GEOLOGY AND STRATIGRAPHY OF SEDIMENTS BELOW THE SIDI HAKOMA TUFF IN THE DIKIKA RESEARCH AREA, LOWER AWASH VALLEY, ETHIOPIA

New geological research from the Dikika Research Area, Lower Awash Valley, Ethiopia, provides a cohesive stratigraphic record of sedimentation and evolution spanning the range of >3.8 to <0.6 Ma, complementing records of surrounding research areas. Most sediments exposed at Dikika belong to the Hadar Formation (>3.4 to <2.3 Ma), famous for its many paleoanthropological finds. However, the definition of the this geological unit was developed in the 1970s at Hadar as the prospecting for hominids expanded from initial discoveries. Current research further afield from Hadar is demonstrating that the Hadar Formation definition needs to be further expanded or better delimited, not only to include or exclude strata above and below, but also to better articulate the tectonic implications of regional sedimentation patterns. While strata from Gona, western Hadar, and Asbole are expanding the younger end of the depositional sequence, the eastern Dikika area exposes variably thick sedimentary sections (~10 to 40+ m) below the Sidi Hakoma Tuff (SHT; ~3.4 Ma), thereby attributable to the defined Basal Member. Along a low ridge in the southern Dikika Area, the Ikini Tuff outcrops in a fault block where it is doubly upthrown by two intersecting normal faults. Major and trace-element abundances in glass shards from this tuff indicate a strong correlation to the Wargolo Tuff in the Turkana Basin, and to the VT-3 in the Middle Awash Region (~3.8 Ma). Furthermore, sections in the eastern and southern portion of the Dikika Area lie directly on tens of meters of basalt, which forms the local sedimentary basement. The upper several meters of these basalts exhibit the C horizon of a deeply weathered paleosol, overlain by juvenile sediments derived from the basalt, including poorly sorted sandstones with abundant angular basaltic rock fragments and other initial weathering products such as reworked calcite vugh-fills and zeolites. These sandstones are overlain by, and in some places partially interfinger with, lacustrine clays, which typify the Basal Member elsewhere. The lacustrine sequence thickens northwards across NE-trending faults, some of which are syndepositional, as indicated by growth faults. These exposures document the early evolution of this segment of the rift basin, the implications of which will be discussed.