For those of you who are interested in finding out about Earth Sciences and what geologists do, we have put together these pages to present some background on the topic, provide a very general summary of the geological history of Western Canada, and show some pictures which illustrate some aspects of what we do.

Earth Science has existed as a scientific endeavour only for about 300 years. For the first century or so it was practised as a sideline by men in other fields (e.g. Engineering, Medicine, Biology). Only in the 20th Century did scientists come to understand the fundamentals of the earth's internal structure, and the tectonic processes that move and shape the great continental land masses. Like any scientific pursuit, Earth Science relies on observational data, either by direct study of the rocks or indirectly by using chemistry, magnetism, seismic waves, or other physical attributes. From these data, models are devised that reconstruct the processes and conditions that have shaped the earth, and permit us to make predictions about the conditions where data are not available. The process is like trying to reconstruct a 1000-piece jigsaw puzzle with only a few dozen pieces. The general features of the picture become apparent fairly quickly, but many of the details are not resolved. These details are the subject of much lively debate.

Western Continental Margin

We begin our history of the great mountain system of western North America known as the Cordillera (Spanish: a chain of mountains) with the breakup of the supercontinent of Rodinia, sometime near 750 million years ago (Late Proterozoic; see (geological time scale at right). At that time, most researchers believe that East Antarctica and Australia rifted away from western North America. As a result, a great belt of sedimentary sequences collectively known as the Windermere Supergroup began to accumulate at the continent-margin. This point is actually about half way along in the history of western North America, but it marks the first stage when we know much of the basic outline, even though many details of this early history are obscured by later cataclysmic events. For much of the next 400 million years, the western margin of the continent was the site of a 'passive' (that is, Atlantic-type) continental margin. Sediments were washed off the continent and collected at its edge. By the early Paleozoic, the continental margin looked quite different in the north than in the south. The southern part consisted of a narrow carbonate shelf, passing in a short distance into deeper water where fine mud collected. The northern part consisted of a broad, shallow carbonate platform, much like present day Bahamas Bank. Thanks to plate tectonics, this part of the continent was in the tropics then, so the processes involved in forming these carbonate platforms were much the same then in northern Canada, as they are now in the Bahamas.

In the Late Devonian to Early Carboniferous periods (380-350 My ago), a major mountain-building event occurred, and is preserved fragmentarily in several widely separated parts of western North America, including Nevada, and southern British Columbia. Other parts of the margin, such as eastern Yukon, seem to have undergone extension and rift faulting, but we do not currently know enough to be certain whether this extension followed the mountain building (as commonly happens), or whether these very different processes were going on at the same time in different places. This is an area of active research. We do know that rift faulting was active locally in the late Paleozoic (Carboniferous and Permian, ca. 350-250 My ago) in northern British Columbia, but new work also indicates that volcanism near the continental margin in southwestern Yukon was forming a chain of island arc volcanoes (Japan or Indonesia are modern equivalents) that would later be pushed into that part of the margin.

Jurassic and Early Cretaceous Mountain Building

During the late Paleozoic and Triassic (ca. 250 My ago), a series of island arcs formed in the ocean west of North America, but the fossil species preserved in the sedimentary rocks, and the magnetic record preserved in the volcanic rocks indicate that they formed up to 3000 km south of their present positions. They slowly migrated northward, attached to the global oceanic conveyor belt system we call plate tectonics. The present Cordilleran mountain system had already begun to develop by about 175 million years ago, during the Middle Jurassic. The initial stage of mountain building began when the first group of these island arc chains, and fragments of intervening oceanic crust, finally collided with western North America. The first collision involved a composite crustal block known as the 'Intermontane superterrane' (see location map) because it is centred on the Intermontane plateaus of today's British Columbia. Their collision produced the Columbian Orogeny, the first of two full scale mountain building episodes. This resulted in the formation of a fold and thrust belt, major crustal thickening and the intrusion of granite plutons throughout the Omineca Belt, the central core of the Cordillera. By Late Jurassic time, sediments were shed eastward from these rising mountains and formed the first layers of 'clastic wedge' rocks deposited as a broad apron onto the older continental margin sediments in what is now eastern British Columbia and Alberta. These rocks would later become caught up in the faulting and folding as the deformation progressed eastward into the Foreland Belt of the Cordillera.

