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

Williams, Martin; Assefa, Getachew; Adamson, Donald

doi:10.1144/gsl.sp.1986.025.01.20

Cited by 7 publications

(3 citation statements)

References 28 publications

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“…such as that which occurs today by the Bouri horst. The dominantly fl uvial nature of deposition at this time is seen closer to Gona in the eastern Middle Awash area (Williams et al, 1986).…”

Section: Busidima Formationmentioning

confidence: 76%

The geology of Gona, Afar, Ethiopia

Quade

Levin

Simpson

et al. 2008

The Geology of Early Humans in the Horn of Africa

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Deposits in the Gona Paleoanthropological Research Project (GPRP) area in eastcentral Ethiopia span most of the last ~6.4 m.y. and are among the longest and most complete paleoenvironmental and human fossil archives in East Africa. The 40 Ar/ 39 Ar and paleomagnetic dates and tephrostratigraphic correlations establish the time spans for the four formations present at Gona: the Adu-Asa (>6.4-5.2 Ma), Sagantole (>4.6-3.9 Ma), Hadar (3.8-2.9 Ma), and Busidima Formations (2.7 to <0.16 Ma). The volcano-sedimentary succession at Gona displays many classic tectono-sedimentary features of an evolving rift basin. The mixed volcanic and fl uviolacustrine Adu-Asa Formation is the earliest expression of rifting at Gona, probably deposited in a small half-graben. The Sagantole and Hadar Formations were deposited in a much METHODS Tephra Analysis Tephras were fi rst treated with 2 M HCl to remove any carbonate, briefl y rinsed in 2% HF, washed in distilled water, and sieved. In most cases, the 60-120 µm size fraction was used for electron microprobe analyses. All analyses were made on a CAMECA SX50 electron microprobe with a wavelength-larger half-graben bounded to the E-NE by an as-yet-unidentifi ed normal fault. The Sagantole and Hadar Formations are both fl uvial and lacustrine, refl ecting periodic shallow impoundment of a low-gradient paleo-Awash River, perhaps by an accommodation zone north of the Ledi-Geraru project area. Starting at 2.9-2.7 Ma, the character of sedimentation changed dramatically throughout the Awash Valley as bed load coarsened and the meandering paleo-Awash River cyclically cut and fi lled. Unlike the Hadar Formation, the Busidima Formation thickens westward, suggesting deposition in a half-graben of the opposite polarity compared to Sagantole/Hadar time. Sedimentation rates decreased 5-fold, from 0.25 mm/yr in the Hadar Formation to 0.05 mm/yr in the Busidima Formation, perhaps in response to slowing extension rates and/or opening of the half-graben north of Gona.

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Section: Busidima Formationmentioning

confidence: 76%

The geology of Gona, Afar, Ethiopia

Quade

Levin

Simpson

et al. 2008

The Geology of Early Humans in the Horn of Africa

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“…The Wonji Fault Belt was probably initiated about 1.6 Ma ago (Meyer et al, 1975) and a major impulse in volcanic and tectonic activity occurred within it at 0.25 Ma (Berhe, 1986). Williams et al, 1986;Adamson and Williams, 1987). activity. This study is concerned with the observation of tensional fissures.…”

Section: Brief History Of Rifting In the Region Of The Main Ethiopianmentioning

confidence: 99%

Tensional fissures and crustal extension rates in the northern part of the Main Ethiopian Rift

Williams

Aumento

2004

Journal of African Earth Sciences

123

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“…Recent seismic data from both Africa (Rosendahl & Livingstone 1983;Rosendahl et al 1986) and the North Sea (Gibbs 1984) have clarified ideas about rift structure. This, together with extensive research on rift basin sediments and sedimentation in East Africa (Frostick & Reid 1979, 1980Cohen et al 1986;Williams et al 1986;Williamson & Savage 1986) has inspired this reassessment of arid-zone continental rift sedimentation. It provides a new model that gives a satisfactory explanation for lithotype distributions that have remained anomalous in existing sedimentary models.…”

mentioning

confidence: 99%

Tectonic control of desert sediments in rift basins ancient and modern

Frostick

Reid

1987

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Desert sediments are frequently associated with continental rifts or aulacogens in the stratigraphic record. In this circumstance, rift structure and the timing and magnitude of tectonic events exert an overriding control on the distribution and character of alluvial sediments. Recent advances in the understanding of rift structure make a reassessment of ideas on the relationship between faulting, drainage patterns and alluvial facies distribution timely. Existing models of symmetrical graben with large alluvial fans shed from the footwalls of planar, normal boundary faults do not accord with observations in recent rifts. For example, half-graben dominate the structure throughout the African rifts. Boundary faults are mostly listric and the associated back-tilting of fault-blocks, combined with the doming that frequently accompanies rifting, causes back-shedding of rivers away from the basins. Alluvial fan development at fault scarps is, therefore, limited. The maximum observed fan radius in Lake Turkana is, for example, only 8 km, or 10~ of the half-graben width at that point. The supply of sediment to rift basins can be dominated by axial rivers (eg Sugata R. flowing into L. Logipi in Kenya, the Awash R. of the Afar in Ethiopia, and the Rhine R. flowing through the Rhine rift), but there are a surprising number of basins that do not have major axial inputs (eg L. Tanganyika and L. Malawi in East Africa and L. Baikal in Siberia). In these cases, the importance of smaller, but more numerous rivers draining across the roll-over becomes obvious. These rivers are subjected to rapid changes in base level and longitudinal gradient: movement of a boundary fault alters the geometry of the lake basin and base-level falls as the lake adjusts. This is accompanied by the aggradation of coarse alluvial deposits across the roll-over. Clast size gradients in these deposits are well defined for the region between the basin margin and the main depocentre, with typical measured values of -3 mm km -1. During the long, intervening periods of tectonic quiescence these and other deposits are reworked and redistributed downslope and offshore. Deltas build out into the lake, but microstratigraphy reveals that they are not of the classic Gilbert type. Short-and long-term fluctuations in lake level cause large-scale migration of the shoreline so that lake muds and alluvial sands are interdigitated over many kilometres in sequences which vary from a few metres to tens of metres in thickness. A synthesis of detailed work carried out on the sedimentology and geomorphology of a modern rift of the common half-graben type provides a new model of sedimentation that will prove useful in interpreting ancient deposits with hydrocarbon potential.

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

Cited by 7 publications

References 28 publications

The geology of Gona, Afar, Ethiopia

The geology of Gona, Afar, Ethiopia

Tensional fissures and crustal extension rates in the northern part of the Main Ethiopian Rift

Tectonic control of desert sediments in rift basins ancient and modern

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