Depositional History of the Niagaran Series in Southern Ontario1

Abstract

ABSTRACT This paper presents a new correlation of the Niagaran Series of southern Ontario (represented by the Neahga, Reynales, Irondequoit, Rochester, DeCew, Lockport and Guelph formations in the Allegheny Basin; and the Fossil Hill, Amabel and Guelph formations in the Michigan Basin). Marked lithological and faunal differences in the two basins, during the Late Llandovery and Wenlock, are attributed to the topography of the Algonquin Arch and to the fluctuation of sea level. Following the withdrawal of the Cataract Sea in Middle Llandovery, the succeeding transgression inaugurated the Niagaran Series. The deposition began in the early Late Llandovery and ended in the Ludlow, with a small interruption in the early Wenlock probably due to local tectonic uplift in the Algonquin Arch. The depositional history may be divided into three stages: 1. Late Llandovery transgression during Fronian and Telychian (C1-C6); 2. Early Wenlock regression; and 3. Late Wenlock transgression which continued till Ludlow time. The Fossil Hill Formation on the north Cataract shelf, and the Reynales Formation on the south Cataract shelf are time transgressive toward the axis of the arch, and are, for the first time, demonstrated on the basis of faunal assemblages. The Rockway Member of the Irondequoit Formation at Niagara Gorge is time--equivalent with the Merritton Member of the Reynales Formation on the arch area. The Amabel Formation of the Michigan Basin is correlated with the Rochester, DeCew and Lockport formations of the Allegheny Basin. These formations are considered to be Late Wenlockian age, and may be partly Early Ludlovian. Three Upper Llandovery brachiopod zones are proposed (in ascending order), the Microcardinalia pyriformis Zone (C1-C2), Pentamerus oblongus Zone (C3-C5) and Plicostricklandia manitouensis Zone (C6). All three zones are present in the Fossil Hill Formation. Several Late Llandovery brachiopod communities are recognized in southern Ontario. The Microcardinalia Community of C1-C2 interval occurs in the Manitoulin Island area. The Pentamerus Community of the C3-C5 interval occurs in the Bruce Peninsula, and an Eocoelia Community characterized by the abundance of Hyattidina exists in the Niagara Gorge area. The Stricklandia Community of the C6 interval is found in both Manitoulin Island and Niagara Peninsula.