The stratigraphy and sedimentology of the late Precambrian St. John's and Gibbett Hill formations and the upper part of the Conception Group in the Torbay map-area, Avalon Peninsula, Newfoundland

摘要

In the Torbay map-area, the St. John’s and Gibbett Hill Formations of the Cabot Group and the upper part of the Conception Group form a continuous and-conformable sequence of Late Hadrynian sedimentary rocks. The upper part of the Conception Group consists of green-gray and variegated graywacke, siltstone, argillite, and volcanic tuff. The St. John’s Formation consists of six lithofacies and is divisible into two members; namely, in ascending order, Middle Cove and Outer Cove Members. The Middle Cove Member is represented by the laminated black shale and the Outer Cove Member by interbedded sandstone (or siltstone) and black shale with locally developed volcanic tuff. The Gibbett Hill Formation, overlying the St. John’s Formation, consists of green-gray arkosic sandstone, siltstone, argillite and volcanic tuffs. This formation is an equivalent of the lower member of the original Signal Hill Formation redefined in this thesis. The names "Battery Member" and "Cuckold Member" are proposed for the middle and upper members of the original Signal Hill Formation. -- The presence of volcanic tuffs in these units indicates intermittent volcanic activity during late Precambrian time. Tentative correlation of the tuff beds in the Torbay map-area suggests a diachronous relationship of the boundary between the Gibbett Hill and Signal Hill Formations. -- Aspidella terranovica Billings 1872 and associated surface markings in the Outer Cove Member of the St. John's Formation are considered to be inorganic sedimentary structures such as load casts and gas- or water-escape structure. -- A model of the depositional environments of part of the Cabot Group and the upper part of the Conception Group is proposed. The upper part of the Conception Group was formed as turbidites in deep marine environment. The Middle Cove Member of the St. John's Formation was deposited in a prodelta and the Outer Cove Member in distal delta front. The Gibbett Hill Formation, transitional to subaerial deposition, was formed in a proximal delta front. -- The source area for these units probably lies to the north of the Torbay map-area.