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The Geological History of Saskatchewan



Geological history is in some ways very like unto biblical history: only part of the questions can be answered. The question of whether the Exodus really took place or whether Joshua really blew down the walls of Jericho can be replaced by geological puzzles such as what was the original rock composition or source(s) or under what conditions the rocks we see today were formed. The clues are not scrolls or stellae, but the rocks themselves, their relationships, index minerals and environments.

Geological time needs to be addressed. Ages are assigned to rocks based upon analysis (- ie- radioactive dating) of rock materials that were created while the rocks cooled and solidified. In effect the last time the rocks were metamorphosed/cooled is recorded, not the first. Thus a Precambrian rock originally cooled 2.8 billion years ago, having been metamorphosed about 1 billion years ago will give the date of only 1 billion years. In other words the ages obtained are related only to the last incarnation/episode of rock formation. The relationships of the rocks are also important as (for example) in sedimentary rocks the "way up", once determined can show you the order of sedimentation. Invasive rocks such as granites may cross cut sediments and in turn are cross-cut by later dykes. The order of deposition and intrusion are thus determined by inherent relationships.

The Precambrian rocks that comprise most of the bedrock in northern Saskatchewan profoundly impacted upon later events. They provided the source (erosion products) for later, adjacent sedimentary formations while their surfaces, configurations and tectonic scars provided the matrix for future major geological structures such as basins and mountain ranges. The continental nuclei represented by the Saskatchewan Precambrian profiles are reflected both geophysically and topographically throughout northern Saskatchewan. The younger volcanic and sedimentary rocks derived from these Precambrian rocks flank the main shield areas. The granitic and volcanic rocks were a prime sources for the infilling of the Williston Basin.

Generally, the story of Saskatchewan geology is the story of the ongoing earth processes that created it. We surmise that a crust forms over the molten surging mass of the early earth. Igneous rock material coalesced into continental plates which formed through the process of cooling. Accretion of this rock material built up and foundered over millennia, leaving a protocontinent adrift on the earth's surface. Over time It succumbed to irresistible earth processes, separated into scattering smaller segments or subcontinents.

About 2.1 billion years ago, in the area that was to become Saskatchewan, two huge sections of Archaean continental plate first separated. An ocean gap, fed by tremendous rivers was infilled from the Churchill continent to the northwest thus Saskatchewan is envisaged to have been a shallow continental shelf. Volcanic islands formed along the continental margins, themselves subducted and eventually eroded. By 1800 million years ago, the Churchill and Superior continents crashed together. Plate subduction formed a series of volcanic islands followed over time by a range of mountains defining the margins of a geosyncline. Much of this continent was gradually covered by shallow seas. By 1700 million years ago the mountains were eroding away into these tropical seas. The Athabasca Basin, primary source of uranium in Saskatchewan, filled with sediments derived from these unnameable Hudsonian mountains comparable to the modern Himalayas. The record of the rocks reveals that shallow seas covered our area for much of the time from at least 1700 million years to 65 million years ago. At first, these seas were equatorial, then much later, temperate and muddy-bottomed (when dinosaurs ruled and our continent had drifted approximately into in its present geographic position). Although no human has observed these formative earth processes we are able to derive this information from a combination of paleontological and paleomagnetic data.

The seas retreated in the late Cretaceous stranding the land which eventually hosted dense tropical forests. A huge variety of species (particularly the dinosaurs) died off about 65 million years ago during one of the great extinctions at the end of the Cretaceous Period. This extinction was caused by (the argument goes) catastrophic atmosphere/temperature changes generated by possible asteroid impacts and/or volcanism. Volcanic activity and burning forests filled the atmosphere with a veritable blanket of smoke and dust. This ancient analog to "nuclear winter"

From 50 million years ago to the beginnings of the Ice Age (about 2 million years ago) our area underwent a major change in climate, and the tall trees of the ancient forests fell to hardy grasses, precursors to today's familiar grasses. (Other sections of this project deal with the paleo-botany of the province).

