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The Ordovician glaciation in Eritrea and Ethiopia, NE Africa
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologi och geokemi.
2007 (engelsk)Inngår i: Glacial Sedimentary Processes and Products, Blackwell Publishing , 2007, s. 321-342Kapittel i bok, del av antologi (Fagfellevurdert)
Abstract [en]

Ordovician (Hirnantian?) glacigenic deposits are described here for the first time from south-central Eritrea. These deposits rest on an almost peneplained Neoproterozoic basement and define, in Eritrea and Ethiopia, a depositional area measuring at least 200 km in an east-west direction and 170 km in a north-south direction. For this preliminary note, five sections through the glacigenic succession were logged in Eritrea and one in Ethiopia. Facies types are described and interpreted. An ice-proximal facies assemblage is located in the Tigray Province of northern Ethiopia, the type area of the glacigenic Edaga Arbi Beds. These proximal deposits, c. 20 m thick, are characterised by melt-out diamictites, with striated clasts, interlayered with sandstone beds displaying horizontal lamination and normal grading (sand-silt). The horizontal lamination in the section is transitional with climbing ripple beds. Ice rafted clasts in sand-granule grade are common in these sandy beds. This ice-proximal section also exhibits some, minor soft-sediment deformation, such as asymmetrically folded beds, south-dipping reverse faults and glacial grooves suggesting transport to the north. This proximal facies grades laterally into a cross-bedded arkosic sandstone, the Enticho Sandstone, which probably represents deposition on subaqueous outwash fans. Cross-beds in this sandstone dip consistently to the north also in south-central Eritrea. Glacial striae and grooves are observed on top of the Enticho Sandstone in two localities in Eritrea. These proximal facies types are overlain by a distally deposited mudstone-dominated unit, 3-40 m thick, most probably deposited from turbid overflow plumes, although it also contains ice-rafted clasts. Only this unit hosts ice-rafted clasts in Eritrea. In Eritrea it also contains some diamictites. The name Edaga Arbi Beds is adopted for this unit in Eritrea. Icebergs were probably responsible for the deposition of the diamictites in south-central Eritrea. The development of this glacigenic succession was probably related to a regular retreat of the ice margin from north to south. It is also probable, that this succession only represents one cycle of deglaciation, the last of the two (or three) recognised in other parts of North Africa. The post-glacial development is initially represented by the deposition of a probable marine dune complex migrating from north to south. Fossil evidence and trace fossils, particularly Arthrophycus alleghaniensis (Harlan) suggest that the age of these glacigenic deposits is Late Ordovician, probably Hirnantian.

sted, utgiver, år, opplag, sider
Blackwell Publishing , 2007. s. 321-342
Emneord [en]
Glaciation, Ordovician, Eritrea, Ethiopia, Edaga Arbi
HSV kategori
Identifikatorer
URN: urn:nbn:se:su:diva-22429ISBN: 978-1-4051-8300-0 (tryckt)OAI: oai:DiVA.org:su-22429DiVA, id: diva2:188956
Tilgjengelig fra: 2007-12-20 Laget: 2007-12-20bibliografisk kontrollert

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