Evolution of North America

• Two hypotheses
• Early evolution of continents - Most or all continental crust formed early in Earth history during primeval molten stage when mantle and core differentiated
• Gradual evolution of continents - Primitive crust like ocean crust; continental crust produced at subduction zones

• Continents are made up of stable interior, orogenic (mountain) belts, and coastal plain (including continental shelves)
• Stable interior — Craton
• No tectonic activity for a long time
• Two parts:
• Shield
• Platform

• Precambrian rocks at or near surface
• Form nuclei of continents (Fig. 20.2)

• Platform - Veneer of relatively undeformed sedimentary rocks covering shield
• Platform deposits record sea level change
• Flooding of continent — transgression
• Marine sandstones, limestones, shales
• Epicontinental seas - inland seas
• Retreat of sea — regression
• Fluvial (river) sandstones, shales; coal

• Orogeny — mountain building
• Younger deformed sedimentary, igneous and metamorphic rock (Fig. 20-2) nearer to margins of modern or ancient continent
• Form by continental collision
• Folding, faulting

Major Structural Features of North America

• Dominated by granitic and high-grade metamorphic rocks
• Composed of Precambrian provinces of mountain belts (orogens) and ancient continental blocks (cratons) (Fig. 20-19)
• Continuous geologic relationships within province; abrupt changes across provinces
• Consistent radiometric ages within province

• Platform covered with thick accumulations of relatively undeformed Paleozoic sedimentary rock
• Overlies shield rocks
• Sedimentary rocks — cycles of shales, limestones, sandstones, coal
• Rocks reflect transgression and regression

• Orogenic belt
• Late Precambrian and Paleozoic mountain-building associated with subduction and continental collision
• Formed from Appalachian Orogeny, a series of three Paleozoic collisions (orogenies)
• Appalachian Orogeny
• Mid-Ordovician collision with S. America (Taconic orogeny)
• Taconic mountains in NY, CT, MA
• Late Devonian collision with S. America (Acadian orogeny)
• Appalachians of PA to Newfoundland
• Late Permian collision with Africa (Allegheny orogeny)
• Central and southern Appalachians

• Extends from Alaska to Guatemala
• Includes
• Mountain belts (e.g. Coast Ranges, Sierra Nevadas, Rocky Mountains)
• Basin and Range Province (Nevada)
• Colorado Plateau
• Reflects later Mesozoic and early Tertiary orogeny plus later rejuvenation

• Accreted terranes
• Island arcs and microcontinents plastered (docked) onto main continent; about 40 in the Cordilleron (Fig. 21-12)
• Originate in Pacific Ocean

• Gently dipping platform
• Relatively thick accumulations of later Mesozoic and Tertiary sediments
• Alternating shales and sandstones with some lignite

Geology of Texas (Geologic map of Texas)

Palo Duro Canyon (http://www.tpwd.state.tx.us/park/paloduro/paloduro.htm)

• Palo Duro Creek and its tributaries are eroding the Ogallala sandstone which forms the Llano Estacado

• Tertiary Ogallala sandstone overlies less resistant Triassic sandstones and softer Permian rocks (mostly clay)

Llano Uplift

Enchanted Rock

• Enchanted Rock is a spectacular granite pluton (square mile stock) in southern Llano County
• Rises some 385 feet above the streambed of nearby Sandy Creek to a maximum elevation of 1,825 feet above mean sea level
• This great granite monadnock (resistant rock or hill conspicuously rising above otherwise flat terrain) is the second largest such mountain in the United States (the largest is Stone Mountain, Georgia)
• It is part of a rough, segmented ridge, which is in turn part of the surface expression of a large igneous batholith of middle Precambrian material intrusive into earlier metamorphic schists and gneiss. (http://www.tsha.utexas.edu/handbook/online/articles/view/EE/rje13.html)