone, much of which is preserved in the dissected flanks of the volcano. The first two cycles are recorded in distal apron deposits but are not well preserved in the core of the volcano. Second-order sedimentary cycles, consisting of fining-upward sequences (5-10 m thick) of granular tuffaceous sandstones and conglomerates that fine into siltstones, stacked floodplain paleosols, and airfall tuff beds, dominate the strata of the first two cycles. These fining-upward sequences represent alluvial aggradation that accompanied and followed eruptive episodes that were much shorter in duration than the main cycles of doming and eruption.