Cretaceous and Younger Events

A second collision occurred sometime before the mid-Cretaceous, when the 'Insular superterrane' slammed into the back end of the 'Intermontane superterrane' at a rate likely in excess of 5 cm per year. This collision resulted in a second major pulse of regional deformation, crustal thickening and uplift, and widespread intrusion of mid-Cretaceous granites, especially in the Coast and Omineca belts. The final major deformation pulse in the Cordillera lasted from the middle part of the Late Cretaceous until the earliest part of the Tertiary. This event, often called the Laramide Orogeny after an event of similar age in the U.S. Rocky Mountains, was responsible for important thrusting and folding in the Omineca and Foreland belts, resulting in as much as 200 km of crustal shortening in the southern part of the Cordillera. These uplifted mountain ranges, the Canadian Rocky Mountains, shed large clastic wedges eastward onto the western plains.

As these shortening events came to a halt in the earliest part of the Tertiary, northward motions of parts of the Cordillera continued, and became more prominent. Major faults accommodated additional northward terrane motions. For example, the Tintina-Northern Rocky Mountain Trench system accumulated at least 450 km of west-side north displacement. The San Andreas Fault of southern California is a modern analogue for these large faults. In the Early Tertiary, the first of several pulses of extension caused the partial collapse of thickened crust beneath the Omineca Belt. Such gravitationally induced collapse is increasingly recognized as part of the evolution of mountain systems globally. Only in the northernmost part of the Cordillera has significant mountain building continued past the Early Tertiary. There, Late Tertiary folding and thrust faulting is widespread, with evidence locally that such deformation continues today. Most of the modern northward motion of the Pacific Plate is accommodated on the Queen Charlotte Fault, but a small amount is transferred to the northern Cordillera, so that the St. Elias Mountains are presently rising, and much of the rest of the Yukon continues to deform slowly.

• in Earth Sciences:
  • Wonderful Life, Stephen Jay Gould
  • The Map that Changed the World
• on the Canadian Cordillera:
  • Where Terranes Collide, by C.J. Yorath, 1990; Orca Book Publishers, Victoria, B.C., Canada.
  • Of Rocks Mountains and Jasper, By C. J. Yorath and B. Gadd, 1995. Dundurn Press, Ltd., Toronto, Ontario.

Coral

Fossils play a very important role in geology. They provide a record of ancient life, help us to assign ages to rock successions, and yield important information about ancient environments. These fossil corals were found in the mountains west of Fort Liard, N.W.T. Because corals live only in salt water, they tell us that the rocks in which we find them were originally formed in an ancient sea.

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Cross-bedded sandstone

Most of the rocks we study in this project are sedimentary rocks. That is, they consist of material that was once loose sediment (such as mud, sand, gravel, or shell fragments). After the sediment was buried, the grains were cemented together by mineral deposits from groundwater and turned to rock. Nearly all the world's coal, gas, and oil are contained in sedimentary rocks. This photograph shows an outcrop of sandstone (sand that has become rock). Notice that between the flat-lying bedding surfaces in the rock there are also surfaces that dip to the right. These extend across the centre of the photograph and are called cross-bedding. You can find the same kind of dipping layers inside the dunes and ripples that form underwater in modern rivers. The cross-beds in this photo tell us that this sand was deposited by a current that flowed from left to right.

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Mt. Withrow syncline

As the Rocky Mountains formed, the strata (rock layers) of the Earth's crust were folded. A U-shaped fold is called a syncline. In this picture, taken near Mount Withrow in northeastern B.C., the syncline is outlined by beds of sandstone that are resistant to weathering and form ridges. The low area in the centre of the fold is underlain by shale, a much less resistant rock that weathers very readily. This picture also shows how geological structures can extend along the Earth's surface: you can see the syncline stretching off into the distance.

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Box anticline

This photograph shows beds of rock that have been folded into an A-shaped structure called an anticline. You may notice that the hinge of this fold has a flat top and two steep limbs, creating a box-like shape. Folds with this shape are called box folds. Both synclines and anticlines can be box folds.