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Fans and fan deltas – precursors to the Armorican Quartzite (Ordovician) in western Iberia

Published online by Cambridge University Press:  01 May 2009

N. McDougall ,
P. J. Brenchley ,
J. A. Rebelo  and
M. Romano
N. McDougall
Affiliation:
Department of Geological Sciences, University of Liverpool, Liverpool L69 3BX, U.K.
P. J. Brenchley
Affiliation:
Department of Geological Sciences, University of Liverpool, Liverpool L69 3BX, U.K.
J. A. Rebelo
Affiliation:
Serviços Geológicos de Portugal, Rua da Academia das Ciências, 19, 1200 Lisbon, Portugal
M. Romano
Affiliation:
Department of Geology, University of Sheffield, Beaumont Building, Brookhill, Sheffield S3 7HF, U.K.
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Abstract

The Armorican Quartzite (Lower Ordovician) is a very extensive sandstone body found throughout a large part of Iberia and Brittany; similar quartzites are present in north Africa and elsewhere. In Iberia it generally lies unconformably on a thick, folded, late Precambrian to Cambrian turbidite sequence (the Complexo Xisto-Grauváquico (CXG)), but there are some places where the quartzite has a conformable relationship with the underlying CXG. Where conformable the whole succession shows varied facies sequences including: (i) a regressive mud–sand sequence as a precursor to the main quartzite development, (ii) submarine fan sediments of the CXG passing up into slope and then shallow marine facies, or (iii) fluvially influenced breccio-conglomerates of a marginal fan delta passing up into shallow marine sediments.

The inferred development of alluvial fans associated with fan deltas prograding into shallow marine environments implies steep slopes and the likelihood that faulting controlled some of the uplift. The mozaic pattern of differential uplift and subsidence throughout the region suggests that the CXG was affected by local block movements rather than by regional folding.

The presence of upstanding blocks during the initial sedimentation of the Armorican Quartzite suggests that the quartz sand might have been derived from multiple sources dispersed throughout the area rather than from sources along a single shoreline at the margin of the depositional area.

Type
Articles
Information
Geological Magazine , Volume 124 , Issue 4 , July 1987 , pp. 347 - 359
Copyright
Copyright © Cambridge University Press 1987

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