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

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.

Fluvial products and processes before the evolution of land plants: Evidence from the lower Cambrian Series Rouge, English Channel region

“…Marine incursions into the fluvial strata resulted in deposition of the Bluestone Bay Sandstone Member of the Alderney Sandstone Formation in the northern basin and the Erquy Quartzite Member and the Port Lazo Formation Upper Member in the southern basin (Went, , ).…”

“…They are finer grained than the repetitively stacked sheets of trough cross‐stratified sandstone that underlie and pre‐date them. The Roche Jagu Formation is fine to medium‐grained with localized very fine sandstone and mudstone beds (Went, ).…”

“…Mudstones occur either as thin beds capping units of trough cross‐stratification, where they are interpreted as abandoned channel fills, or intercalated with planar laminated and ripple cross‐laminated sandstones where they are interpreted as bar top fines (Todd & Went, ; Went, ).…”

“…Each of the coastal localities provides a good opportunity for case study (e.g. Todd & Went, ; Went, , , ; Went & Andrews, ; Ielpi & Ghinassi, ; McMahon et al ., ). However, the true significance of certain facies or architectures may not be appreciated without consideration of the changes that are happening at basin or regional scale.…”

Biotic forcing militates against river meandering in the modern Bonneville Basin of Utah

“…Flume experiments showed that increasing vegetation coverage can be related to restrained braiding and development of planform sinuosity (Gran & Paola, ; Tal & Paola, ). However, all of this evidence has been challenged on several fronts, owing to: (i) non‐unique solutions regarding whether vegetation promoted fluvial sinuosity in the early Palaeozoic or the opposite (Santos et al ., ; and ensuing debate in Davies et al ., ; and Santos et al ., ); (ii) the ever‐increasing documentation of pre‐vegetation alluvial strata that demonstrates a remarkable similarity in planform style (Santos et al ., ; Ielpi & Rainbird, , ; Santos & Owen, ; Went, ; Ghinassi & Ielpi, ; Ielpi, ; Ielpi et al ., ; Went & McMahon, ), channel geometry (Ielpi et al ., , ) and discharge regimes (Long, , , ; Nicholson, ; Ielpi & Ghinassi, ; Ielpi & Rainbird, ) with post‐vegetation rivers; (iii) alternative flume experiments where meander bends are maintained in the absence of vegetation (Peakall et al ., ), or where vegetation itself promotes flow disturbance, channel branching and eventual bar braiding (Coulthard, ); and (iv) the direct observation of modern, vegetation‐devoid fluvial landscapes capable of maintaining aggradational meandering channels (Li & Bristow, ; Li et al ., ; Ielpi, , ). In its broader sense, this debate represents a fundamental trial of uniformitarianism between modern and pre‐Silurian sedimentary landscapes, and could eventually disclose – or rule out – parallels between fluvial systems on Earth and other planetary bodies (e.g.…”