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112lect31 - GY 112 Lecture Notes D Haywick(2006 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Western North America Tectonics Lecture Goals : A) The Triassic B) The Jurassic C) The Cretaceous Textbook reference: Levin (2003) 7 th edition, Chapter 11; Levin (2006) 8 th edition, Chapter 13 A) The Triassic Before we start discussing Mesozoic tectonics in western North America, it’s necessary to remind you of world paleogeography in the Paleozoic. In that era, the big event was the assembly of Pangaea. If you recall, Laurentia (proto- North America) was drifting toward Baltica (proto-Europe) and Gondwanna (proto-Africa/South America/Antarctica/ Australia/India). Laurentia’s motion was toward the west-southwest. The net result (and this is what we spent a bunch of time on) was that the eastern coastline of the continent was an active continental margin ( officially a convergent plate boundary ). We kind of omitted the western side of Laurentia because all of the tectonic “action” was in the east. The western coastline during the Paleozoic was mostly passive. Thick blankets of mature siliciclastic sediment was deposited all along the coast from what is today Alaska to what is today California. Quartz arenite sandstones were deposited in beach deposits whereas siltstones and shales were deposited in deeper marine environments. There were also reef deposits and some oolites. Sound familiar? It should. This was the same type of sediment that was deposited in the east before convergence began. As in the east, much of the sediment deposited along western Laurentia was tropical during the Paleozoic – remember that the equator passed through the continent during much of the Paleozoic. The stratigraphic names of the rock formations in the west are as you might expect completely different that those in the east. The one formation that I remember from my undergraduate days is the Gog Quartzite (pictured to right from http://www.terragalleria.com ). It is a thick, slightly metamorphosed quartz arenite sandstone that was deposited in the early Cambrian in what is today southern British Columbia. The name is kind of unfortunate. Quartzite is a term better left for highly metamorphosed quartz arenite sandstones. I prefer the term Gog Formation for this rock unit. Whatever you call it, it’s a pretty impressive rock. It is thick and was highly resistant to deformational events. It shaped the way the Cordilleran Mountains formed in western Canada during a series of Mesozoic orogenies. The Gog Formation provides a friendly reminder about the importance of sedimentary rock units during mountain building. Mountains form during deformational events from pre-existing rock units. The
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GY 112 Lecture Notes D. Haywick (2006) 2 Cordilleran Mountains formed through deformation of rocks that were initially deposited along a passive continental margin.
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