Deposition of the Cambrian continental rise: the St. Roch Formation near St. Jean-Port-Joli, Quebec
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abstract
Detailed mapping between Plage Victor and St. Jean-Port-Joli has enabled us to add a 550 m section to the top of the existing 450 m section of Hubert, making the longest continuous section anywhere in the Cambrian continental rise, Iapetus margin of North America. Our section can be divided into four main facies. The red mudstone facies makes up 30% of the section, and contains thin classical turbidites with easterly flow directions. The classical turbidite facies include both calcareous and siliceous turbidites, mostly Bouma BCE types, in beds up to about 1 m thick. Flow directions are again mostly to the east. The massive and pebbly sandstone facies occurs in units up to nearly 100 m thick, and contains both massive and normally graded beds with many fluid-escape features and rare trough cross-bedding. Occurrences of this facies normally have channelled bases, and there are several 5–10 m thick thinning- and fining-upward sequences. Paleoflow directions are southeasterly. The slurry facies includes massive to graded slurry beds, interpreted as debris flow deposits, and slurry breccias that incorporate slabs of interbedded sandstone.We interpret the red mudstones and classical turbidites as base-of-slope, continental rise deposits. The generally easterly and east-southeasterly flow directions suggest a local dip of the continental rise in that direction. The massive and pebbly sandstones represent the fill of feeder channels incised into the rise with southeast, oblique-to-maximum-dip, directions. The absence of thickening-upward sequences in the various turbidite facies suggests the absence of conventional submarine fans, perhaps due to the dominantly fine-grained material forming the lower slope and rise in this area.
