GEOLOGICAL MAPPING AND TEPHROSTRATIGRAPHY OF THE HADAR FORMATION NEAR 11.25 N AND 40.75 E (AFAR REGION, ETHIOPIA)

Fossil-bearing strata of the Hadar Formation crop out in a belt bounded on the southwest by the Ourda River, by the Awash River to the south and east, and by the Mille-Elowha Road to the north and west. Geological investigations from two field seasons permit us to present a basic stratigraphic framework (including tephra) for these Plio-Pleistocene units. 250 m of gently north-dipping strata are exposed within the south-central zone (Middle Ledi). Late Quaternary plateau gravels overly them in an angular unconformity of a few degrees. All units but the plateau gravels are cut (<20 m) by the few north-northwest and north-northeast striking normal faults (<2 km long). The strata are dominated by silty claystones with ubiquitous carbonate nodules. The lower 50 m of section is marked by gastropods, limestones, and lignites (lacustrine). The following ~50 meters comprises mudstones and discontinuous km-scale sand bodies several meters thick (floodplain and fluvial). The central 100 m of section alternates from red to brown sands and mudstones (floodplain) to silvery and yellowish mudstones with occasional fish fossils (lacustrine). The uppermost 35 m are green and brown silty claystones interbedded with at least one sand and pebble conglomerate package (floodplain and fluvial). They were the most productive for fossil recovery. Outcrop patterns and comparison with published field descriptions indicate that the base of the section is defined by the Sidi Hakoma Tuff while the top of the section is probably marked by BKT-2 tephra. We traced the upper tuff for >40 km across outcrops spanning the Adayitu-Mille Road. The thickest (8 m) and most diverse components crop out 8 km northeast of Adayitu, extending the trend of eastward thickening and increasing diversity noted in Hadar. Two additional tephra units are so far uncorrelated by field observations. The first is a grey to light tan, ledgy, discontinuous, ashy tuff encased within packages of pebble conglomerates and dark brown mudstones. The second crops out along the Mille River near its confluence with the Weranso. There, an 8 meter section exposes 16 tuffaceous layers. These field observations provide spatial breadth and new tephra descriptions for depositional basin models and the characterization of regional faunal variation as a function of changing paleoenvironment.