Yukon's Triassic Reefs

Across southern Yukon, there are numerous exposures of limestone that contain fossils of organisms that lived during the  Triassic Period  from ca. 251 to 201 million years ago. Some of these exposures are well known and accessible, such as those at Grey Mountain just east of Whitehorse, while others are more remote. These limestones formed in warm shallow marine environments just off the coast of ancestral North America nearly 200 million years ago. In this virtual experience, you will learn about these ancient reef settings and the diverse fauna that populated them. Don't miss our photos of rock outcrops from multiple sites and the 3D models of amazing Triassic fossils at the end of this tour!

The term reef has been used quite broadly and has different definitions in different industries. For example, the term is used by fishermen and boat captains alike to describe a shallow obstruction which may be dangerous to the boat. However, the definition is slightly more descriptive for scientists. A buildup is a term applied to a body of carbonate rock that has built up topographic relief above its surrounding environment. A reef is a type of buildup that is self-generated, built entirely by organisms through the process of secreting calcium carbonate (calcareous) shells and skeletons. Today, these marine ecosystems are bursting with biodiversity, support the culture and economy of nearby communities, and famously attract tourists from all over the world who seek to experience their beauty. Coral reefs such as the Great Barrier Reef, the Rainbow Reef, or the Red Sea Reef are great examples of modern-day reefs that help paleontologists understand fossilized ancient reefs.

An ancient reef is a biologically constructed mound or accumulation that grew on the seafloor and has been lithified into a carbonate rock that contains fossils. Depending on the size and shape, they may also be referred to as ancient bioherms or biostromes.

The first reefs were primarily composed of microbial mats and the earliest calcifying organisms. These grew on shallow-water shelves around ancient continents through the late Archean and Proterozoic, around 2.5 billion years ago! Since then, there have been five major extinction events that have impacted the ocean’s biodiversity and affected the evolution of ocean life. Over millions of years, reef life evolved into complex corals, sponges, and other organisms. Since we did not witness the reefs while they were living, ancient reef deposits such as those in southern Yukon act as time capsules that give geoscientists clues as to what ocean life and marine environments looked like through geologic time.

Reef growth is controlled by many factors, including water temperature, sediment supply, nutrient availability, and water depth (which controls sunlight). Plate tectonic processes have continuously rearranged and rebuilt Earth's continents through time, so ancient reef deposits are found on continents all over the world – from the high Arctic and Antarctic to alpine mountains, and to the shores of the Great Lakes!

Reefs can grow in many different shapes, depending on the environment they form in. For example, if the reef is subject to constant wave energy, the resulting reef forms will have a low rounded or flat profile. If the reef is sheltered from wave action, the organisms will grow in more delicate branching patterns. If reef communities develop in deeper water where sunlight is less intense, they may grow as thin plates to maximize light collection.

Carbonates are sedimentary rocks that are composed of carbonate minerals. Minerals in this family characteristically have a carbonate ion (CO ₃ ) in their formula, such as calcite (CaCO ₃ ) or dolomite (CaMg(CO ₃ ) ₂ ). Carbonate rocks can form through chemical or biochemical precipitation:

Chemical precipitation: the precipitation of minerals from ion-rich water. Biochemical precipitation: the extraction of ions from sea water by organisms to build shells and organic structures.

A carbonate rock may also be classified as a bioclastic carbonate. A bioclastic rock is composed partially or completely of bioclasts that have been cemented together to form a rock. Bioclasts are particles that have a biological origin, such as shells or coral fragments. An example of a bioclastic carbonate rock is a fossiliferous limestone.

Carbonate rocks are very common, comprising ~20% of the sedimentary rocks found on the surface of the Earth. But how do you know if the sedimentary rock you are seeing is a carbonate?

Carbonates usually appear dull on the surface and can vary in colour, typically either white, grey, or brown. They may be massive, meaning they lack layering, or bedded, meaning they have prominent layering features. Carbonate rocks commonly contain fossils, such as body fossils or trace fossils.

A body fossil is the preservation of a biological skeletal structure from an organism. This can include a physical part of the organism, such as a shell, tooth, branch, or bone. It may be a cast or impression of the physical part of the organism that has been naturally filled in and preserved with sediment. Paleontologists use body fossils to reconstruct the size and shape of an organism, study ancient biodiversity, or even determine the relative age of a rock. Biostratigraphy is the study of identifying and organizing rock layers based on the fossils found within the rock unit.

