PALAEOGEOGRAPHY OF THE CAMBRIAN LAURENTIAN MARGIN WITHIN THE QUEBEC REENTRANT

The study of key areas is particularly useful to clarify our understanding of regional palaeogeography. The Anse Maranda Formation is a Cambrian formation previously interpreted as a deep-water turbidite deposit and correlated to other green feldspathic sandstones of the northern Appalachians. However, its location, composition, ichnofacies and sedimentary structure suggest otherwise. The Anse Maranda Formation sits at the base of the Ile d’Orleans Group and is located at the extreme northwest of the external domain of the Humber zone. Unlike other Cambrian green sandstones of the Appalachians, which are chlorite-bearing, the sandstones of the Anse Maranda Formation are glauconite-rich. Study of the glauconite showed that it grew in-situ, thus suggesting a deposition on a sediment-starved environment. Extensive occurrence of trace fossils of the Cruziana ichnofacies and some of the Zoophycos ichnofacies indicates deposition on a shelf near the storm-wave base. Intense biogenic activity left no primary sedimentary structures. This suggests that the Anse Maranda Formation was not deposited at the toe of a continental slope but rather on the adjacent continental shelf. Detailed work within the Anse Maranda Formation allowed the recognition of local features reflecting an asymmetric geometry. To the west, the regular sedimentation pattern suggests a gentle slope. To the east, the occurrence of channelized conglomerates and thick debris flow are the results of a steep and irregular slope. The geology of Anse Maranda Formation is consistent with a sea-level fall during the late Early and Middle Cambrian and a rise during the Late Cambrian. The timing of the lowstand corresponds to the base of the Sauk II para-sequence. The Anse Maranda sandstones are rich in feldspar indicating a nearby continental source. Because its lateral extend is not important, it is proposed that the Anse Maranda Formation was deposited on a narrow shelf adjacent to a promontory. This supports the existence of the Montmorency Promontory, a feature originally proposed to explain lateral changes in Ordovician flysh facies. Our work indicates this that feature was present in Early Cambrian and that a modification to the palaeogeographic model of this portion of the Quebec Re-entrant is required.