Nonmarine Macroborings in Early Pleistocene Algal Biolithites (Stromatolites) of the Turkana Basin, Northern Kenya

Macroscopic bioerosion traces (macroborings) occur abundantly in calcareous algal biolithites (stromatolites) that were deposited in Pleistocene (0. 7 to 1.9m.y. a.) lake-margin environments west and east of Lake Turkana, Kenya, in the northern part of the Gregory Rift Valley. The macroborings are restricted to laminated carbonate substrates, and they include two forms: nearly straight, occasionally branched, cylindrical shafts, referred to the ichnogenus Trypanites; and arcuate, festooned, spreiten borings, referred herein to the new ichnogenus and ichnospecies Sertaterebrites nachukui. This is the first report of Trypanites in a nonmarine setting and the first report of true spreiten borings in any ancient setting. The creators of the borings are presumed to be invertebrate animals, and, in fact, similarities between the two ichnotaxa suggesthat both could have been made by the same organism. However, because no modern animals have been discovered in the process of making such traces, their affinity is unknown. The borings were excavated in algal biolithites developed along the margins of a precursor to modern Lake Turkana. These biolithites are abundant in the Early Pleistocene sequence of the Turkana Basin, occurring in over 25 stratigraphic levels. The biolithites occur variously asovoidal, spheroidal, or hemispheroidal heads or as polygonal pavements of laminated, biochemically precipated, low-magnesium calcite .