Abstract

The Mistaken Point Formation is an Ediacaran fossil-bearing deepwater unit exposed in the eastern Avalon terrane of Newfoundland, and was deposited during a basin transformation associated with the shutdown of an Ediacaran arc system. Facies and provenance analyses constrain basin morphology, hinterland deformation, and the sedimentologic response of submarine fan systems to this basin transformation, and improve our understanding of the paleoenvironmental and tectonic context of this important Ediacaran fossil-bearing unit. Two distinct submarine fan systems are recognized in the St. John’s region, corresponding to the Middle Cove and Hibbs Cove members. The former is dominated by axial and off-axis submarine lobe strata with radial flow orientations recording submarine fan sedimentation fed transversely by a west-facing slope. The latter is dominated by off-axis, distal, and confined lobe strata with dominant flow to the south-southeast, recording fan sedimentation on a structurally confined south-facing slope. The Middle Cove Member to Hibbs Cove Member shift was accompanied by a reduction in fresh volcaniclastic sediment at the expense of quartz, sedimentary, and metamorphic lithic grains. Regional detrital-zircon geochronology shows that sediment was derived from erosion of Ediacaran arc rocks, with the Hibbs Cove Member detrital-zircon ages indicating an increase in erosion of West Avalonian cover successions and basement, coupled with an increase in zircon from synsedimentary igneous activity. Maximum depositional ages suggest that the Middle Cove and Hibbs Cove transition was diachronous, with the switch to delivery of sediment from denuded West Avalonian cover successions beginning in the Mistaken Point region by ca. 565 Ma and progressing northward and eastward over ca. 8 Myr. Conversely, a change in paleoflow that records the structural transformation of the basin was synchronous at ca. 557 Ma, resulting in a change from a transversely supplied submarine basin to a south-facing submarine slope. This transformation is interpreted to represent the transition from a backarc basin to a foreland basin, and is compared to the modern Sierra Pampeanas in South America, where flat-slab subduction during volcanic ridge subduction resulted in craton-ward migration of the arc front and the development of a retroarc foredeep.