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Diamictite and oolitic ironstones, a sedimentary association at Ordovician–Silurian transition in the north Gondwana margin: new evidence from the inner nappe of Sardinia Variscides (Italy)

Oggiano, Giacomo and Mameli, Paola (2006) Diamictite and oolitic ironstones, a sedimentary association at Ordovician–Silurian transition in the north Gondwana margin: new evidence from the inner nappe of Sardinia Variscides (Italy). Gondwana Research, Vol. 9 (4), p. 500-511. ISSN 1342-937X. Article.

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DOI: 10.1016/j.gr.2005.11.009

Abstract

The contact between the Silurian black phyllite and the Cambro–Ordovician underlying rocks has been investigated over different tectonic units, affected by green-schist facies metamorphism, in the inner nappe zone of the Sardinia Variscides. In spite of strain and metamorphism, the field work highlighted the occurrence of diamictic sediments. In the Canaglia Tectonic Unit the diamictite consists of dark, massive metamorphic claystone bearing chamositic ooliths, chamositic nodules and millimetre to centimetre sized clasts, dispersed, or gathered in clusters, within the muddy matrix. In the Argentiera Tectonic Unit the diamictite consists of angular clasts, ranging in size from few millimetres to several decimetres, scattered within a finely laminated black sericitic meta-argillite. Field data, textural and compositional analyses suggest a glacio-marine environment for the formation of the diamictites. The Canaglia diamictite deposited in a protected, glacial-influenced, shore. Compositionally it can be defined as ironstone; in the Upper part it hosts a horizon of clast-supported conglomeratic hard ironstone, mostly made of magnetite, which testifies for sub-aerial reworking. The source of the iron is to be related to local, glacio-eustatic driven, emergence of Upper Ordovician alkaline mafic volcanics. These are widespread in the uppermost Ordovician of the Canaglia Unit, possibly linked to the rifting stage that invested the north Gondwana margin, before the uppermost Ordovician–early Silurian sea level rise.

Item Type:Article
ID Code:4774
Status:Published
Refereed:Yes
Uncontrolled Keywords:Diamictite, upper ordovician, oolithic ironstone, North Gondwana, Sardinia
Subjects:Area 04 - Scienze della terra > GEO/03 Geologia strutturale
Divisions:001 Università di Sassari > 01 Dipartimenti > Scienze botaniche, ecologiche e geologiche
Publisher:Elsevier
ISSN:1342-937X
Copyright Holders:© 2006 International Association for Gondwana Research
Deposited On:05 Nov 2010 13:30

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