Acadian orogeny

The Appalachian Orogeny, a result of three separate continental collisions. USGS

The Taconic, Acadian and Alleghenian orogenies are the three tectonic phases responsible for the formation of the present Appalachian Mountains. The Acadian orogeny is a middle Paleozoic mountain building episode dating back 325-400 million years which should not be regarded as a single event but rather as a chain of mountain building events. The Acadian orogeny is mostly responsible for the deformation in the northern Appalachians between New York and Newfoundland. The tectonic stresses brought on by the collision of the Avalonian tectonic plate with the convergent boundary of northeastern Laurentia began the formation of the early Appalachian orogeny. Its climax occurred in the Late Devonian with deformational, plutonic and metamorphic events extending into Early Mississippian time.

It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of the Great Basin.

Avalon Collision, Closing of the Iapetus Ocean. LISRC

Geodynamics

The Acadian orogeny was caused by the convergence of a small continental landmass called the Avalonia island arc at an active margin on the northeastern portion of the Laurentia plate. The convergence of the Avalonia island arc created and reactivated numerous faults in the region and caused older rocks to be deformed and metamorphosed.

Avalonia islands gradually accreted onto the eastern margin of the Laurentia plate and created magmatic activity which can be found in scattered outcrop belts along parts of Newfoundland and much of the coastal region of New England from Connecticut to northern Maine. Convergence of the Taconic and Avalonia tectonic plates were early stages of the formation of a supercontinent called Pangea. The final process, termed the Alleghanian orogeny, occurred when the African and North American plates collided producing the final tectonic stresses attributed to the formation of the Appalachians.

Acadian Orogeny, Various terranes accreted to the Laurentian plate. LISRC

When the Avalonia tectonic plate was accreted onto the Laurentian plate during the Acadian orogen, the land mass was pushed on top of the Laurentian plate creating a depressed area called a foreland basin or foredeep. The enormous stresses brought on by the collision of these two plates caused the foredeep to migrate westward as the pressures continued from the eastern direction.

Depositional Environments of the middle to late Devonian time period.

Deposition of sediments in the migrating foredeep due to the erosion of the Avalonia terrane and the prevailing Acadian Mountains created an interesting combination of sediments called flysch and molasse. Flysch is deep water sediment indicating that the foredeep was completely inundated forming depositional environments of black shale later recognized as the Needmore and Millboro formations. Deposition of shallow water sediments known as molasse become more prevalent as the foredeep filled with sediment and became progressively shallower. The red rocks from the meandering river channels known as the Hampshire Formation carried and deposited large volumes of sediment directly into the basin as underwater avalanches which are preserved in the Brallier Formation. The marine life preserved in the Foreknobs formation was deposited on the continental shelf of the ancient inland sea.

North American Structure, Various orogenic belts and other terranes depicted in the North American Structure map. USGS

The Avalonia land mass originally included parts of Great Britain and southern Europe but was rifted away from North America during the Triassic opening of the Atlantic Ocean. Avalon added the land lying under the coastal plain province to the edge of the continental shelf in the Mid-Atlantic region of North America. Large volumes of volcanic rocks and younger Iapetus Ocean sediments were accreted onto the North American margin as the land masses converged forming a band of younger terrane between the older North American and Avalonian terranes. The majority of this metamorphosed material can be found in the states of Maine, New Hampshire, Vermont and major portions of Massachusetts and Connecticut. These Avalonian rocks typically consists of schist, phyllite, gneiss and granite which can also be found in parts of South Carolina, Newfoundland, Nova Scotia as well as Morocco, Portugal, Spain, Ireland, Wales, southern England and the northern coasts of France, Belgium, Holland and Germany.

Early Devonian, Placement of continents during middle to late Devonian Period. LISRC

Avalonian tectonic plate converged on Laurentia obliquely from the northeast. The Laurentian coast had irregularities that the Avalonia island arc hit the hardest which resulted in a ragged type of mountain building that began in the north and migrated south through time as the Iapetus Ocean closed from northeast to southwest in direction. The northern collision in Canada is recognized by the severely deformed igneous and metamorphic rocks of Nova Scotia with sediments spreading southward.

