Geological History of Western Canada Study Notes

Overview and Learning Objectives of Western Canada's Geological History

• General Scope: Western Canada features a diverse geological record spanning from the Archean Eon to the Holocene. This history encompasses nearly every geological process, producing a wide array of rock types, world-class fossils, and vast mineral resources (beryllium to uranium).

• Core Learning Objectives:

  • Identify the composition and ages of the provinces within Laurentia.

  • Trace the movement of Laurentia over the last $650\,Ma$.

  • Understand the accretion of additional landmasses on the eastern, northern, and western margins of North America during the Phanerozoic.

  • Explain the depositional environments of Proterozoic, Paleozoic, Mesozoic, and Cenozoic sedimentary rocks in the Western Canada Sedimentary Basin (WCSB).

  • Summarize the origin and migration of British Columbia and Yukon terranes.

  • Relate Mesozoic terrane accretion to the formation of the Rocky Mountains.

  • Analyze the effects of Pleistocene glaciations and ongoing volcanic/seismic activity.

The Genesis and Journey of Laurentia

• Definition of Laurentia: The largest and oldest of Earth’s cratons (stable ancient crust), forming the core of North America.

• Age and Assembly:

  • Some constituent rocks are older than $4\,billion\,years$.

  • Laurentia has existed in its current form for approximately $1\,billion\,years$.

  • Provinces were assembled via plate-tectonic processes between $1\,Ga$ and $3\,Ga$.

• Chronology of Ancient Rocks:

  • Acasta Gneiss: Located north of Yellowknife, Northwest Territories. It is the oldest undisputed rock on Earth at $4.03\,Ga$.

  • Nuvvuagittuq Greenstone Belt: Located on the east coast of Hudson Bay, Quebec. Isotopic dating suggests $4.28\,Ga$, though its reliability is debated; it is widely accepted as at least $3.75\,Ga$.

  • Oldest Cratons: Slave and Superior Cratons (generally >3\,Ga), Wyoming Craton (>3\,Ga), and Hearne and Rae Cratons (>2\,Ga).

• Tectonic Evolution (650 Ma to Present):

  • Movement: A zigzag path from the southern hemisphere to the North Pole.

  • Supercontinents: Laurentia was part of Rodinia ($1,100\,Ma$ to $700\,Ma$), Pannotia, and Pangea.

  • Continental Margins Addition:

    • The Appalachian, Innuitian, and Cordilleran fold belts were added since $500\,Ma$.

    • Appalachian Fold Belt: Formed $\sim 350\,Ma$ when Gondwana (modern Africa) collided with the east coast.

    • Innuitian Fold Belt: Formed $\sim 350\,Ma$ when the small continent Pearya collided with the north coast.

  • Interiors and Subsidence: By $450\,Ma$, the interior of Laurentia was depressed by a subducting plate, leading to marine sedimentation in the Ordovician, Silurian, and Devonian periods ($450\,Ma$ to $350\,Ma$).

Precambrian Foundations of Western Canada

• Exposure and Lithology: While Laurentia extends to eastern BC, ancient cratonic rocks are mostly buried by younger strata in BC, Yukon, and Alberta (except the northeast corner). Shield rocks are highly varied, often strongly metamorphosed due to deep burial.

• Superior Province:

  • Dominated by granitic and gneissic rocks (pink on maps) with interspersed greenstone belts (metamorphosed sea-floor basalt and sediments).

  • Mineral Resources: Hosts large volcanogenic massive sulphide (VMS) deposits and nickel deposits (e.g., Thompson, Manitoba) formed from mantle-derived mafic magma.

• Trans-Hudson Orogen (THO):

  • Represents a continent-continent collision zone between the Superior Craton (south) and the Churchill Craton (north) at $\sim 1.9\,Ga$.

  • Current remnants include highly metamorphosed sedimentary/volcanic rocks and large granitic bodies (e.g., Flin Flon VMS deposits).

• Churchill Craton and Sedimentary Basins:

  • Includes the Athabasca Basin (Saskatchewan) and Thelon Basin (Nunavut), both aged $\sim 1.7\,Ga$.

  • Uranium: Athabasca is economically vital for unconformity-type uranium deposits.

  • Carswell Crater: A $40\,km$ diameter impact site ($\sim 115\,Ma$) that brought metamorphic rocks to the surface from beneath $2,000\,m$ of sandstone.

• Wopmay Orogen and Muskox Intrusion:

  • Wopmay Orogen: Site of ancient continent-continent collision west of the Slave Craton.

  • Muskox Intrusion ($\sim 1.1\,Ga$): Created by a mantle plume. It features repetitive layering from heavy metal minerals settling in low-viscosity magma (nickel, copper, chromium; potential for platinum and palladium). This type of intrusion is rare today as the mantle has cooled.

• Western Precambrian Successions:

  • Monashee Complex: Oldest rocks in BC ($\sim 2\,Ga$) near Revelstoke.

  • Purcell Supergroup: Mid-Proterozoic ($\sim 1,400\,Ma$) clastic rocks deposited in rivers/lakes during the supercontinent Columbia phase.

  • Windermere Group: Late Proterozoic ($\sim 700\,Ma$) clastic sediments deposited after Columbia rifted. Includes the Toby Formation, a marine mudstone with glacial dropstones from "Snowball Earth" glaciers.

Paleozoic Sedimentation and the Emergence of Exotic Terranes

• Passive Margin Deposition: In the early Paleozoic ($542\,Ma$), Laurentia was near the equator. Passive margin sedimentation produced limestone, mudstone, and sandstone.

