undifferentiated rock units

Paleontology and geology

Throughout the Ordovician, most of Quebec was above sea level and exposed to erosion. Southeastern Quebec, however, was covered by a very shallow lagoon filled with stromatolites, trilobites, brachiopods, and other invertebrates that left their traces in the sand. Farther from the coast, where the waters were deeper, graptolites thrived. By the Middle Ordovician, subduction began off the coast of Laurentia, creating volcanic islands offshore. The shallow shelf over southeastern Quebec became a narrow, deep depression in between the islands and the continent. These waters were home to trilobites, brachiopods, bryozoans, corals, cephalopods, graptolites, and crinoids. Their fossils are preserved in limestones, shales, and sandstones in the region and on Anticosti Island. In the Late Ordovician, shallow seas spread over the middle of Laurentia, including part of western Quebec. Limestone, sandstone, and shale deposited in these seas can be seen in a few small areas in the west.