Biogenic structures in exhumed surfaces around saline lakes: An example from Lake Bogoria, Kenya Rift Valley

Five overprinted suites of animal and plant traces around saline, alkaline Lake Bogoria, Kenya, are described from a set of interfingering Late Pleistocene and Holocene exhumed surfaces that reveal a complex story of changes in lake level and environmental controls on trace assemblages through time. These stratigraphic surfaces are amalgamated and form a co-planar surface. The sediments were first deposited during intermediate lake levels, then reworked and cemented during low lake levels, and were subsequently overlain by transgressive lacustrine sediments during higher lake levels. Ongoing exhumation of the surface near the present shoreline has reactivated the sediments as a substrate for the activities of epifaunal and infaunal animals, as well as plant growth. The modern environmental setting at Lake Bogoria is very similar to that of the preserved exhumed surfaces, which allows comparisons between observed sets of environmental factors affecting animal behaviour and their representation in the fossilized examples. Pronounced lateral environmental heterogeneity around saline and hypersaline lakes may lead to concentrations of life around sites with vital resources, such as freshwater. At Lake Bogoria, groups of fault-controlled hot springs represent these "oases", and have been a relatively consistent feature in and around the lake during the development of the exhumed surfaces. Together, the diverse assemblage of animal trace suites from Lake Bogoria represent the Mermia and Scoyenia ichnofacies, but can be divided into five suites that reveal details of laterally heterogeneous sets of environmental factors, and are useful indicators of lake level, substrate moisture content, substrate cohesion, etc. Suite 1 comprises the traces of chironomid larvae (Diptera: Chironomidae) formed in subaqueous lacustrine settings. Suite 2 includes the traces of flamingos (Phoeniconaias minor and Phoenicopterus ruber) formed at the shoreline. Suite 3 compares well with the Mermia ichnofacies and comprises trails and burrow systems associated with relatively fresh, saturated to extremely shallow subaqueous substrates. Suite 4 is comparable to both the pre- and post-desiccation suites of the Scoyenia ichnofacies, is the most diverse suite at Lake Bogoria, and mainly comprises the burrows of beetles, earwigs, and their larvae as well as a diverse set of rhizoliths and vertebrate tracks. Suite 5 contains a moderately diverse set of traces attributed to termites and ants.