The premier East African hominid site of Hadar recently reopened for research after a 15 year lapse. Major geologic opportunities are offered by the Pliocene Hadar Formation because of its setting along the tectonic margin between the Ethiopian Plateau and the Afar Depression with an outstanding badlands exposure that extends well beyond the studied site of Hadar. We will build a facies model of deposition for the formation pegged to a very good stratigraphic/geochronologic framework. The previous sedimentological work stressed stratigraphic correlation and suggested only very generalized environments of deposition. Being just 50 km east of the presently 2.5 km high Plateau escarpment, these Pliocene deposits remarkably consist of 70% mud units and only 30% sands. Most of the muds are massive with incipient arid paleosols. Minor laminated mud sequences were deposited in a lake we hypothesize transgressed from the east (central Afar). While the massive mud sand sequence seems to represent deposits of a meandering river the sands are too laterally extensive and too gradational into cobble conglomerates (westward toward the escarpment) to be explained by presently available meandering stream models. By delineating and mapping detailed subfacies, especially in the sands and by cataloging lateral and vertical sequences and the first systematic paleocurrent data, we can assess sandflat versus various channelized flow options. For the muds we hope to delineate paleosols of varying maturity, and whether the lake was saline or fresh. Careful facies mapping along the several isochronal tuff surfaces will provide environmental mosaic time-slices across Hadar. This research should provide: (1) a significant segment of the history of the Afar tectonic boundary; (2) knowledge of the environments where the hominids lived and were preserved, and a strategy for exploring for them in the vast exposures east and west of Hadar, and; (3) a contribution toward a general facies model for thick rapid alluvial deposition where sheets sands exist in association with thick muds.
