DIAGENESIS OF THE FOOTPRINT TUFF (PLIOCENE), LAETOLI, TANZANIA

A thin Pliocene tuffaceous unit at Laetoli, Tanzania, the Footprint Tuff, was made famous by the paleontologic, taphonomic, stratigraphic, and petrologic studies of Mary Leakey, Dick Hay, and their many colleagues. As noted by Hay, early post-depositional partial lithification of the tuff was influential in the preservation of the remarkable assemblage of footprints made by insects, birds, early hominids and other mammals on depositional surfaces within the tuff. Our study utilizes samples comprising a nearly continuous section from the lower Footprint Tuff (approximately 9.5 cm thick), collected and sent to us by Dick Hay. We have applied a range of modern electron microbeam techniques to reinterpret the Footprint Tuff as a series of rapidly deposited air falls of crystal-vitric melilite nephelinite ash, not carbonatite as previously described. The calcite-rich composition of the tuff relates to pore-filling cement that fills the interstices between rounded pellets of altered glass and crystal fragments. The earliest calcite cement is botryoidal microcrystalline, and is enriched in Fe, Sr, and Ba. This early calcite predates the formation of root traces or burrows and through-going fractures that, in turn, predate the growth of phillipsite cement. Precipitation of this earliest microcrystalline calcite cement was sufficient to preserve the footprints. The final stage of cementation by sparry calcite predates the substantial clay alteration of the pellets. The sequence of diagenetic events in the ash points to an early history characterized by largely water-saturated conditions, followed by drying and shrinking of the smectitic clays.