Geologic Events Affecting Eastern North America



The Grenville orogeny in eastern Laurentia (ancestral North America) about 1 b.y. ago apparently resulted from assembly of the supercontinent Rodinia which contained ancestral cores of today's continents.  This continental scale collision built a great orogenic (mountain) belt.  Rocks that were thrust into the deeper part of the orogenic belt were subjected to high pressure and temperature and were metamorphosed.  The preexisting igneous and sedimentary rocks were metamorphosed into gneiss.  Eventually, the mountains eroded away, leaving only a vast area of Grenville age metamorphic rocks (mostly gneiss), the roots of the former mountains.  Grenvillian metamorphic rocks underlie most of eastern North America and are exposed in places, such as in the Bronx.

Breakup & Passive Margin:  By about 600 to 700 m.y. ago Laurentia broke out of Rodinia.  Passive margin subsidence yielded a thick sequence of Upper Proterozoic and Lower Paleozoic sedimentary strata along the present day U.S. east coast.  Limestones were deposited on a shallow continental shelf during the Cambrian Period.  Limestone gave way to the deposition of muds (which lithified into shale) in the later Cambrian and the Ordovician periods. 

Return to Convergence:  During the Ordovician a volcanic island arc developed off the eastern margin, indicating that the period of passive margin subsidence was over and the margin had become one of active convergence.  Basaltic lavas erupted from arc volcanoes.

The Taconic Orogeny:  The volcanic arc collided with eastern Laurentia in the Late Ordovician (around 450 m.y. ago).  Deformation occurred from New York to New Foundland.  Another mountain belt was formed and the previously deposited sedimentary rocks became metamorphosed and foliated in the core of those mountains.  Over time the Taconic highlands were worn away by erosion, leaving only the metamorphic roots.  Immature sediments shed westward from the Taconic highlands resulted in a clastic wedge deposited in the Taconic foreland basin.

The Acadian Orogeny in the Devonian (around 380 m.y. ago) was the result of collision of Europe with northern portions of Laurentia (Greenland and Newfoundland) and added another permanent terrane to the eastern cratonic margin (the Avalon Terrane added portions of New England and Maritime Canada.  This collision overprinted Taconian metamorphic ages, intruded abundant granite (like in New Hampshire, "The Granite State"), built another high eastern mountain range that shed another clastic wedge that coarsens to the east called the Catskill clastic wedge.  The new tectonic unit composed of Laurentia and Europe is called Laurussia

The Appalachian orogeny (also called Alleghenian orogeny) in the Pennsylvanian and Permian periods resulted from collision of Africa and North America, as part of the collision of Laurussia and Gondwana.  Orogenies on many continents attest to the coalescence of the supercontinent Pangea.  In eastern North America the Appalachian Orogeny resulted in the emplacement of massive thrust sheets and widespread folding.  This was the final phase of compressional deformation in the Appalachian region.

Rifting:  Normal faulting and the formation of half-grabens throughout eastern North America indicates that the supercontinent Pangea began breaking up in the Triassic Period (around 225 m.y. ago).  The Newark Basin filled with sediments and, toward the end of deposition around 190 m.y. ago in the Jurassic period, basaltic lava flows and intrusions (Palisades sill).  This period of stretching and basin formation was followed by the breakup of Pangea and the onset of seafloor spreading in the new-born mid Atlantic ridge by about 165 m.y. ago. 

"The Ice Age:"  Glaciers spread down to the New York area during the Pleistocene epoch, scouring the surface bedrock exposures, producing many erosional landforms (striations, roches moutonŽes) and depositional landforms (moraines, outwash plain).