Darrah Healey


The Acadian Orogeny: An Overview

The Acadian Orogeny was one of three major orogenic phases in the creation of the Appalachian mountain chain. The first of these phases is called the Taconic orogeny and occurred during the middle to late Ordovician period. This phase is generally attributed to the subduction of North America under an island arc. The second is the Acadian and briefly it is associated with the collision of North America with the Avalonia terrane. The third was the Allegheny orogeny and deformation due to it is most visible in the central Appalachians. This last phase is generally accepted as being due to the collision of Africa with North America.

Avalonia was a microcontinent made up of Late Precambrian to Early Paleozoic rocks. When the terrane was accreted onto North America, it created an area called a foredeep. This phenomenon is due to the strongly damped harmonic wave created by the weight of the terrane on North America. When the large mass was pushed on top of North America, the rocks responded viscously. Because of this, a depression was created in front of the terrane. Due to erosion of both Avalonia and North America and deposition of the sediments in the foredeep, a trace remains of where the foredeep was. In the case of the Acadian orogeny, the foredeep created migrated westward as pressure from the East continued. An interesting set of sediments resulted from the filling of the migrating foredeep called flysch and molasse. Flysch is a deep water sediment and thus indicates that the foredeep was inundated. As it filled with sediment it got progressively shallower and the next phase of deposition consisted of the shallow water sediment, molasse.

The Avalonia terrane was actually a composite of several terranes, some of which had pre-Cambrian basement rocks. In Maine at the time of the orogeny, a long standing magmatic arc shut off and a thick succession of Silurian through Lower Devonian volcanic and plutonic rocks remained. The Acadian deformation and metamorphism in Avalonia were mild relative to the activity in the Kearsage-Central Maine basin that lay to the West. The Kearsage-Central Maine basin was essentially trapped in between the Avalonia terrane and North America and thus experienced the greatest deformation.

Another interesting feature of the Acadian sediments is the fossils. Among these features are Brachiopods, Corals, and Stromataporoids. The rocks that these fossils are found in are mainly quartzites and impure limestones. Brachiopods, Corals, and Stromataporoids are also shallow water depositions. The presence of all of this marine deposition leads into one of the major controversies over the Acadian orogeny which is whether or not the Kearsage-Central Maine Basin was a second ocean or not. By calling it a second ocean, I am referring to the closing of the Iapetus Sea which was the equivalent of the present day Atlantic and began closing during the Taconic orogeny continuing until Africa and North America collided in the Allegheny orogeny.

The rate of convergence of the plates involved in this orogeny must have been at least five km per million years. This is compared to present day plate movement that is on the order of ten-twenty km per million years.

There are essentially four major areas of terranes in the Northern Appalachians: the miogeocline, the Central Mobile Belt, Avalonia, and Meguma. To the West of the miogeocline is the deformation front. The Central Mobile Belt consists of the deep water sedimentary units, including ophiolites, that were trapped during accretion. Meguma is a small slice of rock formed at the African continental margin. The rocks in Meguma are Early Paleozoic in age. Meguma was accreted onto what is now Nova Scotia.

The deformation of the Eastern margin of North America was extensive and the resulting Appalachian mountains were at one time quite mighty. In the present day, much of the volume of this is lost due to erosion and the full scope of deformation is lost from the geologic record. What remains, however, can offer much insight into how terranes moved and where and what types of deposition took place in the spaces created by the orogenic processes. Much of the sediment resulting from the Acadian orogeny is marine, but not all of it is. The stratigraphic record indicates that in some areas the Acadian foredeep was superimposed on an active magmatic arc. This is an area of particular interest in the study of Acadian orogeny.



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