CEN 514: Summary of Notes for September 11, 1997

Orogenies That Have Affected Eastern North America

• 1.3-1.0 billion years ago: GRENVILLE Orogeny
• • Prior to this orogeny, a westward-dipping subduction zone existed along the east coast of Proto-North America, adjacent to the Pre-Grenville Ocean.
  • As subduction of the Pre-Grenville Ocean occurred, a huge continent from the opposite site of this ocean slammed into Proto-North America, creating a Grenville supercontinent.
  • This was a very active tectonic environment, affecting an area from Labrador, Canada, south through Georgia and Texas into Mexico, forming mountains as high as the Himalayas.
  • The resulting mountains, the Grenville Plateau, were a source for sediment all over North America.
  • By 700 million years ago the supercontinent was leveling off, and the giant insulating blanket of sediment and other rocks caused mantle material to rise and parts of the crust to melt
  • This upwelling magma forced the supercontinent to rift, forming the Iapetus Ocean.
  • Grenville age rocks outcrop in the Adirondacks, eastern Quebec and the Hudson Highlands, the New Jersey Highlands, and other locations along the east coast of North America. The Hudson and New Jersey Highlands were affected by later orogenies, but the Adirondacks were not.
  • In the High Peaks region of the Adirondacks, metanorthosite is exposed, which was originally formed 25 to 30 kilometers below the surface.

• 660-550 million years ago: RIFTING of the Grenville Supercontinent.
• • A rift zone formed along the suture zone where continents had joined during the Grenville Orogeny.
  • The splitting of the Grenville Supercontinent formed the Iapetus Ocean.
  • About 550 million years ago, the Taconic Island Arc formed in the Iapetus Ocean, as a result of a subduction zone that developed and dipped to the east.

• 460-440 million years ago: TACONIAN Orogeny
• • According to Isachsen et al, about 200 million years after the Iapetus Ocean opened, subduction of oceanic crust in an eastward direction beneath the Taconic Island Arc, led to the eventual collision of proto-North America and the island arc. Gilbert Hanson (personal communication) and others believe that subducion was actually to the west, and that the volcanics developed at the eastern edge of North America rather than in an island arc.
  • This arc is thought by its proponents to have had a continental crust base, and extended from Newfoundland to Alabama
  • As the collision proceeded rocks in the accretionary wedge and basement were pushed westward on thrust faults, these rocks now make up the Taconic Mountains in eastern New York State and western New England. Rock units metamorphosed by this event include gneisses, schists, and marbles.
  • Cameron's Line is a demarcation that divides the rocks to the west, of continental origin, from the rocks to the east, of oceanic origin. The Manhattan Schist, Fordham Gneiss, and Inwood Marble are examples of units of continenental origin, while the Harrison Gneiss and Hartland Formation lie east of Cameron's Line, and are of oceanic origin. The Hartland Formation is believed to form part of Long Island's bedrock.
  • The paleotectonic environment espoused by Gilbert Hanson somewhat resembles modern day Japan and China, where the Pacific Plate is subducting in a westward direction.

• 410-380 million years ago: ACADIAN Orogeny
• • Renewed subduction of oceanic crust beneath enlarged proto-North America, led to the collision of a microcontinent called Avalon with proto-North America.
  • Avalon attached to proto-North America and part of it can be found today in easternmost New England.
  • A shallow Devonian sea on the interior of proto-North America continent developed, as a new magmatic arc continuely grew as Avalon approached.
  • The collision built high mountains along the eastern part of the continent.
  • Rocks in central Connecticut were metamorphosed. This area is not cut by the Connecticut Valley.
  • 374-360 million years ago the Acadian Mountains were being eroded and deposited what is now the CATSKILL DELTA, which partially filled the shallow sea.

• 330-250 million years ago: ALLEGHANIAN Orogeny
• • The last orogeny recorded in the Appalachians was caused by proto-Africa slamming into North America.
  • This orogeny produced the Appalachian Mountains the rejuvenated remnants we still see today, the chain extends from Alabama to Newfoundland.
  • Alleghanian faults along Eastern North America indicate that proto-Africa probably slid southward along a transform margin, there was little subduction involved.
  • As proto-Africa slid it also rotated clockwise, pushing westward into the southern part of proto-North America, forming the Valley and Ridge Province. The Kittatinny Ridge in western New Jersey and the Shawangunks in New York State are part of this region.
  • As a result of the rotation of Africa against the southern part of Proto-North America the, Appalachians were higher in the south, and only portions of New York State were deformed.
  • Rocks in southeastern Connecticut and western Rhode Island were metamorphosed.
  • A supercontinent was formed called PANGEA.
  • the concentration of continental mass acted as a thermal blanket and causes instability in the asthenosphere, which lead to what kind of tectonic regime?? Read on to find out if you got it right.

