Precambrian Beginnings: Roots of the Northwest Central

The oldest known materials in the world are 4.4-billion-year-old zircons from rocks in Western Australia.

The Earth is estimated to be approximately 4.6 billion years old—an age obtained by dating meteorites. Rocks dating to around four billion years old are found on almost every continent, but the oldest rocks known on Earth are 4.3 billion-year-old rocks found along the eastern shore of Hudson Bay in northern Quebec. These are part of the Canadian Shield, the ancient core of the North American continental landmass, which has experienced very little tectonic activity (faulting and folding) for millions of years. Shields, or cratons, are the stable cores of all continents and are often covered by layers of younger sediments. They formed and grew during pulses of magmatic activity, as bodies of molten rock deep in the Earth’s crust contributed to form new crust. In the Northwest Central US, the main cratonic elements are referred to as the Wyoming Province (Wyoming and eastern Montana), the Medicine Hat Block (northwestern Montana), and the Superior Province (Dakotas, Minnesota, Wisconsin, and Michigan) (Figure 1.4). Outcrops of these rocks are exposed mainly as uplifted blocks in mountain ranges throughout Wyoming, Montana, and South Dakota. The oldest rocks identified so far in the Northwest Central US are 3.6 - to 3.8-billion-year-old granitic gneisses found in Wyoming’s Wind River Mountains.

Figure 1.4: Cratonic elements and belts of deformation.

Figure 1.4: Cratonic elements and belts of deformation.

The shape and position of North America has changed dramatically over the last billion years, and geologic processes continue these changes today. Compression from colliding plates, tension from plates pulling apart, the addition of land to North America, weathering, uplift, and erosion have combined to slowly sculpt the form of the continent. As such, it is very difficult to reconstruct the size, shape, and position of continents during the Precambrian. Fewer rocks are preserved from this time, and those that remain have been highly altered. Nevertheless, available evidence suggests that the proto-North American continent, also called Laurentia, had its Precambrian beginnings in a supercontinent that existed around 2.6 billion years ago. From this proto-North America, sediment was eroded and transported by rivers and streams across the ancient continental margins and then into the adjacent oceans. The sediment deposited in the ocean waters on the western margin of Laurentia can be found today in southern Wyoming’s 2.2 - to 2.4-billion-year-old Snowy Range Supergroup, where thick sequences of sandstone, conglomerate, and limestone were deposited near what is now the southern margin of Wyoming.

These sediments contain 2.3-billion-year-old stromatolites (mounds of sediment formed by mats of photosynthetic cyanobacteria), indicating that they were deposited in a continental shelf environment. During this time period, at least two episodes of glaciation occurred, represented by rocks formed from glacially derived sediments (tillites) found in Idaho and Montana.

See Chapter 2: Rocks to learn more about stromatolites.

Around two billion years ago, a second supercontinent, often called columbia or Nuna, began to assemble from major cratons and other fragments of land. In the Northwest Central, the zones of collision between the cratons and fragments are preserved as deformed metamorphic rocks in the Little Belt Arc of northern Idaho and Montana, the Selway Terrane of southern Idaho, and the Trans-Hudson Orogen of the Dakotas and Canada (see Figure 1.4). The breakup of this supercontinent began around 1.5 billion years ago.

The remainder of the Precambrian period saw the formation of a third supercontinent, which geologists call Rodinia, about 1.1 billion years ago (Figure 1.5), and its eventual breakup about 750 million years ago. Preserved remnants of the continental collisions that formed this supercontinent are found widely across modern North America, but very few of these elements are recognizable in the Northwest Central US.

Figure 1.5: The supercontinent Rodinia, circa 1.1 billion years ago. Laurentia represents proto-North America.

Figure 1.5: The supercontinent Rodinia, circa 1.1 billion years ago. Laurentia represents proto-North America.

The breakup of Rodinia was associated with the formation of rifts throughout North America, with igneous activity occurring in rifted zones and continuing slowly and irregularly until about 600 million years ago. North America’s rifted edges formed passive margins, where sediments were deposited on continental shelves into the early Paleozoic era.

A rift occurs when tectonic plates move away from each other. Magma rises up into the margin, cooling to produce new oceanic crust. The resulting action is similar to two conveyor belts moving away from each other. A failed rift occurs when the existing crust is stretched thin and magma begins to well up, but the plate is never completely broken.