Geology of NPs: North American Craton Gary Oberts, Nov. 15, 2017 1. 2. The North American Craton (NAC) is the “basement” Precambrian bedrock forming the heart of the North American continent. The Canadian Shield is the largest exposed part of the Craton's bedrock. The NAC formed from volcanic activity, erosion/sedimentation, and later, accretion, all ~4.3-0.5 BYA. It was the first part of No. Am. to be above sea level and first appeared ~2.5 BYA as part of Supercontinent Kenorland (Arctica), and still exists with lithosphere ~150 miles deep. 3. Lithosphere thickness (miles) 14 5 12 0 92 66 39 4. Baltic Shield (~4.3 BY) Superior Craton at 4.28 BY forms the core of the NAC 5. Continental Shields – “…large stable areas of low relief in the Earth’s crust that are composed of Precambrian (older than 540 MY) crystalline rocks” or “continental nuclei” around which accreted material builds to form a continent • Occur on each continent • Major shields include: Canadian (Laurentia), Amazonian, Baltic (Fennoscandian), African (Ethiopian), Indian, Australian, and Antarctic • Minor shields include: Angaran (Russia), China-Korean • Canadian Shield can also be called a “peneplain” (a surface of low relief and great areal extent and age. 6. Canadian Shield (reds, exposed part of the No. American Craton) 7. The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent. It is the Earth's greatest area of exposed Precambrian rock. The oldest whole rocks found so far date back about 4.28 BY. CS originally 39,000’ high, but eroded now to 1k – 2k feet. Ice-spreading center during Pleistocene (2.6 MY – 11,600 Y). Hudson Bay glacial erosion remnant. 8. Hudson Bay: Formed over several MY as glaciation re-worked NAC/CS surface. The “Last Glacial Maximum (LGM)” was the last period in the Earth's climate history (25k – 21k YA) when ice sheets were at their greatest extension. ... Vast ice sheets covered much of North America, northern Europe, and Asia during the Wisconsin(an) glaciation of 85k – 11k YA. Laurentide ice sheet during Last Glacial Maximum (LGM) 9. 10. 11. Build-up of NAC 1. The original North American continent, Arctica (Kenorland), which started to form about 2.5 BYA from smaller continents and was completed by about 1.9 BYA. 2. Added to the North American continent during the formation of Nena (Columbia) about 1.8 BYA. 3. Added during the formation of Rodinia about 1.3 BYA. 4. Added during the formation of Pangea about 600 million– 300 MYA. 5. Added after the breakup of Pangea about 250 MYA. 12. 13. Crustal thickness in mi. (USGS) 6.5 13 20 26 33 39 14. Gondwana 15. 450 MYA – mid-Gondwana 16. Kenorland/Arctica Columbia Rodinia 2.7 – 2.1 BYA 1.8 – 1.35 BYA 1.3 – 0.65 BYA Pannotia 0.65 – 0.57 BYA Laurasia 0.54 – 0.06 BYA Pangaea 0.33 – 0.18 BYA 17. Rich CS ore sample, containing a mixture of several Fe, Cu and Ni sulfide minerals. Also rich in precious (Au, Ag) metals. Reflective of metamorphosed igneous deposits (>150 “volcanic belts”) and possibly a meteor impact in Sudbury Ont. 18. Morton Gneiss (MN) 3.5 BY 19. .