Module 5

The Paleozoic Platform


The south coast of the Georgian Bay is renowned for the iconic sheer cliffs of the Niagara Escarpment which form the spine of the Bruce Peninsula and Manitoulin Island. But the Niagara Escarpment is comprised of just one in a series of layers of sedimentary rock which blanket the older rocks of the Canadian Shield. They represent a time when the continent of Laurentia (proto-North America) lay at the equator, and was almost entirely covered by shallow epicontinental (meaning 'upon the continents') seaways. Epicontinental seas are rare globally nowadays, in part because sea level is somewhat lowered by the presence of large ice sheets over Antarctica and Greenland, which draw water out of the oceans globally. The Hudson's Bay in northern Canada, as well as the Caspian Sea and Black Sea on Eurasia are epicontinental seas, though with the important distinction of being comparably high-latitude (and therefore colder), and smaller than the seaways that covered Laurentia in the Paleozoic. The Gulf of Mexico is perhaps the best analogy since it is a warm-water seaway, full of coral reefs and other life.

The Paleozoic (meaning 'ancient life') is a period of geologic time when organisms developed an array of hard body parts and body plans diversified as many new species evolved. This meant that for the first time in Earth's history, they left behind an extensive fossil record, and also started to influence the shape of the seafloor by building up debris. As the environment shifted, becoming hotter or colder, wetter or dryer, life evolved to adapt to those changing conditions. The character of sedimentary layers, or strata, building up on the seafloor changed too. The examination of the character of these layers is called sedimentology, while the examination of the order in which they are deposited is called stratigraphy. By tracking these changes, both in time and space, we can deduce the way in which paleoenvironments shifted over time, giving us a useful baseline for understanding environmental change today.

The Paleozoic platform is what we call this series of successive layers of relatively flat-lying sedimentary rocks that built up upon the Precambrian core of Laurentia. Unlike the rocks in the Canadian Shield, these have not been directly deformed by orogenic processes, and are therefore structurally simple, and unmetamorphosed. It is a great place to learn about some of the basic principles of geology and how to 'read' the pages of the rock record. We read this book from bottom to top according to the Law of Superposition: younger sedimentary layers can only be deposited on top of older layers. Since life was prolific throughout the Paleozoic, and continually evolved, we find different fossils in successive layers (the Law of Fossil Succession). We can therefore use which fossils are found in a given layer to correlate strata across wide distances. When the landscape is exposed to erosion (for example by uplift, or by falling sea level), pages are ripped out of this book to create Unconformities, or gaps in the rock record.

These rocks are also important to the economy of Southern Ontario and to the history of Canada as a country. Their investigation began with the work of Sir William Logan under the auspices of the newly formed Geologic Survey of Canada in 1842. Canada, at that point consisted only of "Canada West" (Southern Ontario and the area north of the Great Lakes), and "Canada East" (Quebec on either side of the Saint Lawrence Valley and up into Labrador). Of paramount political importance was the attraction of settlers and industry into southern Canada West, which would require an inventory of resources and farmland. In particular, there were hopes of discovering coal, which could be used to fuel industry, and questions about why that didn't seem to be happening. Now, we know that the rocks of the Paleozoic Platform in southern Ontario are too old to contain coal - plants, which compact to make the burnable rock, simply hadn't evolved yet. It would take the confederation of Canada in 1867 and addition of coal-rich Nova Scotia to provide a reliable supply. However, throughout the 1850's a number of petroleum discoveries were made in the Paleozoic Platform, including at Craigleith on the Georgian Bay, and at Petrolia, where one can find North America's first oil well. Petroleum is derived from the decayed remains of organism, so its presence tells us about the importance of ancient biological activity. Thousands of wells continue to operate across southern Ontario. Other important resources within Paleozoic rocks are building stone, which is used to make the outer facade of buildings, aggregate, which is crushed from rocks like limestone and used in concrete, and salt, which originates is mined from evaporitic rocks.

Today, scientists are furiously working on three-dimensional models of the Paleozoic Platform rocks in Southern Ontario. Their primary goal is to characterize groundwater movement, which has important implications for the distribution of contaminants and safety of the water supply for many communities. Water flows through different rock types differently: some units are aquifers, which allow water to pass through readily, while some are aquitards which suppress water movement. Such hydrogeologic assessments are crucial in safeguarding the provinces groundwater resources.

The purpose of this lesson is to understand how the strata of the Paleozoic Platform arise, and what that tells us about Earth's history.


By the end of this lesson, you will be able to
  • Differentiate between clastic and carbonate rocks and their paleoenvironmental significance.
  • Outline major changes in the fossil record through the Paleozoic.
  • Identify factors that influence the deposition of different sediment types.

  • Key terms: sedimentary rock, strata, stratigraphy, paleoenvironments, transgression, regression, fossils, reefs, bioherms
Part One

The Precambrian-Paleozoic Transition

Part Two

Tracking Paleoenvironmental Change

Part Three

A Walk Through Time: Paleozoic Stratigraphy

Self Guided Learning: Virtual Field Trip
The Paleozoic Coast of Georgian Bay