The Early Silurian Gun River Formation of Anticosti Island, eastern Canada: A key section for the mid-Llandovery of North America

The 85–100 m thick carbonate succession of the Gun River Formation is revised on the basis of nearly continuous cliff sections measured on the northeast coast, and a re-analysis and re-measurement of the stratotype sections on the southwest coast of Anticosti island, Québec. The Gun River Formation (Llandovery: lower Aeronian) is herein formally divided into four members, from the base up, the Lachute, Innommée, Sandtop and Macgilvray members. The formation characterizes a generally deepening phase on the northwestern shelf of Laurentia, bordering the Iapetus ocean in this region. Since the base of the formation is located ca. 150 m above the O/S boundary, it provides fresh data as to the nature of the Early Silurian transgression, radiation, and re-invasion of shelf communities following the Late Ordovician mass extinctions. The Gun River strata were deposited in a mid- to distal, tropical, very gently sloping (< 1–3 m per km), carbonate ramp or shelf, some 40–80 km offshore from the stable Laurentian craton, and at water depths of 30 to 60 m. Much of the formation consists of rhythmically deposited, light grey micrites (with fissile muddy partings), and minor calcareous mudstones, punctuated by abundant hardgrounds, intraformational conglomerates, and shell-beds. The fauna of brachiopods and corals is generally of low diversity, marking a turnover from a latest Rhuddanian community (Merrimack – lower Gun River Fm) to a more diverse midAeronian fauna (top Gun River – lower Menier Fms) when large pentamerids become abundant. This indicates gradual changes rather than abrupt invasions, sustained deposition, and lack of major sedimentary gaps between stratigraphic units. The absence of shallow water reefs, skeletal stromatolites, and ooid or oncoid shoals, also suggests deposition near or periodically below the storm wave base on a carbonate platform in southern tropical latitudes at 20°–25° S.