Search

Abstract: This research reconstructs paleoenvironments of late Pleistocene tufas at Olorgesailie in the southern Kenya Rift and examines the relationship between climate, tectonism and tufa precipitation. The deposits crop out at three main sites, forming at 290kyr and at least twice between 220 and 12kyr ago. Ten carbonate facies comprise the bulk of the tufas, which accumulated at spring vents, in swamps and shallow lakes, along fissures, and along fluvial channels. The tufas contain diatoms dominated by shallow-water, swamp and fluvial taxa associated with fresh to mildly saline conditions. δ13C isotopes are relatively enriched (0.25 to 1.96‰) compared with other East African tufas, but uniform within each set of tufa deposits, suggesting similar sources of CO2 derived from similar plants and soil organic matter during each interval of deposition. δ18O ranges from −7.17‰ to +5.16‰ suggesting variable evaporation and rainfall/groundwater sources or changing water temperatures. Variable REE patterns suggest spatial and/or temporal contrasts in groundwater chemistry. The former springs may have developed in response to climate changes, but there are no clear correlations with known East African wet phases. The deposition of tufa probably also relates to basin breakup and faulting, which likely influenced groundwater movements. Increased permineralized silica in plants in tufas and non-carbonate sediments reflects the introduction of deeper groundwaters than those during earlier Olorgesailie Formation times. Whether climatically or tectonically controlled, the tufa spring sites would have provided a supply of potable water, at least intermittently, during the Pleistocene, and would have provided attractive places for wildlife and possibly for Middle Stone Age human inhabitants of the region, especially during relatively dry times. [Copyright &y& Elsevier]

Abstract: This research develops a detailed palaeoenvironmental history for sediments at Munya wa Gicheru (southern Kenya Rift) and explores the roles of tectonics and climate in the formation of palaeolakes and their use in regional climate correlation. The lacustrine facies of the Munya wa Gicheru deposits were formed from ~1.9 to 1.65Ma and consist of massive and laminated diatomites and diatomaceous silts. Fresh water diatoms include Stephanodiscus transilvanicus, Stephanodiscus niagarae, Aulacoseira agassizi, Aulacoseira ambigua, Aulacoseira goetzeana, Aulacoseira granulata var. valida, A. granulata var. angustissima and Pseudostaurosira zeilleri. One moderately saline phase is marked by the dominance of Cyclotella meneghiniana and by the presence of rare Thalassiosira rudolfii. Twenty-five diatom-based zones and subzones can be distinguished, representing eight high-lake periods and eight shallow-water phases. Erosion surfaces and desiccation cracks indicate two periods of emergence. Lake waters ranged in pH from 7.2 to 9, with conductivities of ~200–11,000μScm−1, but mostly <1000μScm−1. In contrast, the uppermost sediments are comprised of fluvial silts, sands and gravels formed in fluvial, sheetwash and terrestrial settings and represent termination of lacustrine conditions. REE data suggest that the Limuru Trachytes are the dominant source of siliciclastics materials. Geochemical information indicates that catchment weathering was moderate and that CaO concentrations in the sediments are very low. Carbonate rhizoliths are absent. SEM images show minimal corrosion or alteration of the diatom silica. [Copyright &y& Elsevier]