Stratigraphy, Depositional Environments and Basin Structure of the Hadar and Busidima Formations at Dikika, Ethiopia

Sediments exposed in the Dikika Research Project area form a nearly continuous sequence spanning the period from older than 3.8 Ma to younger than 0.15 Ma. By developing a stratigraphic framework of sedimentary basins, we are able to reconstruct a regional geological history that illuminates environmental changes resulting from tectonic events in the Afar triple junction region. The sequence begins with the Basal Member of the Hadar Formation, which was deposited on a dissected and deeply weathered surface of the uppermost flow of Dahla Series Basalt (8–4 Ma). This contact signals an increase in sediment accumulation rate due to active extension along faults parallel to the Red Sea Rift system. Sediments of the Hadar Formation indicate the progressive infilling of the Hadar Basin and migration of the shoreline northward or northeastward toward the axial depocenter, with several brief transgressions southward. After 2.9 Ma, the Dikika Research Project area was uplifted, and the Hadar Formation was faulted and eroded on an angular unconformity. Subsequent to 2.7 Ma, sedimentation returned, although the character and position of the newly developed Busidima half-graben had changed. This basin was formed by the rotation of an asymmetric marginal half-graben around a border fault that paralleled the western escarpment of the Ethiopian Rift. The Busidima Formation deposited in this basin records the migration of the paleo–Awash River across its floodplain in response to a changing tectonic setting. These local paleoenvironmental changes are primarily the response to regional tectonics and are superimposed on the global and regional records of climate change