D. A. Adamson

Strontium isotope evidence for late Pleistocene reestablishment of an integrated Nile drainage network

Evidence of late Pleistocene closed-basin conditions in equatorial African lakes has far-reaching implications for the history of the Nile River. There has been widespread acceptance that overflow of Lakes Victoria and Albert into the Nile occurred some time during the terminal Pleistocene, but recent suggestions that Lake Victoria remained a closed basin until ca. 7.2 ka imply a much longer period of greatly reduced White Nile discharge. This hypothesis has major consequences for current ideas about the river9s paleohydrology, early human exploitation of riverine resources, Nile delta sedimentation, and late Quaternary genesis of sapropels in the eastern Mediterranean. Here we resolve the timing of Lake Victoria9s overflow by using strontium isotopes as a tracer for water from the principal lakes and rivers of the Upper Nile. Our data demonstrate that overflow of Lakes Victoria and Albert into the Nile drainage network occurred no later than 11.5 14 C ka.

Early Pliocene marine sediments, coastline, and climate of East Antarctica

We record the first onshore late Tertiary fossiliferous marine deposit from the 8000-km-long coastline of East Antarctica. Glaciomarine sediments containing abundant diatoms and molluscs were deposited in shallow waters off an open coast during the early Pliocene (∼4.5-3.5 Ma) and are now exposed less than 15 m above sea level on Marine Plain in the Vestfold Hills (lat 68°359S,long 78°009E). Marine diatoms suggest early Pliocene as the most probable age of the sediments. Fossil sand wedges show that the deposit was above sea level at least briefly before it was covered with water and subsequently capped with glaciomarine till. The coastline at the time of deposition of the marine sediments is inferred to have been close to but farther inland than that of today. The ice margin may have been ∼50 km farther inland, considerably reducing the ice volume of Antarctica. Climate at the time of deposition was interglacial and perhaps warmer than the present.

New light on the age of the White Nile

Landsat 5 Thematic Mapper and 7 Enhanced Thematic Mapper imagery reveal lake shorelines in the White Nile valley as far south as lat 10°N. The highest shoreline is at 386 m elevation and was eroded when the White Nile formed a lake as wide as 70 km and >500 km long. Finely laminated green clays laid down on the floor of this lake are overlain by alluvial sands and clays, dated by optically stimulated luminescence as 15 ka to older than 250 ka. The alluvium was deposited during interglacial episodes of stronger summer monsoons and very high White Nile floods. The White Nile paleolake is much older than marine oxygen isotope stage 7 and may have formed ca. 400 ka, during the exceptionally long stage 11 interglacial.

The Physiography of the Central Sudan

The geological history of the Nile in the Sudan has been reviewed comprehensively by Berry and Whiteman (1968), Butzer and Hansen (1968), and more recently by Whiteman (1971). Recent fieldwork by the authors in central Sudan has more than trebled the number of radiocarbon dates available from that region, and has added significant detail to knowledge of the age and origin of the Gezira plain and the late Quaternary history of the Blue and White Niles. This paper discusses the prior-stream channels and associated sand deposits in the Gezira, and the evidence for higher Nile levels in late Pleistocene and Recent times. It is proposed that the hydrological changes which took place in central Sudan during the past 12 millennia reflect climatic changes of regional extent. Before a consideration of the Pleistocene sequence of events, the present-day climate, vegetation and river regimes are briefly reviewed.

Late Quaternary glaciation and deglaciation of the Bunger Hills, Antarctica

The Bunger Hills were covered by the Antarctic Ice Sheet during the last glaciation. During deglaciation the ice sheet margin collapsed into the marine inlets and the sea entered the oasis before 7.7 ka BP. Raised beaches occur widely below 8.5 m and indicate uplift at 1.4 m ka −1 during the middle and late Holocene. After the coastal inlets were formed, the Edisto Ice Tongue and Apfels Glacier still impinged on land margins in the west of the oasis. Two sets of marginal moraines were formed; the Older Edisto Moraines after 6.2 ka BP and the Younger Edisto Moraines during the last few centuries. The margins of the Antarctic Ice Sheet and Apfels Glacier in the south have maintained their present positions since at least 5.6 ka BP and probably 10 ka BP.

