The Early Cambrian Bradore Formation Of Southeastern Labrador And Adjacent Parts Of Quebec: Architecture And Genesis Of Clastic Strata On An Early Paleozoic Wave-swept Shallow Marine Shelf

摘要

Strata of the Bradore Formation have been interpreted in terms of a marine transgression developed at the onset of spreading of Iapetus Ocean, with a basal member of fluvial origin, a middle member of lagoonal origin and an upper member of tidal-channel origin. In Labrador the formation rests directly on a low relief surface, developed on metamorphic rocks of the Grenville Orogen, with no well developed paleosoils. Re-examination of the architecture of pebbly sandstones in the basal (Blanc-Sablon) member indicate that these formed on an open, storm-influenced wave-swept shelf as part of a low relief sand sheet, with evidence of multiple accretionary events. Structure in sandstones in the middle (Crow Head) member is largely obscured by a high density of Skolithos linearis domichnia. Abundance of hummocky cross stratification indicates deepening, with a transition to an open, storm-influenced, shallow shelf. Large-scale cross stratification in the glauconite-bearing upper (L'Anse-au-Clair) member reflects shoaling, with a return to deposition of medium-scale marine sandwave complexes, with no clear evidence of any tidal signature. Strata of the Bradore Formation were at least partly lithified prior to deposition of reefal and platformal carbonates in the overlying Forteau Formation. Paleocurrent indicators in the upper and lower members suggests that during the initial drift phase of Iapetus this segment (St Lawrence Promontory) of the Laurentian margin was swept by both geostrophic and storm-influenced currents, largely oriented parallel to the paleocoastline.