Revision and correlation of late Middle Ordovician stratigraphy northeast of Quebec City

Abstract

Autochthonous late Middle Ordovician rock units between Neuville (near Quebec City) and La Malbaie (160 km to the northeast) have been revised lithologically and correlated by means of graptolites. The following rock units have either had a slight modification of their lithologic definition or have been found to extend into a region where they were not previously recognized: Cap-à-l'Aigle Formation, Saint-Irénée Formation, Utica Shale, and Lotbinière Formation. In addition, two new rock units have been named: Beaupré Formation and Moulin River facies (an informal term) of the Trenton Group.The depositional environments of each of the late Middle Ordovician rock units have been briefly identified (although the bulk of evidence is reserved for a later report). Thus, when the various facies are correlated, some important facts emerge about the southern margin of the Laurentian Platform and the rise of the Taconic Orogen to the southeast of Logan's Line. The Deschambault limestone and underlying Cap-à-l'Aigle sandstone were deposited during a marine transgression, after which a structural feature (Montmorency Promontory) developed in the Quebec City to Beaupré area. The promontory was asymmetrical, with a gentle gradient to the southwest, and a steep gradient and a basin to the northeast. Sedimentary thicknesses increased three- to fourfold in the basin northeast of the promontory, and the facies within the basin are markedly different from those over the promontory and southwest. Within the basin there developed deep-water limestone (Moulin River facies), and submarine-fan facies (Saint-Irénée Formation and 'new facies') with interfan microflysch (Lotbinière Formation). Trenton limestone facies on the promontory were deposited in a carbonate bank environment. No Utica Shale is found northeast of the Montmorency Promontory. In contrast, the 'typical' succession of Trenton–Utica–Lotbinière–Lorraine was developed in the region of the promontory and south-west in the St. Lawrence Lowland. An exception is the Beaupré submarine fan that developed over the promontory. All three submarine fans were shed by the rising Taconic Orogen which can be shown by graptolites and the presence of Mg-rich chromite to have been deeply eroded as early as ruedemanni zone time. No Lorraine Group facies are known from the area of our investigation.