Paleoenvironmental and basin analysis of the late Neoproterozoic (Ediacaran) upper Conception and St. John's groups, west Conception Bay, Newfoundland

Abstract

The Conception and St. John's groups consist of deep-marine, fine-grained siliciclastic–volcaniclastic rocks with abundant impressions of Ediacaran body fossils. Based on previous tectonic reconstructions, these strata are interpreted to have accumulated during the transition from a fore arc (phase 1, ca. 630–560 Ma) to a strike-slip pull-apart basin (phase 2, ca. 560–540 Ma). The Drook and Mistaken Point formations accumulated during phase 1 and are characterized by tabular, thick- to medium-bedded turbidites (east-southeast paleoflow directions), contourites (southwest paleoflow directions), and volcanic ash beds. The high proportion of each bed consisting of TE (Bouma turbidite division) mudstone, and the lack of correlation between bed thickness and basal grain size, suggest ponding of the turbidity currents. The differences in paleoflow directions and texture between Conception Bay and equivalent units at Mistaken Point imply that the two areas may have been geographically isolated from each other; phase 1 deposition in the study area is inferred to have taken place in the basin-plain portion of a transversely supplied, partially confined sub-basin. This "west Conception sub-basin" is bounded to the east by outcrops of the older Harbour Main Group that we speculate may have been a topographic high separating the sub-basin from the main fore-arc basin located to the present-day east. The Trepassey and Fermeuse formations represent phase 2 and are characterized by tabular thin-bedded turbidites, slump deposits, and volumetrically minor amounts of volcanic ash that accumulated on a southwesterly prograding slope. The regional consistency of lithologies and paleoflow directions throughout the Avalon Peninsula suggest that basin differentiation was minimal at this time.