MAGNETIC PROPERTIES OF ALTERED MUDSTONES FROM EARLY PLEISTOCENE LAKE LORENYANG, NORTHERN KENYA

Modern Lake Turkana is shallow, saline-alkaline, and well mixed by persistent winds of the Turkana Jet. A precursor however, Lake Lorenyang, accumulated mudstones that preserve evidence of strongly anoxic bottom waters, suggesting greater stratification than today. Previously, these mudstones have been interpreted to contain a suite of alteration minerals that indicate reduction diagenesis of pyrite at the time of early Pleistocene deposition, followed by the formation of gypsum and hematite at the time of Holocene(?) tectonic exhumation. Outcrop data collected from ~90 m of section exposed at West Turkana reveal stratigraphic trends correlated with depositional facies. Lake Lorenyang offshore mudstones dated to 2.2-1.9 Ma have magnetic susceptibilities and natural remanent magnetization intensities one to three orders of magnitude less than deposits from the underlying alluvial interval or the superjacent heterogeneous interval. For East Turkana, similar attributes of the Lake Lorenyang mudstones are observed. Isothermal remanent magnetization (IRM) acquisition studies indicate that offshore mudstones carry a hematite-dominated or a mixed hematite-magnetite assemblage of magnetic minerals, whereas nearshore and alluvial mudstones are magnetite rich. This is confirmed by thermal demagnetization of a three-axis IRM, which also suggests a pervasive presence of goethite that is probably a consequence of recent weathering. Three hypotheses are offered to explain the origin of the altered lacustrine mudstones and their magnetic properties: (1) orbital climate change increasing stratification due to density differences between monsoonal freshwater input via the ancestral Omo River and saline-alkaline lake water, (2) tectonic structure compartmentalizing the basin with embayments sheltered from mixing winds, and (3) the Turkana Jet being a geologically recent phenomenon, evoked by post early Pleistocene regional uplift and funneling of winds through the lowland corridor between the Ethiopian and Kenyan domes.