Taphonomic controls on animal tracks at saline, alkaline lake bogoria, Kenya rift valley: Impact of salt efflorescence and clay mineralogy

The morphologies of subaerial vertebrate tracks and invertebrate traces in surficial sediments can be used in paleoenvironmental reconstructions by providing details about the initial substrate conditions (e.g., water content) and any changes experienced during trace preservation. A qualitative field study was undertaken to identify the factors affecting track taphonomy in saline lake-margin environments, supported by experiments to isolate selected factors that modify track morphology and substrate characteristics. Flamingo tracks, ubiquitous around saline, alkaline Lake Bogoria in the Kenya Rift, were the focus of field study, although other vertebrate tracks and surficial invertebrate trails and tunnels were also considered. A complex and interacting set of environmental factors were identified, such as the effect of lake-level fluctuations on substrate grain size and capillary evaporation. Several factors affect the early taphonomic modification, destruction, stabilization, and/or cementation of tracks and their associated substrates. The laboratory experiments tested the effects of pore-water salinity, clay mineralogy (e.g., swelling or non-swelling clays), drying method (e.g., solar radiation or wind), and wetting-and-drying cycles on tracks. These data were supported by statistical analyses of impression depth and planimetric area. The results show that track morphology in smectitic substrates is altered rapidly by wetting and drying, particularly if the substrates contain saline pore waters, but morphology is less affected in non-swelling clays. The degree of morphologic alteration and/or resistance to wetting and drying varies with the rate of substrate drying and the type of the resulting salt efflorescence (e.g., interstitial vs. surficial crust). These short-term, process-related taphonomic effects help to explain the morphologic variants of tracks in lake-marginal settings, and provide insights into the preserved distribution patterns of subaerial ichnofossil assemblages.