112lect32 - 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 Mesozoic Sedimentation (North America and Alabama) Lecture Goals : A) Triassic Sedimentation (Breakup of Pangaea) B) Jurassic Sedimentation (Birth of the Atlantic Ocean) C) Cretaceous Sedimentation (Creation of the Coastal Plain Province) Textbook reference: Levin (2003) 7 th edition, Chapter 11; Levin (2006) 8 th edition, Chapter 13 A) Triassic Sedimentation The formation of Pangaea was the most important tectonic event in the Paleozoic. It was also instrumental in major paleoclimatic change and evolution. But alas, Pangaea was only a temporary supercontinent. It, like most marriages these days, was doomed to eventual breakup. That separation began in the Triassic and really took off in the Jurassic. The evidence for initial breakup is now buried beneath the Coastal Plain Province of the Appalachian Mountains or off the east coast of North America (thousands of metres below the surface of the Atlantic Ocean). In order to study these rocks, you need to either drill down to them (e.g., get cores ) or use seismic techniques (this is the discipline of geophysics ). Fortunately, we have a lot of both types of data because the oil companies have been actively studying the region looking for $$$$ (i.e., oil and gas). Seismic profiles of the east coast of Canada and in the Gulf of Mexico reveal fault structures that we have previously discussed in GY 112 (refer back to the Wopmay Orogeny lecture): The faults are all normal which indicated that the area was experiencing tension during the Triassic. The multiple normal faults are typical of rift valleys which I’m sure you’ll remember, is the initial phase of divergence ( divergent plate boundary ). As divergence Note: Depending upon the semester (and how verbose Doug gets), these notes may comprise two full lecture periods rather than one.
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GY 112 Lecture Notes D. Haywick (2006) 2 proceeds, the rift will widen enough to allow new oceanic crust to form, but this takes time. To the best of our knowledge, no new oceanic crust formed until later in the Mesozoic. In the Triassic, all that rifting did was to form linear canyons very much like the East Africa Rift today. Nevertheless, this was a very important event that signaled the eventually breakup of Pangaea and the eventual formation of new oceans. The North Atlanta and Gulf of Mexico started to open first. The Southern Atlantic opened a bit later. The initial rifting was accompanied by a great deal of sedimentation. The crack that formed developed along the middle of the Appalachian Mountains 1 and produced a steep topographic gradient (see diagram below). As I have said in this class before, holes don’t last very long in geology and this long linear “hole” was no exception. It started to get filled by sediment. The type of sediment that initially was deposited into the rift was, not surprisingly, coarse
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112lect32 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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