Volcanosedimentary paleoenvironments of Ediacaran fossils in Newfoundland

A new perspective on paleoenvironments of Ediacaran fossils of the upper Conception Group (Newfoundland) comes from geochemical and sedimentological study of volcanic tuffs and sedimentary rocks. Tuffs in the Conception Group have major- and trace-element compositions and U-Pb ages comparable with those of source volcanics on the nearby Burin and Bonavista Peninsulas and the islands of St. Pierre and Miquelon. Loss of silica and alkalies in some ashes indicates weathering on land, not marine diagenesis. Volcanic crystal and lapilli tuffs fail to show grading and have lapilli and highly vesicular scoria scattered in a fine-grained matrix, and so they were deposited on land, not in water. These as well as block-and-ash flows, volcanic spindle bombs, and degassing features are evidence of eruptions from nearby subaerial volcanic edifices. The fossiliferous Conception Group accumulated within a forearc basin, formed on continental crust, inboard of the Holyrood horst, and uplifted as part of an ancient subduction complex or accreted terrane. Like analogous forearc basins in Oregon-Washington, southern Chile, and Japan, the Conception Group includes not only marine bay turbidites, but also a variety of intertidal and terrestrial tsunamites, seismites, tempestites, and paleosols. Traditional marine turbidite models explain deposition of the Mall Bay, lower Drook, and lower Briscal Formations of the Conception Group, but the Gaskiers, upper Drook, upper Briscal, and Mistaken Point Formations were deposited in coastal plains and intertidal zones. Paleoenvironments of vendobiont fossils preserved in life position in Newfoundland were terrestrial to marginal marine, not deep sea.