Résumés
Abstract
The Ratcliffe Brook Formation of the Saint John Group of the Avalon Terrane of New Brunswick forms the type Etcheminian Series for sub-trilobitic Cambrian strata. The formation, which is of variable thickness but up to 240-263 m, consists of six lithofacies which are generally arranged in stratigraphically ascending order, and are:- RBI basal conglomerate, RB2 rippled and non-rippled sandstone, RB3 layered siliceous claystone and fine-grained sandstone, RB4 greenish-grey chamositic sandstone and siltstone, RB5 maroon, thick-bedded, medium- to coarse-grained sandstone and conglomerate, and RB6 greenish and maroon shale, siltstone, sandstone, and volcanogenic sandstone.
Fades RB1 is interpreted to have been deposited in braid channels associated with distal portions of an alluvial fan, possibly in a coastal setting. Overlying fades are interpreted as having been deposited in transitional and shallow marine environments. Fades RB2-RB4 were deposited in a continuously deepening basin related to the early Cambrian eustatic marine transgression. Fades RB5 is subdivided into two subfades, RBSa and RB5b, with RBSa believed to represent shallow marine foreshore-shoreface deposits and RBSb deposits formed within distributary channels on the delta plain of a prograding delta complex. Fades RB6 was deposited in a lagoon of probable uneven width and depth, and which received sediments from varied local sources.
The considerable variation in thickness and spatial development of the six lithofacies within the Saint John Basin suggest that initially the basin had an uneven topography. In early stages of the early Cambrian transgression only basin lowlands were drowned and were subject to sedimentation. Complete coverage of the basin was not accomplished until late in the development of the Ratcliffe Brook Formation with the deposition of fades RB6.
RESUME
La Formation de Ratcliffe Brook (Groupe de Saint John, Laniere d'Avalon, Nouveau-Brunswick) consume la serie-type de rEtcheminien pour les strates cambriennes sub-trilobitiques. La formation, dont l'epaisseur varie mais atteint 240 a 265 m, comporte six lithofacies arranges generalement en ordre stratigraphique vers le haulet qui sont:- RB 1 poudingue basal, RB2 gres avec ou sans rides, RB3 gres fin intercale' de claystone siliceux, RB4 gres et siltite chamositiques gris-vert, RBS gres moyen a grassier et poudingue marrons en bancs epais, et RB6 shale, siltite, gres et gres volcanogenique verdatres et matrons.
On interprete le fades RB 1 conune s'ltant accumutes dans des chenaux tresses associ6s aux portions d is talcs d'un cone de dejection, possiblement en milieu littoral. On interprete les fades sus-jacents conune s'eiant accumules en milieux paralique et marin peu profond. Les fades RB2 a RB4 se sont accumules dans un bassin s'approfondissant de facon continue et reli£ a la transgression marine eustatique du d£but du Cambrien. On subdivise le fades RB5 en deux sous-fades, RBS a et RB5b; on croit que RBSarepresente des depots marins peu profonds d'avant-plage et d'estran, et RBSb des depots formes dans des chenaux de plaine deltaique au sein d'un complexe deltaJque en cours de progradation. Le depot du fades RB6 s'ef fectua dans une lagune dont la largeur et la profondeur variaient, et qui recevait des sediments de sources diverses et locales.
La forte variation de l'epaisseur et du dlveloppement dans l'espace des six lithofacies presents au sein du Bassin de Saint John suggere que la topographie initiate de ce dernier etait irrdguliere. Seules les basses-terres de bassin furent ennoyees et soumise aune sedimentation durant l'amorce de la transgression eocambrienne. L'inondation totale du bassin ne fut r£alisce que vers la fin du deVeloppement de la Fonnation de Ratcliffe Brook, avec le depot du fades RB6.
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