EDIACARAN BIOTA ON BONAVISTA PENINSULA, NEWFOUNDLAND, CANADA

Journal of Paleontology, Jan 2008 by Hofmann, H J, O'Brien, S J, King, A F

The Murphy's Cove Member is dominated by medium-bedded, gray and green sandstones and gray, green, and red mudstones. Variegated structureless and laminated mudstones are present at the top of the member and may correlate with the Hibbs Cove Member, uppermost Mistaken Point Formation, at its type locality on the Avalon Peninsula (King, 1990). Fine-grained, white, gray to light brown tuff forms thin beds and laminae throughout the member and commonly preserve Ediacaran fossils underneath.

ST. JOHN'S GROUP

Conception Group strata in the core of the Catalina Dome pass conformably into fossiliferous marine mudrocks and interbedded sandstones of the St. John's Group (Williams and King, 1979), which consists of, in ascending stratigraphic order, the Trepassey, Fermeuse, and Renews Head formations. These sediments formed initially in a deep basinal slope environment, which, in response to a combination of sea-level changes and to seaward advances of a large prograding delta, shallowed over time and in two major cycles (King, 1980, 1990; O'Brien and King, 2005).

Trepassey Formation.-The Trepassey Formation lies conformably above the Mistaken Point Formation and shows a coarsening- and thickening-upward succession. It is divided into a lower, mud- and silt-rich Catalina Member and an upper, sand-rich Port Union Member (O'Brien and King, 2005). The Catalina Member is a succession of repetitive, thin-to medium-bedded, greenish-gray mudstone, siltstone, and pyritiferous sandstone. The beds are tabular with a sharp base, laterally persistent, and commonly graded, with a basal laminated silty sandstone passing upwards into structureless mudstone and tuff; they are interpreted as Bouma DE turbidites. Ripple cross-lamination and sole marks are rare in these beds. In the upper part of the member, medium beds of cross-stratified sandstone are locally developed. Ediacaran fossils occur below distinctive brown ash layers at several stratigraphic levels in the upper part of the member. The Catalina Member correlates with most of the Trepassey Formation at its type locality in the southern Avalon Peninsula. The upper part of the Catalina Member coarsens and thickens upward and passes transitionally into the Port Union Member. Thick to very thick, 1-3 m tabular beds of fine- to very coarse-grained gray quartzofeldspathic sandstone and granule conglomerate, and interbeds of mudstone, siltstone, agglomerate, and tuff characterize this member. The coarse beds are immature, volcanogenic, and formed under combined rapid downslope subaqueous flow of pre-existing sediment mixed with new pyroclastic debris, which might explain why no fossils have been found within these units. A comparable coarse sandstone facies forms a unit only 5 to 10 m thick at the very top of the Trepassey Formation on the Avalon Peninsula. Other extensive facies within the Port Union Member include thick-bedded fine- to medium-grained sandstone with distinct parallel lamination, associated thick beds of fine- to medium-grained sandstone with a massive or structureless appearance, and intercalated fossiliferous units of thin-bedded mudstones and very fine-grained sandstones that are comparable with those of the underlying Catalina Member.