LOWER MIOCENE PALEOENVIRONMENT AND PALEOCLIMATE AS RECORDED BY THE LOCHERANGAN FORMATION, TURKANA BASIN, KENYA

In order to trace the development of terrestrial ecosystems and their corresponding paleoclimatic transition across the Miocene-Pliocene boundary, it is essential to provide baseline conditions for the Miocene. Here we present revised stratigraphy and paleoenvironmental data for a Lower Miocene record from Locherangan, Turkana Basin, Kenya. The 50 m section is comprised of alternating lacustrine clays and pedogenically modified mudstones (paleo-Vertisols with various degrees of maturity) separated by flooding surfaces. Locherangan is highly fossiliferous, containing invertebrates, vertebrates, trace fossils including coprolites, and one discrete bed of plant macrofossils. A fluvial sand deposit caps the top of the section. This unit was excavated in the 1980’s and yielded hominoid fossils of Afropithecus turkanensis and Simiolus enjienssi. For this preliminary study, samples were collected in the summer of 2016 in order to further the reconstruction of the paleoenvironment and paleoclimate recorded at Locherangan. Samples were processed for stable isotope analysis of ostracods, bulk total organic carbon measurements, and presence/absence description of the macrofossils. Comparing ostracod δ¹⁸O data to modern values, the paleoenvironment corresponds to that of shallow, seasonally flooded Sanderson’s Gulf located in the northwestern end of modern Lake Turkana. Work is underway to further characterize the paleosols and to apply geochemical proxies to estimate mean annual precipitation to quantify how precipitation changed throughout the section. Taken together, this record indicates deposition on a dynamic, fluctuating lacustrine margin, although questions remain about the lateral extent of this system. Was it localized or did it represent a broader depositional regime in the Turkana Basin during this time? What was the structural configuration of the Turkana Basin prior to the significant rearrangement ~4 Ma and how did that impact the paleorecord? While the Locherangan section is limited in area, when combined with other records from both the Miocene and Pliocene from the Turkana Basin and beyond, it presents a compelling snapshot into the conditions in which early hominoids were first evolving.