SEDIMENTARY PATTERNS IN THE PLIOCENE HADAR FORMATION, AFAR RIFT, ETHIOPIA

The sedimentary strata of the Hadar Formation preserve a high-resolution record of environmental character and change from the late Pliocene, roughly 3.5 – 2.3 Ma. Detailed analysis of the fluvial and lacustrine strata indicates that tectonic and climatic signals are evident in patterns of sedimentary character and shifts in accumulation history. Aspects of the Pliocene Hadar landscape, important as habitat to the Australopithecus afarensis populations well-represented in the fossil record, can also be reconstructed from the sedimentary sequence.

A composite sequence of some 175 m of sedimentary strata is exposed in the badlands of Hadar. An interlocking framework of isotopic age control on volcanics (8 dated levels), magnetic polarity transitions (4 chron or subchron boundaries), geochemically characterized vitric tephra (N≥ 22) and numerous lithostratigraphic markers provides tight temporal control on the sequence. A distinct shift at ca. 2.9 Ma, marked by a dramatic change in accumulation rates, stratigraphic completeness, and facies character demonstrates a major tectonic reorganization within this segment of the Main Ethiopian Rift. This likely reflects the initiation of down-rift translation of the primary terminal depocenter. The earlier high-accumulation rate (ca. 30 cm/ky) phase of Hadar Formation history is largely responsible for the geologically and palaeontologically rich early record at Hadar. The subsequent tectonic reorganization is reflected in the down-rift migration of lacustrine facies, and a shift in fluvial character to successive cut-and-fill cycles dominated by coarse (conglomeratic) lithologies.

Within the early phase of accumulation at Hadar, a strong cyclicity to the fluvial system, and regular intercalations of lacustrine or lake margin facies, reflects significant periodicity in the control of sedimentary character. This control is interpreted to be dominantly climatic in nature. After 2.9 Ma the stratigraphic record becomes much more complex. In spite of the abundant tephra markers in this latter time interval, highly localized stratigraphic preservation associated with cut-and-fill cycles makes interpretation of driving forces here more difficult, though climatic factors may still dominate.