The Hudson Bay Basin: sedimentary deposits from a tropical sea

Modern-day Churchill and the surrounding area are characterized by sub-arctic conditions. In places around the coast and rivers of this area you can find outcroppings of carbonate rocks that were deposited some 450 million years ago. Back then, much of northern Manitoba was blanketed by a shallow, tropical sea. These rocks contain a diverse assemblage of fossils and record the first mass extinction event that occurred on this planet.

Carbonate rocks from the Portage Chute Formation overlie Proterozoic granites along the Churchill River,
over 100km south of the town of Churchill.

The Hudson Bay Basin

During the Late Ordovician (450 million years ago) to early Silurian (440 million years ago), carbonate rocks were deposited in the Churchill area of Manitoba, which is part of the Hudson Bay Basin. At this time, the continent of North America was positioned in the southern hemisphere and extended to the equator. The area around modern-day Churchill was positioned very close to the equator, and was covered by shallow, tropical seas.

The carbonate rocks around Churchill (and elsewhere in the Hudson Bay Basin) contain a diverse assemblage of fossils; corals, cephalopods, trilobites, bryozoans, trilobites, and brachiopods are some of the common types of fossils found in these rocks. An important evolutionary radiation occurred during the Ordovician Period. This radiation event, termed the Great Ordovician Biodiversification Event, represents a huge increase in marine biodiversity. This event essentially set up the structure of marine ecosystems for the next 250 million years.

At the end of the Ordovician Period environmental and climatic conditions changed dramatically. A sudden and intense cold climate led to the destruction of shallow marine environments which hosted the diverse communities that resulted from the Great Ordovician Biodiversification Event. This marks the first mass extinction event which occurred on our planet, and killed off 85% of marine species in a very brief period of time. At the beginning of the Silurian Period (~443 million years ago) marine communities began to recover as environmental conditions improved. The strata of the Hudson Bay Basin in northern Manitoba record the peak of diversity in the late Ordovician, the extinction event, and the beginning of the post-extinction recovery in the earliest Silurian.

The Churchill River

Along the northern section of the Churchill River there are spectacular outcrops of Late Ordovician carbonates. Travelling by helicopter, we spent a few days flying to these outcrops to study and examine these excellent stratigraphic sections and the diverse fossils within these rocks.

Surprise Creek is a tributary of the Churchill River. Beautiful views
like this are a perk of doing geological field work!

At the localities we visited, we were able to observe the diverse marine communities that existed in these areas 450 million years ago. They represent the peak of that Ordovician diversification event. The diversity and abundance of these fossils is very impressive!

From top left, clockwise: a strophomenid brachiopod, which is a clam-like animal; a mold of the outside of a large, coiled gastropod (snail) shell; a cross-section of a small, coiled cephalopod (squid) shell; an internal mold of a bivalve, a clam-like animal, that coincidentally looks heart-shaped. Scale bar is in centimeters.

The fossils from these rocks give us many clues about the environments these sediments were deposited in. For example, a high diversity of animals frequently indicates very favourable marine conditions for life. Specific groups of animals can give us even better clues regarding their original environment. Corals, both tabulate and rugose, are common in some of the formations of rock in northern Manitoba. Corals typically require shallow, calm, warm, and nutrient-rich waters to thrive.

Two tabulate corals found in Late Ordovician strata of the Hudson
Bay Basin. Left: a small fragment of Calapoecia. Right: a small
fragment of Paleofavosites.
 

By using a combination of the paleontology and lithology (rock type) of the Hudson Bay Basin outcrops in Manitoba, geologists have been able to determine the environments these rocks were deposited in. For the most part, these carbonates were deposited in shallow, warm, calm sea waters. In the latest Ordovician, when the global climate cooled, conditions became less favourable however. We are able to see a marked decrease in diversity and abundance in the fossil assemblages of these latest Ordovician rocks. This represents the first mass extinction event, referred to as the end Ordovician mass extinction. The lithology also reflects a period of very shallow water and restricted conditions (which means that there was probably less oxygen in the water and higher rates of evaporation).

Chasm Creek, another tributary of the Churchill River. Over time this creek has eroded through several
meters of Late Ordovician carbonates, providing geologists with an ideal stratigraphic section to
study and take samples from. 

Modern-day Churchill and northern Manitoba are characterized by sub-arctic conditions; winters are long and cold, and trees are sparse. The marine waters of the Hudson Bay are cold all year round. However, this is in stark contrast to the environmental and climatic conditions of that area 450 million years ago. If you ever find yourself in Churchill, just remind yourself that in the Early Paleozoic, the entire area would have been a tropical paradise!