At least, five continent-wide glaciers successively crushed their way across Saskatchewan, and, in turn defeated by temperature increases, retreated northwards. The glaciers that dominated the most recent 2 million years in Saskatchewan were so deep and extensive that the entire province was covered (with the known exception of the topographically highest Wood Mountain and Cypress Hills plateaus). For the sake of scale it should be noted that humans came to North America while glaciers from the most recent ice age still covered much of what is now Saskatchewan.

Beneath the glacial and sedimentary rock that cover the southern half of the province are older Precambrian rocks that contain evidence of the first major events referred to above. The rock record records the plate collision, extensive island arc volcanic activity and mountain building of 2 billion to 1700 million years ago. Finally, thousands of years after the last glaciation, the prairie originated.

Our present landscape and environment results from a series of geological events, temperature changes and atmospheric influences intertwined into an arabesque of unimaginable complexity. A process that rolls on despite humanity.

The earth is postulated to be approximately (4.5 to 4.8) billion years old. The Precambrian rocks that are found in northern Saskatchewan represent only the first three billion years of the earth's history, or nearly two-thirds of the time that has passed since accretion of the earth and moon.

Essentially, the Precambrian rocks of northern Saskatchewan are a structurally complex and metamorphosed assortment of igneous rocks that were both deposited above and intruded beneath the surface. Sedimentary rocks formed by their erosion and metamorphic rocks formed by folding and melting of these igneous and derived sedimentary deposits are also a large component. While, certainly, there were several cycles of tectonism (faulting and mountain building) and later deformation only the latest episodes emerge for our viewing. What is important to keep in mind is the process. The original protocontinent saw plate separation, opening of an ocean basin, creation of two continents, their inevitable collision, building of volcanic island arcs, new folding, shearing and intrusion and finally full-scale mountain building. Still within the Precambrian, these unnamed, sharply peaked mountains suffered corrosive weathering under intense weather systems, shallow seas encroached over eons, covering the now rounded mountain tops: the rocks of the younger Precambrian and Cambrian to Cretaceous times represent these eons.

The younger Precambrian in Saskatchewan is recorded only the Athabasca group, a series of formations (primarily sandstones, conglomerates and minor silt), unconformably overlying the older granitic basement in the Athabasca Basin in which all of the major sources of Saskatchewan's uranium occur. These sedimentary formations have remained unmetamorphosed since they were deposited a billion and a half years ago. The emergence of land in the late Cretaceous, climatic change and emergence of grassland savannah in the Tertiary and the advance and retreat of 5 possibly 6 glaciers in the Quaternary provide us with the landscapes over which we walk.

Our remaining Saskatchewan rocks, present in those areas south of the Precambrian outcrops (See attached outcrop maps) represent the Phanerozoic Eon, which is defined as that time interval during which complex (metazoan) life forms prevailed. All of the Phanerozoic periods except the Permian are represented in Saskatchewan Phanerozoic rocks. They are all sedimentary. No Phanerozoic volcanoes or igneous intrusions are demonstrable in the rock record. It is a measure of the long term geological stability of our region that tectonism of sedimentary formations was never severe enough to cause thermal metamorphism although some burial metamorphism is noted. It is apparent that these southern Saskatchewan Paleozoic rocks were formed in the warm, shallow seas that dominated the continental interior,. The rock record contains areas denoting major hegiras of time wherein missing surfaces were apparently caused by temporary withdrawals of the seas from our area due to topographical changes in source areas, and subsequent non- deposition or erosion over time.

The shallow, salty epicontinental seas permanently withdrew from our province at the end of the age of the dinosaurs.

The Western Canada Sedimentary Basin (a long, trough shaped structure southwest of the Precambrian Shield) contains the Phanerozoic rock record. This basin (the Athabasca) has been forming since the erosion of the Hudsonian mountain ranges in the late Precambrian and has deepened throughout much of its history.

The origin of the Western Canada Sedimentary Basin is, to date, still an area of dispute. Theoretical approaches vary. Regional downwarping due to pressure at the plate edges or steady metamorphism and compression of deep bedrock units postulated by Nisbet are some possible explanations. Topographic highs, to the southwest of the basin (eventually engulfed by the basin sediments) include the Swift Current Platform, Alberta Ridge and Peace River Arch: these may have been the original upwarps.


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Modified on 19 Mar 96