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Alluvial fan, braided river and shallow-marine turbidity current deposits in the Port Lazo and Roche Jagu formations, Northern Brittany: relationships to andesite emplacements and implications for age of the Plourivo-Plouézec Group

Published online by Cambridge University Press:  03 August 2016

DAVID J. WENT
DAVID J. WENT*
Affiliation:
Ostralegus Ventures Ltd., Silver Birches, Fox Hill Village, Haywards Heath, RH16 4QZ
*
*Author for correspondence: david.dwent@talktalk.net
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Abstract

Facies and stratigraphic analysis of the Port Lazo and Roche Jagu formations, together the lower part of the Plourivo-Plouézec Group, suggests deposition in three distinct depositional systems. The lower part of the Port Lazo Formation comprises red conglomerate, sandstone and shale of alluvial fan to alluvial plain origin. A conformable interval of grey sandstone and shale succeeds the lower Port Lazo red beds and records a period of subtidal sedimentation dominated by river-fed, shallow-water turbidity currents (hyperpycnites). The succeeding Roche Jagu Formation comprises red sandstones and shales of braided fluvial origin. It is intercalated with, and succeeded by, andesites. The andesites succeeding the fluvial strata overlie a prominent erosion surface and are lava flows, whereas those intercalated with the fluvial strata are intrusions. Rb–Sr radiometric dating of the andesites at 472+/−5 Ma is commonly used as evidence for the whole Plourivo-Plouézec Group being Early Ordovician in age. However, the stratigraphic relationships and patterns of sedimentation in the Port Lazo and Roche Jagu formations, together with the localized presence of Arumberia, suggest they are most likely of early Cambrian age and related to a phase of post-Cadomian rifting. The facies deposited show both similarities to and differences from neighbouring strata of equivalent age, and highlight the control exerted by sediment load on alluvial and nearshore processes on early Palaeozoic environments.

Keywords

hyperpycniteslower CambrianArumberiapre-vegetation environments
Type
Original Articles
Information
Geological Magazine , Volume 154 , Issue 5 , September 2017 , pp. 1037 - 1060
Copyright
Copyright © Cambridge University Press 2016 

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