Stephenville on the Port-au-Port Peninsula, Newfoundland

Day 5

Welcome to the Humber Zone
Stephenville is a town on the Port-au-Port Peninsula on the west coast of Newfoundland. In geologic terms, Stephenville is part of the Humber Zone. The Humber zone has Grenvillian age basement rocks (~1200 million years old rock) overlain by Cambrian and Ordovician sandstones, limestones, and shales. These sedimentary rocks were all metamorphosed and deformed during the Taconic and Acadian orogenies in the Late Ordovician (~440 million years ago) and Early Devonian (~410 million years ago) respectively.

The basement rocks in the Humber Zone formed during the Grenvillian orogeny about 900 to 1200 million years ago. During the Grenvillian orogeny, the precursor to the North American plate collided with another plate and the collision of these two plates formed a large mountain range similar to the modern Himalayan Mountains. The precursors to the Appalachian and Adirondack Mountains in the eastern United States were built during this orogeny. The collision between the two plates also strongly metamorphosed the existing rocks, forming the high-grade and strongly deformed metamorphic gneiss seen today.

Following this collision, continental rifting occurred. An ocean formed where the continents split apart. This ocean, known as the Iapetus Ocean, existed from the Cambrian through the Ordovician (438 to 570 million years ago). The Cambrian and Ordovician sandstones, limestones, and shales found in the Humber Zone today were deposited along the coast of the Iapetus Ocean.

The Iapetus Ocean began to close during the Ordovician. When it closed completely, the proto-North American continent again collided with another continent. This collision attached crust and rocks unrelated to the Grenvillian Orogeny on the central and eastern parts of what is modern Newfoundland. As a result, Newfoundland can be divided into three sections based on its geology. The western section is the Humber Zone and is associated with the Grenvillian orogeny. The middle section is made up of the Dunnage and Gander Zones and is associated with volcanic rocks from the Iapetus Ocean basin. The eastern most section is called the Avalon Zone and is known as an exotic terrane because the rocks from the Avalon Zone were attached onto Newfoundland only about 410 million years ago.

An outcrop of magnetite banded granitic gneiss
on the Port-au-Port Peninsula.

A close-up look at the magnetite bands in the gneiss.

Magnetite banding
Magnetite (Fe3O4) bands occur within some gneisses that outcrop around Stephenville. The Stephenville magnetite was used as a source of iron during World War II. About 16,000 tons of magnetite were mined for the war effort (Geological Highway Map).

The magnetite occurs as veins and lenses within mafic gneisses. Magnetite veins can also be found in diorites, igneous rocks with a silica composition between granitic and mafic rocks. Feldspar, pyroxene, and hornblende are the other major minerals in the mafic gneiss. The mineralogy of the diorite is mostly feldspar, quartz, and pyroxene (Geological Highway Map).


The Geological Highway Map of Newfoundland and Labrador