Acadian orogeny

The Acadian orogeny is a long-lasting

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

Palaeogeography

During the time of the Acadian orogeny, Middle Devonian (385 Ma), the

The collision initiating the Acadian orogeny resulted in the closing of the southern Iapetus Ocean and the formation of a high mountain belt.^[6] After the Acadian collision took place, Gondwana began to retreat from Laurentia with the newly accreted Avalonian terranes left behind. As Gondwana moved away, a new ocean opened up, the Rheic Ocean, during the Middle to Late Devonian, and subsequently its closure resulted in the formation of the Alleghanian orogeny.^[7]

Laurentia

The evidence for the Acadian orogeny is abundant and widespread in the northern Appalachians, recorded by the plutonism and the migration of the northern Appalachian deformation front toward the craton. In the central to southern Appalachians, evidence for the Acadian orogeny is poor and is found primarily in the plutonism of the Blue Ridge and metamorphism of the Cat Square terrane.^[10]

The Acadian orogeny experienced at least three major phases of deformation, and in places, unconformities are recognized.^[4] These phases are called tectophases and represent the sequence of collisions that occurred from the Avalonian terranes accreting to Laurentia. As a result of these tectophases, deltas developed on the adjacent parts of the stable craton, eastern margin of Laurentia.^[11] These deltas are described as foreland-basin, delta-complex clastic wedges, which are responsible for the large volumes of sediment input into the Appalachian basin.^[9]

Collision

The collision of Avalonia with Laurentia initiated a sequence of events where the older rocks were subjected to deformation, plutonism, metamorphism, and uplift that occurred over a large area of eastern Laurentia.^[12] During the course of the orogeny, new faults formed, while older faults were reactivated.^[2] Acadian deformation and metamorphism were asymmetric across the strike of the orogen. The Acadian plutons intruded every belt, unlike the deformation/metamorphism, of Avalonia which did not undergo much of the alteration shown in other localities.^[13] During the Middle Devonian, centers for volcanoes and uplift formed in the New England region and shed fine-grained clastic material into an inland seaway that covered a large part of southern and central Appalachia. 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, where it is known as the Coastal Lithotectonic Block.^[2]

The collision between Laurentia and Avalonian terranes is actually more complex than described above. The collision is broken into three or possibly four tectophases which represent a successive collision of the Avalonian terranes with eastern Laurentia.^[4][11]

Tectophases

Because major clastic wedges and basinal deposits are distributed in a southwestward progression, it is assumed they originated from areas near

Foreland basins are a product of tectonic deformational loading, or crustal thickening along the orogen, a consequence of overthrusting and folding. The Acadian foreland basin is categorized as a retroarc foreland basin, which occurs on the overriding continental lithosphere, adjacent to a foreland fold-thrust belt behind a continental margin arc.^[4] The initial result of loading is a bulge move out and uplift of the foreland, which generates a localized unconformity. The distributions of unconformities display an asymmetric pattern in relation to the promontories.^[14] Subsidence follows bulge movement and uplift and is produced on the cratonic side of the orogen due to regional isostatic adjustment to the load by the lithosphere.^[4] Once thrust propagation declines, substantial relief and drainage nets have had time to develop, and the as a resulting coarser clastic sediment is eroded and transported to the foreland basin.^[14]

The basement structures of the Appalachian foreland basin at the onset of the Acadian orogeny were reactivated during foreland lithospheric flexure. These structures affected the foreland basin evolution and sedimentation patterns, and the preexisting faults partitioned the basin into regions of fault-controlled uplift and depocenters.^[15]

Delta complex

The Appalachian basin, during the Middle Devonian and Early Mississippian, is characterized by large volumes of deltaic sedimentary rocks that were deposited in the Acadian foreland basin as a response to the Acadian orogeny. These deposits extend from central New York and Pennsylvania westward to Ohio, and south along the Appalachian Mountains through Virginia and Tennessee to Alabama. The Acadian delta complex is categorized into two deltas, the Catskill Delta of Middle and Upper Devonian age, and the Price-Rockwell in the Pocono Mountains delta of Late Devonian and Early Mississippian age.^[16] The Acadian delta complex is coupled to the four tectophases of the Acadian orogeny, both in terms of provenance and depositional settings. The relief resulting from the orogeny was the fundamental source of the delta sediments.^[11]

The Catskill Delta complex consists of a coarsening upward sequence of rocks. Its thickness is greatest in eastern Pennsylvania and thins westward into Ohio. The Catskill paleogeography appears to consist of many small streams, which deposited their sedimentary load along a coastal alluvial plain that was hundreds of miles long.^[16]

The Middle Devonian to Lower Mississippian

• Central Pangean Mountains

References