Mountain Building 1: The Proterozoic record

Following the formation of the Superior Province around 2.7 billion years ago during the late Archean, the Proterozoic of the Midwest was characterized by the deposition and presence of banded iron formations. With the evolution of photosynthetic organisms (still single-celled at this time), their waste product, oxygen, was released into the oceans and ultimately the atmosphere. The oxygen reacted and combined with dissolved iron particles to produce bands of iron oxides such as hematite and magnetite.

Figure 1.3: Precambrian outcrops within the Midwest.

Figure 1.3: Precambrian outcrops within the Midwest.

Figure 1.4: The accretion of the Pembine-Wausau island arc and the Marshfield terrane led to the Penokean Orogeny of the Proterozoic.

Figure 1.4: The accretion of the Pembine-Wausau island arc and the Marshfield terrane led to the Penokean Orogeny of the Proterozoic.

Between 1.8 and 1.9 billion years ago, an orogenic event produced the Penokean Mountains, which extended from Minnesota into northern Wisconsin and Michigan. The Penokean Orogeny occurred as the Pembine-Wausau island arc and Marshfield terrane collided from the south, increasing the size of the Superior landmass (Figure 1.4). Erosion of the Penokean Mountains led to deposits of sandstone along the margin of a shallow sea. Around 1.7 billion years ago, after millions of years of heat and pressure, this sandstone produced the metamorphic Sioux Quartzite of Minnesota and Iowa. The Baraboo Quartzite of the Devil’s Lake area in central Wisconsin also dates to this time.

Figure 1.5: Active for 20 million years, a great rift valley extended through the Midwest from today’s Lake Superior to Kansas.

Figure 1.5: Active for 20 million years, a great rift valley extended through the Midwest from today’s Lake Superior to Kansas.

Around 1.1 billion years ago, a rift valley called the Midcontinental Rift—extending from the eastern edge of modern-day Lake Superior to Kansas—began to split North America apart (Figure 1.5). Intense volcanism occurred along the rift, producing igneous deposits with a thickness of some 7.6 kilometers (25,000 feet). The spreading occurred for 20 million years, after which the rift zone began to sink and was gradually filled with sediment.