• Burgess Shale (Cambrian):

  • Located in Yoho and Kootenay National Parks, BC.

  • Includes the Walcott Quarry and the recent Marble Canyon discovery ($2012$).

  • Unique for spectacular preservation of soft-bodied organisms from ancestors of modern life.

• Western Canada Sedimentary Basin (WCSB):

  • Much of the interior continent was submerged in inland seas linked to the ocean.

  • Stratigraphy: Includes $15$ Paleozoic formations from Ordovician to Carboniferous. $11$ are marine carbonate-dominated (limestone/dolomite).

  • Prairie Evaporite Formation (Devonian): Formed in an isolated basin where evaporation rates exceeded precipitation/inflow; source of potash in Saskatchewan.

• Exotic Terranes:

  • Blocks of crust with exotic origins (often from the southern hemisphere) moved toward North America via subduction.

  • Evidence of southern origin: Fossils and magnetic orientations in volcanic rocks.

  • Example: Wrangellia Terrane (includes Carboniferous Mt. Mark Formation on Vancouver Island).

Mesozoic Tectonics: Mountain Building and Basin Evolution

• Timeline: Triassic ($252\,Ma$) to end of Cretaceous ($65.5\,Ma$).

• Terrane Accretion Sequence:

  1. Intermontane Superterrane: Formed from Quesnel, Cache Creek, and Stikine terranes; collided in the early Triassic. Acts as BC’s interior plateau.

  2. Insular Superterrane: Formed from Alexander and Wrangellia terranes; collided $\sim 100\,million\,years$ later, forming Vancouver Island and Haida Gwaii.

• Formation of the Rocky Mountains:

  • The Intermontane Superterrane acted as a "bulldozer," pushing, folding, and thrusting Proterozoic and Paleozoic sediments eastward.

  • Thrusting: Older rocks pushed over younger ones (e.g., McConnell Thrust at Mt. Yamnuska where Cambrian carbonates moved $40\,km$ over Cretaceous mudstone; Lewis Thrust where Paleozoic rocks moved $80\,km$ over Cretaceous rocks).

• Igneous Activity:

  • Subduction created massive volcanism. Eroded magma chambers remain as batholiths (Coast Plutonic Complex).

  • Uplift: Some granitic bodies have risen $\sim 8,000\,m$ over the past $100\,Ma$.

• The Mesozoic WCSB:

  • Erosion of the Rockies provided sediment for the basin. The basin is a foreland basin, formed by the weight of mountains (isostatic depression) and subducting slabs.

  • Sediment Volumes: Peak sediment production occurred in the Upper Cretaceous ($100\,Ma$ to $65\,Ma$).

  • Hydrocarbons: Paleozoic sediments, buried deeply by Mesozoic layers, were heated to form oil and gas reservoirs.

  • Dinosaurs: Terrestrial Cretaceous formations host the Dinosaur Park Formation (at least $50$ genera) and the Hilda Bone Bed (approximately $1,500$ ceratopsians killed in a flood).

Cenozoic Accretion, Volcanism, and Glaciation

• Paleogene Accretion:

  • Pacific Rim Terrane ($\sim 55\,Ma$): Forced under western Vancouver Island.

  • Crescent Terrane ($\sim 42\,Ma$): Accreted sea-floor pillow basalt/gabbro to southern Vancouver Island.

  • Historical impact: Accretion pushed Vancouver Island closer to the mainland, uplifting Nanaimo Basin sediments to form the islands in the Strait of Georgia.

• Volcanism and Seismicity:

  • The subduction of the Juan de Fuca Plate (remnant of the Farallon Plate) is the modern driver for Cascades volcanism (e.g., Mount Baker) and BC volcanics (Garibaldi, Meager).

  • Igneous belts show a westward shift over time (Pliocene/Oligocene complexes are $40\,km$ east of modern ones).

• Cenozoic WCSB:

  • Deposition of terrestrial fluvial/deltaic strata: Paskapoo Formation (Alberta), Ravenscrag Formation (Saskatchewan/Alberta boundary), and Turtle Hills Formation (Manitoba).

  • Mammalian Fossils: Primitive ungulates, pangolins, colugos, and primates (Plesiadapiformes).

  • Paskapoo Stats: Average thickness $500\,m$; covers $90,000\,km^2$; volume $\sim 45,000\,km^3$.

• Pleistocene Glaciations:

  • Began $\sim 2.64\,Ma$ in the Klondike region (Yukon).

  • Cordilleran Ice Sheet (west) and Laurentide Ice Sheet (east) covered most of Canada.

  • Legacy: Massive erosion features in mountains and vast glacial sediment features (drumlins, eskers) on the plains.

Questions & Discussion

• Oldest Parts of Laurentia: Slave and Superior Cratons.

• Innuitian Fold Belt Formation: Pearya collided with North America $\sim 350\,Ma$.

• Stability of Athabasca/Thelon Basins: They are unmetamorphosed because they sit within a stable craton away from tectonic margins.

• Archean vs. Phanerozoic Magma: Ultramafic Muskox-style intrusions require higher mantle temperatures available in the Archean; the mantle is currently too cool.

• Prairie Evaporite Change: Accumulation resulted from the isolation of the basin from the open ocean, where evaporation exceeded input.

• WCSB as a Foreland Basin: It formed due to the isostatic depression of the crust caused by the weight of the Rocky Mountain fold-and-thrust belt.

• Paskapoo Thinning: The Formation thins toward the northeast because the source material (the Rockies) was located to the southwest, and sediment supply decreased with distance.