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The Ayn Formation of the Mirbat Group, Dhofar, Oman

By
Philip A. Allen
Philip A. Allen
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
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Ruben Rieu
Ruben Rieu
TOTAL E&P Nederland, Bordewijklaan 18, Den Haag, The Netherlands
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James L. Etienne
James L. Etienne
Neftex Petroleum Consultants Ltd, 97 Milton Park, Abingdon, Oxfordshire OX14 4RY, UK
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Albert Matter
Albert Matter
Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland
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Andrea Cozzi
Andrea Cozzi
ENI Angola, Rua Nicola Gomes Spencer, 140, PO Box 1289, Luanda, Angola
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Published:
January 01, 2011

Abstract

Glacial deposits are found in the Ayn Formation and Shareef Formation of the Mirbat Group close to Mirbat in Dhofar, southern Oman. The Mirbat Group is most likely a correlative of the Abu Mahara Group of the Huqf Supergroup of northern Oman. The Ayn Formation, the main subject of this chapter, comprises <400 m of mainly coarse-grained glaciogenic deposits, ponded in 2- to >8-km-wide N- to NW-oriented palaeovalleys eroded into crystalline basement, with few or no deposits preserved on intervening palaeohighs. The Shareef Formation occurs as thin, lenticular, erosional remnants beneath the unconformably overlying Cretaceous. The Ayn Formation is overlain by a thin (<3 m), discontinuous cap carbonate that passes from carbonate-cemented talus on the basin margin to stromatolitic carbonate on palaeohighs and resedimented gravity flows on palaeovalley flanks.

The Ayn Formation is younger than its youngest detrital zircons and the youngest late plutons in crystalline basement, constraining it to <c. 720 Ma, but its exact age is unknown. The detrital zircon population comprises exclusively Neoproterozoic sources, suggesting derivation from the juvenile Neoproterozoic crust of the Arabian area.

The composition of fine-grained matrix in glaciogenic diamictite units and of non-glacial mudstones, plotted using the chemical index of alteration (CIA), suggests strong variations in the intensity of palaeoweathering on contemporary land surfaces between the mechanical weathering-dominated Ayn Formation, and the chemical weathering-dominated overlying Arkahawl Formation, which supports the notion of major glaciation followed by rapid climatic transit as basin margins were flooded and buried with sediment during post-glacial transgression. The carbon isotopic ratio (δ13C) of the post-glacial carbonate is strongly variable from −3.5‰ to +5.8‰, whereas carbonate fissures in the underlying basement range between +4.1‰ and +5.7‰.

Two independent palaeomagnetic studies have yielded low palaeomagnetic latitudes for the Mirbat Group.

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Contents

Geological Society, London, Memoirs

The Geological Record of Neoproterozoic Glaciations

Emmanuelle Arnaud
Emmanuelle Arnaud
University of Guelph, Canada
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Galen P. Halverson
Galen P. Halverson
McGill University, Canada
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Graham Shields-Zhou
Graham Shields-Zhou
University College London, UK
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Geological Society of London
Volume
36
ISBN electronic:
9781862394117
Publication date:
January 01, 2011

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