Geochronology and Field Observations from Oligocene Nameriyek, Turkana County, Kenya

The ~29.5 Ma Topernawi Formation of the northern Turkana Depression, Kenya, is among the few Oligocene syn-rift sedimentary sections exposed in the East African Rift System. We present new 40Ar/39Ar geochronology and field reconnaissance that expand both the spatial extent and paleontological potential of the Topernawi Formation. Feldspar ages from volcaniclastic sediments in the Ekitale Basin yield a weighted mean of 29.668 ± 0.054 Ma, confirming the Oligocene depositional chronology. The first geochronology from Nameriyek, a previously undated set of outcrops ~5 km west of the Ekitale Basin, demonstrates that Oligocene sedimentation may extend beyond previously known exposures. Beneath the Nameriyek ignimbrite (29.322 ± 0.092 Ma), a newly identified fine-grained lacustrine section preserves vertebrate fossils in a low-energy depositional setting with no lithologic analog in the coarse volcaniclastic facies of the Ekitale Basin. These lacustrine facies represent a new prospective in situ Oligocene site from one of the most sparsely documented intervals in the Afro-Arabian terrestrial vertebrate record. Further fieldwork is needed to test these hypotheses and explore the geology and paleontology of Nameriyek.