Piero Bruni

A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea

One of the most contentious topics in the study of human evolution is that of the time, place and mode of origin of Homo sapiens. The discovery in the Northern Danakil (Afar) Depression, Eritrea, of a well-preserved Homo cranium with a mixture of characters typical of H. erectus and H. sapiens contributes significantly to this debate. The cranium was found in a succession of fluvio-deltaic and lacustrine deposits and is associated with a rich mammalian fauna of early to early-middle Pleistocene age. A magnetostratigraphic survey indicates two reversed and two normal magnetozones. The layer in which the cranium was found is near the top of the lower normal magnetozone, which is identified as the Jaramillo subchron. Consequently, the human remains can be dated at ∼1 million years before present.

GEOLOGY OF THE HOMO -BEARING PLEISTOCENE DANDIERO BASIN (BUIA REGION, ERITREAN DANAKIL DEPRESSION)

This paper deals with the geological context of the northernmost site in the East Africa Rift system which has yielded Homo erectus -like remains. They are dated ca. 1 Ma and have been found in the deltaic deposits of the Alat Formation belonging to the Dandiero group. This newly defined group crops out extensively in an elongated belt from the Gulf of Zula to the North to the Garsat area to the south. In the Buia-Dandiero area it ranges in age from the Early to the Middle Pleistocene, and incorporates six formations, from bottom up: the fluvial Bukra Sand and Gravel, the deltaic and lacustrine Alat Formation, fluvial Wara Sand and Gravel, the lacustrine Goreya Formation, the fluvio-deltaic Aro Sand and alluvial Addai Fanglomerate. This succession is bounded by two major unconformities, which separate it from the Neoproterozoic basement and from the overlaying Boulder Beds fanglomerate, and has been designated the Maebele Synthem. The latter is the result of two lacustrine transgression and regressions evidenced by two depositional sequences. The unconformities bounding the Maebele Synthem are related to the tectonic history of the basin fill and its substrate. The development of the two sequences was, instead, mainly controlled by lake level fluctuations and, hence, by climatic variations connected with the weakening and strengthening of the monsoons in the northwestern Indian ocean. The environment where the Buia Homo lived was a savannah with some scattered water pools. This environment probably extended farther north along the western coastal plain of the Red Sea, and was a preferential pathway for the dispersal of the hominids from East Africa toward Eurasia.

Geology of Ethiopia: A Review and Geomorphological Perspectives

The Ethiopian region records about one billion years of geological history. The first event was the closure of the Mozambique ocean between West and East Gondwana with the development of the Ethiopian basement ranging in age from 880 to 550 Ma. This folded and tilted Proterozoic basement underwent intense erosion, which lasted one hundred million years, and destroyed any relief of the Precambrian orogen. Ordovician to Silurian fluviatile sediments and Late Carboniferous to Early Permian glacial deposits were laid down above an Early Paleozoic planation surface. The beginning of the breakup of Gondwana gave rise to the Jurassic flooding of the Horn of Africa with a marine transgression from the Paleotethys and the Indian/Madagascar nascent ocean. After this Jurassic transgression and deposition of Cretaceous continental deposits, the Ethiopian region was an exposed land for a period of about seventy million years during which a new important peneplanation surface developed. Concomitant with the first phase of the rifting of the Afro/Arabian plate, a prolific outpouring of the trap flood basalts took place predominantly during the Oligocene over a peneplained land surface of modest elevation. In the northern Ethiopian plateau, huge Miocene shield volcanoes were superimposed on the flood basalts. Following the end of the Oligocene, the volcanism shifted toward the Afar depression, which was experiencing a progressive stretching, and successively moved between the southern Ethiopian plateau and the Somali plateau in correspondence with the formation of the Main Ethiopian Rift (MER). The detachment of the Danakil block and Arabian subcontinent from the Nubian plate resulted in steep marginal escarpments marked by flexure and elongated sedimentary basins. Additional basins developed in the Afar depression and MER in connection with new phases of stretching. Many of these basins have yielded human remains crucial for reconstructing the first stages of human evolution. A full triple junction was achieved in the Early Pliocene when the MER penetrated into the Afar region, where the Gulf of Aden and the Red Sea rifts were already moving toward a connection via the volcanic ranges of northern Afar. The present-day morphology of Ethiopia is linked to the formation of the Afar depression, MER, and Ethiopian plateaus. These events are linked to the impingement of one or more mantle plumes under the Afro-Arabian plate. The elevated topography of the Ethiopian plateaus is the result of profuse volcanic accumulation and successive uplift. This new highland structure brought about a reorganization of the East Africa river network and a drastic change in the atmospheric circulation.

UNUSUAL ICHNOFOSSILS IN HOMO ERECTUS-BEARING BEDS OF THE PLEISTOCENE LAKE DEPOSITS IN CENTRAL-EASTERN ERITREA, EAST AFRICA

Abstract The continental Pleistocene Alat Formation in Eritrea hosts one-million-year-old Homo erectus remains and includes lacustrine calcareous beds with exceptionally numerous and well-developed ichnofossils. They are referred to two different types with distinctive morphologies based on the shape of their casts on the bed soles. Ichnofossils of the first type, termed donuts, have a pronounced ring shape with an average diameter of ∼13 cm. Those of the second type have a rosette shape with a diameter of up to 35 cm and more than 20 short fingers arranged around a flat, circular area. We interpret the donut-shaped structures as casts imprinted by the conical pedal disk of an unknown organism on the soft floor sediment. A similar organism with a more complex pedal disk, characterized by protrusions, may have made the rosette-shaped ichnofossils. Both ichnofossils are interpreted as resting traces of soft-bodied animals anchored in a vertical position within the sediment. After their deaths, the traces were preserved as casts on the lake floor and subsequently filled by calcareous sand. The peculiar plug-shaped resting traces, the lacustrine habitat of the tracemakers, and the lack of similar traces in the literature suggest that these fossils represent two new ichnotaxa left by invertebrate organisms of unknown affinity.

Deposits of ephemeral and perennial lakes in the tertiary Daban Basin (Northern Somalia)

Abstract The Daban Basin is filled with 2700 m of Middle Eocene to Oligocene clastic deposits. From the bottom of the sequence upward six sedimentary environments are distinguished: restricted lagoon, delta, lagoon, alluvial plain, ephemeral and perennial lakes. The latter show a lateral transition into thick fan-delta sequence (Kalajab fan-delta). A general vertical trend from an ephemeral, saline lake to perennial fresh-water lake has been recognized. The lower portion of the lacustrine sediments, which is 1100 m thick, consists of red and green siltstones, interbedded with cross-laminated sandstones representing dry mudflat (playa) deposits. An interval of saline playa sedimentation, interbedded in the playa mudflat sequence, includes red siltstones, green siltstones and laminated gypsum arranged in cycles; these were deposited in dry mudflats and in a chemically stratified lake. The upper portion of the lacustrine deposits, which is 900 m thick, is composed of green siltstones, cross-lamimated sandstones, chalky laminated limestones and some tuff layers. Fresh-water fishes, ostracods, gastropods and tree trunk remains are locally present. These sediments were laid down in a fresh-water perennial lake. The Daban Basin was a rapidly subsiding depression during the period that rifting was taking place in the adjacent Gulf of Aden. Lacustrine deposition was controlled by the paleogeographic and paleoclimatic changes induced by this event.