Extent of the Catskill clastic wedge in VA and WV

The second impact, known as the Catskill, happened in the regions of New Jersey, southeastern New York and eastern Pennsylvania producing a large, deep Catskill foreland basin. The foreland basin deepens toward the north and is influenced by two processes, initial rapid basin subsidence and sediment filling the basin from bottom to top, incorporating distal sources in the mountains to the east and proximal meandering river systems of the Hampshire Formation transporting sediment toward the Kaskaskia Epicontinental Sea to the west. The mountains in the southeast are the major source of sediment, therefore the Catskill clastic wedge prograded toward the northwest into the foreland basin. As the foreland basin filled with sediment, the shelf environment widened progressively. Actually, as time passed, each environmental belt became wider and more distal environments usually were buried under the depositional environment prograding behind it. The Catskill clastic wedge in Central Pennsylvania is several thousands of feet thick and thins out steadily southwestward through Virginia and West Virginia indicating the diminishing effects of the collision. .

Extent of the Pocono clastic wedge in VA and WV

The third and final phase, known as the Pocono or Price impact, was smaller and took place in southern Virginia. The collision created a small foreland basin with a moderate volume of sediment spreading north and west from the collision. The sediments of the Price/Pocono clastic wedge are not very widespread or thick indicating the collision was small compared to the Catskill impact.

The cause of this great period of deformation is a result of the plate-docking of a small continental landmass called Avalonia (named after the Avalon Peninsula of Newfoundland). The docking of Avalonia onto the composite margin of Ganderia and Laurentia resulted in the closing of a portion of the Rheic Ocean.

Avalonia was gradually torn apart as plate tectonic forces accreted the landmass onto the edge of the larger North American continent. Today, portions of the ancient Avalonia landmass occur in scattered outcrop belts along the eastern margin of North America. One belt occurs in Newfoundland, another forms the bedrock of much of the coastal region of New England from eastern Connecticut to northern Maine.

A period of lithospheric thinning that followed the Acadian orogeny created volcanoes, such as the large Mount Pleasant Caldera in southwestern New Brunswick, Canada.

See also

• Orogeny - 'mountain building'
• Iapetus Ocean
• Taconic orogeny
• Central Pangean Mountains

References

5. Dictionary of Geological Terms, 3rd. Edition,1984, Robert L. Bates and Julia A. Jackson, Eds., prepared by The American Geological Institute

6. International Tectonic Dictionary, 1967, Memoir 7, Am. Assoc. Petroleum Geologists, p. 114

7. USGS document “Migration of the Acadian Orogen and the Foreland Basin across the Northern Appalachians of Maine and Adjacent Areas” (retrieved on 2011-04-02). [1]

8. (LISRC) Long Island Sound Resource Center document “Connecticut Geology” (retrieved on 2011-04-02). www.lisrc.uconn.edu/lisrc/geology.asp?p2=History&p3=glhct

9. This article incorporates copyrighted material from the website by Fichter, L.S.; Baedke, S.J. (1999). “The Geological Evolution of Virginia and the Mid-Atlantic Region” (retrieved and used for educational purposes on 2011-04-08). [2]

10. This article incorporates public domain material collected and used for educational purposes from the website Geological History of Jamestown Rhode Island document “Building New England: The Taconic and Acadian Orogenies” by John Carter (retrieved and used for educational purposes on 2011-04-10). www.jamestown-ri.info/acadian.htm

11. Healey, D.; “The Acadian Orogeny: An Overview”; www.oberlin.edu/healey 204 project

12. Ettonsohn, F. R.; “The Sedimentary Record of Foreland Basin, Tectophase Cycles: Examples from the Appalachian Basin, USA”; from the publication “Cyclic Development of Sedimentary Basins”; copyrighted 2005.

13. Murphy, J. B.; van Staal, C. R.; Keppie, J. D.; July 1999; Geology; v.27; p. 653-656.

14. Dorais, M. J.; Wintsch, R. P.; Nelson, W. R.; Tubrett, M.; “Insights into the Acadian Orogeny, New England Appalachians: a provenance study of the Carrabassett and Kittery formations of Maine"