A trace fossil is a structure left in or on a substrate (e.g. the sea floor) by a living organism that is preserved as a fossil. These fossils record the organism's behaviour, and can include footprints, tracks and trails, burrows, borings, and even excretions. Trace fossils are made in the substrate in which they are found, meaning they were not transported from a different location. This is an important feature because it allows ichnologists (trace fossil scientists) to gather data about the environment in which the organisms lived, how they moved, or even how they fed!

If you were a geologist trying to identify a carbonate rock, you would have a handy tool in your backpack: a bottle of dilute hydrochloric acid! The carbonate ions present in the carbonate minerals are very reactive, and when you drop the acid onto the rock surface, the surface effervesces (fizzes) as the following reaction occurs:

CaCO ₃  + 2 HCl → CaCl ₂  + CO ₂  + H ₂ O

calcite + dilute hydrochloric acid → calcium chloride + carbon dioxide + water

Carbonate minerals have varying reactivity to acid. For example, calcite reacts vigorously upon immediate contact, while dolomite requires you to grind it into a powder before the reaction will occur.

Do you want to see how geologists perform an acid test? Watch the video below!

As you just learned above, carbonate rocks readily dissolve in the presence of acid. On a small scale, the rock effervesces and no visible damage is done to the rock surface. Now, imagine if a limestone unit was exposed to large volumes of acidic water for thousands, or even millions of years...

Karst topography is a distinct type of landscape that develops from the dissolution of carbonate rock in the presence of acidic rainwater. When it rains, CO ₂  ions from the atmosphere and soil dissolve into the rainwater and react to produce a weak carbonic acid (H ₂ CO ₃ ) that acidifies the water. When weakly acidic water comes in contact with the carbonate bedrock and infiltrates into fractures in the rock, the rock is slowly dissolved - just as you saw in the video above! As more bedrock dissolves, the fractures become larger, which allows infiltration of greater volumes of weakly acidic water and encourages more dissolution. Eventually, the bedrock that was once solid and strong becomes a weak network of fractures and caves, and the overlying soil begins to slump. Karst weathering most commonly occurs in limestone, and typical karst features include pitted rock surfaces, vertical fractures, sinkholes, sinking streams and springs, and subsurface drainage systems. As these systems get larger through time, this process can also form caves. These caves can be any size, from meters to kilometers wide!

Triassic-age reefs have been found all over the world, including Europe, Asia, Australia, South America, and North America. They provide a physical record of reef community recovery after a major mass extinction event at the end of the Permian (~252 Ma) known as "The Great Dying." It took approximately 25 million years after the extinction before complex reef communities returned in Late Triassic time. After this reef gap, a period of time in the fossil record where there is no recorded presence of reef growth, ocean life was thriving once again! However, a subsequent mass extinction event at the end of the Triassic Period (~201 Ma) deeply affected and damaged these communities once again. Less than 1% of coral species and less than 10% of sponge genera survived, severely reducing ocean biodiversity, and leading to another restart for reef communities.

Finding Triassic reef deposits is important for geoscientists because they record a critical interval in the fossil record. The ~ 50 million years of the Triassic is a transitional time where some early organisms still exist (such as microbial mats or benthic forams), and the early ancestors of modern organisms have begun to evolve (such as scleractinian corals and planktonic forams). Triassic reefs connect ancient Paleozoic reef systems with modern-day reef communities through multiple mass extinction events.

It is difficult to determine all the organisms that lived in the reefs because some are more likely to be preserved than others, a phenomenon called preservation bias. For example, soft-bodied organisms such as a jellyfish or Hallucigeniidae worms do not have bones or calcium-carbonate exoskeletons. These types of organisms completely decay when they die since they do not have any hard body parts to become fossilized. Other organisms, such as molluscs or gastropods, have bones or calcium-carbonate exoskeletons that become buried by sediment and fossilized after the organism dies.

There are two main types of organisms that affected reef formation during the Triassic: reef builders and reef remodellers.

• Reef builder: an organism which contributed material to the formation of the physical reef. In the Triassic common reef builders were corals, coralline sponges, microbes, coralline algae, and serpulid tube worms.
• Reef remodeller: also known as a microborer and macroborer, is an organism that erodes and reworks the physical reef. These organisms, such as certain bivalves and gastropods, infested reefs and bored holes in the corals, bryozoans, sponges, and shells of reef builders. In some Triassic reefs, about 30% of preserved corals exhibit evidence of macroborer effects!

