Geology of Yukon

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Geology of Yukon: a concise, easy-to-understand overview

Yukon’s rocks tell a very long story. They record a time when the region lay at the edge of the ancient continent Laurentia, and over billions of years it was built up by many pieces of crust (terranes) that arrived from different places, erupted with volcanism, and collected sediments as the land moved and changed shape. The result is one of the longest and most complex sedimentary and volcanic records on Earth, spanning Precambrian to recent times.

Precambrian foundations and early life
- The oldest parts of Yukon sit on the western edge of Laurentia, dating back more than 1.7 billion years. In this era, rocks record early crust growth and large-scale volcanic and sedimentary activity.
- Major early units include the Wernecke Supergroup (a thick mix of sandstone and carbonate rocks) and the Pinguicula Group (sandstone, dolomite, siltstone, shale). These rocks show ancient volcanoes, intrusive activity, and long periods without much change.
- During the breakup of the supercontinent Rodinia about 850–750 million years ago, the region began to drift away from other landmasses. This breakup is seen in basalt flows and rift-related rocks in the Mackenzie Mountains.
- The Cryogenian period (late Precambrian) includes a glaciation (Sturtian) evidenced in the Rapitan Group, and by the end of the Precambrian, early multicellular life appears in carbonate-rich rocks of the Windermere Supergroup.

Early Paleozoic: building a continental margin
- As Laurentia settled into a passive continental margin, Yukon saw substantial sedimentation, including thick carbonate deposits along platforms such as the Mackenzie Platform.
- The Tintina Fault and related structures mark major tectonic rearrangements as newer blocks were added or moved along the edge of the continent.
- Several ancient crustal blocks, or terranes, joined Yukon from different settings. The Cassiar Terrane and parts of the Selwyn Basin record alternating layers of limestone, sandstone, and shale, with evidence of marine transgressions (sea levels rising).
- The region developed rich carbonate platforms (Bouvette and Sunblood Formations) and later clastic deposits as seas fluctuated. The Devonian period brought basalt and other volcanics, as well as zinc-lead deposits hosted by ancient brine activity in some basins.

Mid-Paleozoic to late Paleozoic: collision, accretion, and deeper belts
- The Arctic Alaska Terrane lies in the north, while the Intermontane terranes (including Cache Creek, Yukon-Tanana, and Slide Mountain) arrived from the south and west as the Panthalassa Ocean closed.
- The Yukon-Tanana and Slide Mountain terranes show a history of subduction-related metamorphism and later collision, with evidence of eclogite and other high-pressure rocks from deep burial.
- The Saint Elias and Craig subterranes (Alexander terrane) formed part of the growing Yukon crust, and the Wrangellia terrane linked with Alexander added more arc and ocean-floor material into the mix.
- In the Mesozoic, thick sedimentary and volcanic sequences filled inland basins as the Stikinia and Quesnellia terranes interacted with Cache Creek. The Whitehorse Trough accumulated three kilometers of sediments, including coal, sandstone, and deep-water turbidites, overlain by Jurassic-age volcanism.
- The Laberge Group, Dezadesh Formation, and other units record continued deposition and volcanic activity through the late Triassic and into the early Cretaceous. Metal deposits (copper, gold, molybdenum, and nickel) formed in several of these intrusions and related rocks.

Late Mesozoic to Cenozoic: major plutons, belts, and fresh terrane additions
- By the mid-Mesozoic, the convergence and collision of major terranes (Stikinia, Cache Creek, Quesnellia, and others) created powerful mountain-building events along the Yukon Plateau.
- The Denali-Tintina zone and related faults (including the Teslin Fault) controlled later sedimentation and igneous activity, hosting significant mineral belts and copper-gold porphyry systems (notably near Whitehorse and Dawson).
- The Alexander and Wrangellia terranes continued to modify Yukon’s crust, with large granitic intrusions (plutons) and associated ore deposits appearing from the Late Triassic into the Early Cretaceous. The Tombstone, Mayo, and Tungsten suites are examples of major intrusions produced during this time.
- In the Paleocene to Oligocene, the Amphitheatre Formation recorded basin-fill sediments with placer gold that later moved into new deposits as erosion redistributed material.
- In the Miocene, ongoing subduction beneath Alaska and Yukon produced volcanic rocks (andesite, basalt) and shaped the Coast Plutonic Complex, including the Ruby Range suite, which hosts important copper-molybdenum mineralization.
- The late Cenozoic saw continued volcanic activity near the surface, with the Skukum Group (Eocene) and younger basalt flows near Mount Skookum and around the Yukon. The Yakutat and Chugach terranes added to the western Cordillera, with the Border Ranges Fault marking shifts in subduction zones.
- The youngest volcanic rocks are the less than three-million-year-old basalt flows at sites like Watson Lake, Rancheria, and Selkirk in Yukon.

In short
- Yukon’s geology records a long journey from ancient Precambrian crust to the present, built from many crustal blocks (terrane accretions) that joined the North American margin over hundreds of millions of years.
- It features a chain of events: long sedimentary records, repeated volcanism, mountain-building episodes, and various glaciations and sea-level changes.
- The region hosts rich mineral belts tied to its complex tectonic history, including copper, gold, nickel, and other metals, reflecting the dynamic interactions of the planet’s crust here through time.