A RIVER RUNS THROUGH IT: HOW SEDIMENTOLOGY CAN RECONSTRUCT THE DYNAMICS OF A 17-MILLION-YEAR-OLD RIVER IN KENYA

We investigated sediments from Loperot, Kenya to determine the role of climate and tectonics on the Miocene landscape in east Africa. Previous work indicated that a large perennial river meandered across a semi-arid savannah. Within a 30m exposure, the sedimentology changes above Unit 17; evaporites (e.g., gypsum) in paleosol units disappear and soil chroma values decrease, both attributed to increased humidity. Cross-beds indicate that the river’s flow direction changes from eastward to northward upsection. We hypothesize that a tectonic event caused the Loperot River to capture a nearby river. We analyzed quartz-feldspar-lithic (QFL) composition and texture of sand units, speculating that changes would reflect a shift in source drainage and/or local stream capture. Low in the section (Unit 25), sandstone units are fine-grained with 65%Q, 5%F, 30%L; upsection (Unit 11) feldspar disappears (99%Q, 0%F, 1%L) and the sandstones are much coarser grained (Unit 3). Loss of feldspar could be due to the wetter climate (K-feldspar weathers away) or a difference in the captured river’s source area. Coarser grain size results from increased flow velocity due to river amalgamation. Roundness and sorting remain unchanged; only a significant change in distance from source (>100km) could affect these textural components, which is unlikely within a single rift basin. We conclude that the Miocene landscape in Kenya was certainly dynamic and responded to both climate and tectonics.