Depositional context of Plio-Pleistocene hominid-bearing formations in the Middle Awash valley, southern Afar Rift, Ethiopia

Summary The Awash flows from the Ethiopian Highlands via the Ethiopian Rift into Lake Abhe in the Afar Rift. Over 8000 km2 of Neogene fluvio-lacustrine deposits crop out in the Middle Awash valley, of which we have surveyed 300 km2 E of the river between 10°25′N and 10°45′N. Pleistocene sediments adjacent to the Awash floodplain consist mostly of unfossiliferous brown clays with sand and gravel interbeds which contain vertebrate (including hominid) fossils and prehistoric stone artefacts (Oldowan and Acheulian). Palaeocurrent directions in these fossiliferous sands invariably parallel the ephemeral sandbed channels that flow W from the basalt escarpment to the Awash. We interpret the brown clays as palaeo-Awash floodplain deposits, in contrast to the fossiliferous gravel-sands, which are mainly of local provenance. Away from the river, alternating Pliocene clays and diatomites, with minor fossiliferous gravel-sands, are capped by a 3.8–4.0 Ma primary air-fall tuff. Facies changes are more frequent above this ‘Cindery Tuff’, indicating tectonic disruption of the early Pliocene lakes, reflected also in a progressive increase in fluvio-volcanic gravels from Pliocene to early Pleistocene.

Geological setting of Pliocene rifting and deposition in the Afar Depression of Ethiopia

Abstract Tectonic extension of the Afar amounts to 10–30 km since 1–2 Ma and to 40–60 km since 3–5 Ma ago, or rougly 1–2 cm per year. Active faulting, volcanism, and development of the Pliocene and younger Afar grabens with their rich hominid and other vertebratefaunas, have been controlled by ENE and WNW oriented lineaments or ancient zones of structural weakness. These lineaments also controlled the alignment of the Gulf of Aden and Red Sea, respectively. Crustal stresses resulting from the late Miocene (Messinian) salinity crisis in the Mediterranean basin may have triggered renewed tectonic movements along certain pre-existing lineaments such as the Levant Shear. Separation of Africa from Arabia (and hence the inception of Afar) was controlled by tectonic events far removed from NE Africa, including possible Miocene fusion of the Indo-Arabian plates. During the early Pliocene, the Ethiopian uplands were far lower (possibly by 1000 m) and the southern Afar-Middle Awash region was higher, so that topographic and climatic contrasts between plain and plateau were less pronounced and the Afar climate was less arid than today. There was a major change from lacustrine to fluviatile deposition in the Middle Awash valley soon after 4·0–3·8 Ma ago, caused by extensional tectonics. Within the topographic constraints imposed by volcano-tectonic activity, regional climatic oscillations have controlled the detailed pattern of Pliocene (and later) sedimentation in Afar.

Former glacial lakes of the Bunger Hills, Antarctica

SUMMARY The Bunger Hills in East Antarctica occupy a land area of approximately 400 km2. They have been exposed by Holocene retreat of the Antarctic ice sheet and its outlet glaciers. The accompanying sea level rise flooded the marine inlets that now separate the northern islands and peninsulas from the major part of the hills. During deglaciation the continental ice sheet margin retreated south‐eastwards with several temporary halts, during which ice‐dammed lakes were formed in some valleys. These lakes were maintained long enough to permit formation of beaches of sand and gravel, and for the erosion of shore platforms and low cliffs in bedrock. Around the western end of Fish Tail Bay impressive shoreline features 20 m above sea level define a former ice‐dammed lake that was 5.5 km long. A similar 7 km long former ice‐dammed lake was formed at Lake Dolgoe. The more extensive and deeper glacial lake is revealed by well‐developed and preserved shoreline features cut at 29 m which is 16 m above present lake...

Landform and Vegetation Change in the Greaves Creek Basin: An asymmetric hanging valley in the Blue Mountains, New South Wales

Greaves Creek has cut a hanging valley through the entire Triassic sandstone sequence near Blackheath in the western Blue Mountains, New South Wales. Downstream of Beauchamp Falls, it cuts into Permian strata in the Grose Gorge. The hanging valley has a valley-in-valley structure. The narrow inner valley is bounded by high cliffs and its floor is cut by a deep narrow slot canyon where stream incision has occurred without valley widening. The course of the creek is related to joint directions. Intense jointing, minor faulting and sapping influence the stability of cliffs but up to 30m of incision has occurred without valley widening in the slot canyon. Topographic asymmetry expressed as unequal slopes of the valley sides is related to differential insolation, erosion, vegetation cover, bioturbation and fire intensity. In the western Blue Mountains and elsewhere in the Sydney Basin asymmetric slopes occur in many other valley-ridge systems, particularly those whose long axes are oriented between about east-west and north-east-south-west. Vegetation structure and floristics within Greaves Creek valley are related to physiography of the valley and to aspect through their effects on fire, moisture availability, light availability, soil depth and temperature.