Explore the gallery below to find photographs of various fossils and an artistic interpretation of the marine environment the organisms may have lived in. (Artwork by Esther Bordet of Yukon Graphic Recording, 2022)

Scleractinian corals, also known as stony hexacorals, first appear in the fossil record in the mid-Triassic (~ 240 Ma) and are still the dominant reef coral in today’s oceans. These corals occupy the same ecological niche as their Paleozoic stony coral relatives, rugose and tabulate corals, which went extinct at the end of the Permian. Scleractinian corals are very diverse, growing in many shapes, sizes, and environments. These corals have survived multiple environmental catastrophes, including major extinction events at the end of the Triassic and Cretaceous periods. These hearty corals are now the dominant reef builders in Earth’s modern oceans from tropical coastlines in the Pacific to the Caribbean. Studying the evolution of scleractinian corals provides insight into how these reef builders adapt to changing environmental conditions and may prove instrumental in helping preserve these communities in the future!

Similar to modern reefs, ancient Triassic reefs are extremely complex, and only a small percentage of their biodiversity has been studied and recorded in detail. Paleontologists who study Triassic reef deposits research topics including organism biodiversity, reef structure and morphology, and even the effects of environmental change through time – including the effects of ocean acidification and climate warming or cooling on ancient reef development and survival. Unlike modern biologic examinations, the study of ancient reefs allows scientists to see how these environments change over long periods of time and enables them to create informed predictions of future change in our modern reef communities.

During the Triassic Period, cold, dense oceanic crust from the paleo-Pacific Ocean (Panthalassa) was subducting under the west coast of ancestral North America. This subduction zone created a series of island arc volcanoes that appeared offshore of North America. Warm shallow seas formed at the margins of these new volcanic islands, creating an optimal environment for reefs to develop. This geologic setting would be analogous to the island arcs found in modern-day Japan or New Zealand. Over millions of years, the Panthalassa Ocean floor continued subducting under North America, consuming the oceanic crust and causing the ocean basin to close.

Paleogeographers are scientists who study the evolution of the Earth's tectonic processes through geologic time by analyzing the changing patterns of continents, oceans, climate, and biology. The area of the Panthalassa Ocean in which Yukon's Triassic reef organisms thrived, called the Cashe Creek Ocean, closed through a complex system of subduction zones and large-scale fault systems, and there are many interpretations of the paleo-reconstruction of this area. Scientists who research the same topic commonly create disagreeing hypotheses based on differing factors, such as the amount of data gathered, equipment used, or techniques employed. As scientists discuss their findings, review the data, and challenge each other's ideas, the hypotheses and interpretations become more refined.

• Evolution of Canada's western mountains; Geological Survey of Canada, Open File 5804.  https://doi.org/10.4095/225581 .
• Phanerozoic Tectonic Evolution of the Circum-North Pacific; USGS, Professional Paper 1626.  https://pubs.usgs.gov/pp/2000/1626/ .
• The Canadian Cordillera: Geology and Tectonic Evolution; Geological Survey of Canada, Contribution Series 2001181.  https://74.3.176.63/publications/recorder/2002/02feb/feb02-canadian-cordillera.pdf .

A geologic terrane is an extensive package of fault-bound rocks that have distinctive geologic characteristics and geologic history compared to the neighbouring rocks in the region. By defining a terrane using distinctive geologic features, geologists are able to better distinguish the origin of one group of rocks from another. A geologist can then reconstruct the movement Earth's crust during tectonic events such as mountain building (orogenesis), the opening of a new ocean basin (continental rifting), or the closing of an ocean basin.

The carbonate reef outcrops at Lime Peak are exceptionally preserved and well-exposed. This reef deposit has attracted both local and international scientific interest by those who are keen to study this complete reef cross section, showing multiple stages of reef-growth and the surrounding inter-reef areas. Take a closer look at this world-class reef exposure by watching the video below!

In our digital Triassic reef fossil collection, we have both 2D photos taken in the field and 3D models of collected samples. The annotated 2D field photos are located in the gallery below. Keep scrolling down to explore our 3D models!

Click on each 3D image to rotate and manipulate. For best viewing results, click on the square in the upper right corner to make each image full-screen. Where present, click on numbered annotations to read labels.