• 220 million years ago to present: Rifting of Pangea and Growth of the Atlantic
• • A divergent margin developed along the Appalachian Mountains, creating long steep-sided valleys, such as the Hartford Basin of Connecticut and the Newark Basin in New Jersey.
  • Sediments were deposited on the margins of these valleys, and as rifting continued volcanoes erupted and covered the sediments with lava.
  • The Palisades Sill was part of this magmatic regime, only this large mass of molten rock cooled underground, and today the sill forms cliffs on the west side of the Hudson River near New York City.
  • In the central portion of the rift, new oceanic crust began to form, creating the Atlantic Ocean.
  • The east coast developed into a passive margin, and sediments eroded from the continent over millions of years and built the continental shelf.
  • Continental glaciation eroded rock and seiement from some localities and deposited it in others.

TRIASSIC-JURASSIC RIFT BASINS

About 220 million years ago Pangea was beginning to break up, rifting caused Gondwana and North America (Laurasia) to eventually pull apart.

• Rift basins were created over a broad band along the present coasts of North America, Africa and Europe. One series of these rift basins continued spreading and became the Atlantic Ocean.
• Rifting occurs in stages:
• • At first there is bowing up of continent and lithosphere
  • The continent then pulls apart and rift basins form, that is blocks of the crust move down along faults.
  • In those basins above sea level sediments derived from the surrounding highlands are deposited in the basins by rivers.
  • Basins below sea level fill with ocean water. If the basin is shallow and evaporation in the basin is rapid enough compared to the flow of sea water into the basin, evaporite (gypsum and salts) deposits occur.
  • The upwelling asthenosphere below the rift basin melts resulting in basaltic volcanic rocks at the surface and diabasic intrusive rocks as sills and dikes in the basins and surrounding basement rocks.
  • Large lakes commonly formed in the basins
  • As the basins subside sedimentation continues, leading to greater than 10,000 feet of sediments in some basins.
• Some basins on the continental shelf in the vicinity of Long Island are : the New York Bight Basin just south of western Long Island and the Long Island Basin just south of eastern Long Island
• • Marine basins are characterized by: evaporites near the base, overlain by Jurassic sands and shales, finally covered by Cretaceous and younger sediments. As the evaporites get buried deeper they begin to form salt domes due to their low density relative to that of the overlying sediments.
• Basins presently above sea level in the New York Metropolitan area include the Newark Basin and Hartford Basin
• • These basins were filled with sediments deposited subaerially. They are characterized by: conglomerates at the base which are overlain by river deposits consisting of sandstone and mudstones. The sandstones are channel deposits the mudstones are overbank deposits upon which soils were developed. About 200 million years ago basaltic magma intruded forming dikes and sills and some of the magma reached the surface and formed basaltic lava flows. The Palisades Sill in the Newark Basin and West Rock Ridge sill in the Hartford Basin are representatives of the intrusive phases. About this time the basins developed large lakes which regulary dried and expanded over periods of some 400,000 years.
  • • ___ROCK TYPES OCCUR IN CYCLES
      ______Beginning at the bottom
      ______________________________________STAGE___ __________________________________________________
      gray to gray-red sandstone or siltstone
      siltstone
      cross bedding
      footprints
      fossil soil traces of roots_____________Lakes at Minimum Depth
      black to green, gray siltstone
      calcium carbonate layers_____________Lakes at Maximum Depth
      grey siltstone______________________Lakes Expand____

    Following the Jurassic is the CRETACEOUS

    • The Atlantic Ocean is large.
    • Greenland has pulled away from North America.
    • Development of a passive continental margin.
    • Thick blankets of sediment is beginning to be deposited along the continental margin or shelf which is subsiding as the underlying mantle cools.

Last modified September 9, 1997

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