Sedimentology, lithostratigraphy and depositional history of the laetoli area

Abstract

A review of the stratigraphy and paleoenvironmental context of the main sedimentary units that outcrop at Laetoli is presented here, with a primary focus on the Upper Laetolil Beds. The lithological sequences at many of the numbered paleontological localities, as designated by Leakey (1987a), are described and sedimentary logs for many of these localities are presented. The litho-facies as described by Hay (1987) are reassessed in the light of recent developments in the understanding of volcano-sedimentary processes. Hay (1987) recognized the majority of sediments in the Laetolil Beds as aeolian tuffs, but it is possible that some of these sediments are the product of lahars or hyper-concentrated flows. Gullies and minor channels are relatively common in the Upper Laetolil Beds, indicating that a well-developed drainage system, larger than the present-day Garusi River system, was in place during the Pliocene. Streams and rivers were probably seasonal, but would have been associated with a complex vegetational mosaic, including woodland along the river courses. The general direction of the paleo-flow suggests that run-off originated as precipitation in the elevated areas towards the volcanic highlands in the east and that it flowed southwest across the Laetoli area towards the Eyasi basin. Ponds dotted the landscape during the rainy season, but were dry for much of the year. Primary ash fall deposits periodically blanketed the Laetoli area, forming distinctive marker tuffs. These inundations of volcanic ash would have had an adverse effect on the local ecosystem, leading to a landscape dominated by grasslands and open woodlands. However, these periods of disruption in the climax vegetation were relatively short-lived, with grasslands being quickly replaced by a mosaic of woodland, bushland and grassland.