TERRESTRIAL TRACE FOSSILS AS A TOOL FOR THE SEQUENCE STRATIGRAPHIC INTERPRETATION OF THE PLIOCENE CHEMERON FORMATION, BARINGO, CENTRAL KENYA RIFT VALLEY

Terrestrial traces fossils, produced by air-breathing invertebrates such as termites, provide valuable insights to the interpretation of the 3.3 to 2.6 Ma lacustrine and fluvial-alluvial Chemeron Formation of the central Kenya Rift Valley. We used termite trace fossils as an ichnological proxy for minimum water table position in the 227 m BTB13 drill core and nearby outcrops. This technique facilitated the identification of sequence boundaries, the delineation of stratigraphic packages, and the estimation of the extent of base-level fall associated with lake regression events. In core, termite traces occur as circular or elliptical burrows with sharp margins and meniscate backfill or spreiten-like tunnel fill. This active infilling material often differs in grain size and colour from the host lithology and corresponds to the uppermost lithology of each stratigraphic unit. Accordingly, these deep-tier trace fossils commonly cross-cut sediments deposited from greatly different settings from the colonization surface, including profundal to littoral lacustrine sediments. Bioturbation intensity increases upwards to the colonization surface, which may also preserve other indicators of exposure (e.g., roots, pedogenic structure). Trends in the degree of base-level fall during periods of low lake-level were observed between the stacked stratigraphic packages, ranging from ~0.5 m to ~9.5 m depth. The trends correspond well to lake-type (i.e., underfilled or balanced fill) interpreted from sedimentology, to the degree of pedogenesis associated with the colonization surface, to the interpreted depositional gradient of the system, and to the amount of time represented by the stratigraphic package derived using the age model developed for the core. The cross-cutting nature of these trace fossils also helped to demonstrate the relative timing of incision and fault-brecciation and injection events seen in the core. Integrated with sedimentology, sequence stratigraphy, and paleopedology, terrestrial trace fossils are an effective tool for tracking significant fluctuations in base-level between lacustrine and terrestrial environments in the BTB13 core.