Acadian orogeny

During the time of the Acadian orogeny, Middle Devonian (385 Ma), the paleolatitude of the Laurentia was in the Southern Hemisphere near the equator, between 0° to 30°S latitude. Laurentia did not change much with respect to paleolatitude during the Devonian. Gondwana, on the other hand, traveled a large distance, such that in the Ordovician the South Pole was located in northern Africa, where it then moved west of southern Chile during the Silurian, and moved back to central Africa during the Devonian. However, more recent research, from Scotese & McKerrow, suggests that in Late Devonian, the South Pole was in north-central Argentina rather than northern Africa, which was supported with paleoclimatic evidence. Avalonian terranes Avalonian terranes that constitute Avalonia are the following modern-day regions: northern France, Belgium (the Ardennes), England, Wales, southeastern Ireland, eastern Newfoundland, Nova Scotia, southern New Brunswick and some coastal parts of New England. The basement consisted of Late Precambrian age arc rocks and is believed to come from the margin of Gondwana, sometime in the Early Ordovician. Avalonia rifted from Gondwana during the onset of igneous activity in the Ardennes, Wales, and southeast Ireland that consumed the Tornquist Sea oceanic crust. It drifted in a northerly direction and probably collided with Baltica in the Late Ordovician, and then with Laurentia in the Late Devonian. Evidence for this is consistent with paleomagnetic data which place Avalonia at a temperate latitude during the Ordovician and in a subtropical latitude during the Late Ordovician through the Devonian. ==Overview of events==

The Acadian orogeny resulted from oblique convergence or major transcurrent movement along a large strike-slip fault which represents the zone of convergence between Laurussia/Laurentia and Avalon terranes. 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. The Acadian orogeny experienced at least three major phases of deformation, and in places, unconformities are recognized. 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. During the course of the orogeny, new faults formed, while older faults were reactivated. 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. 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. 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. 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. 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. nomenclature for the Middle Devonian strata in the Appalachian Basin The Middle Devonian to Lower Mississippian siliciclastic strata, deposited by the Catskill Delta, includes black shale, gray shale, sandstone, red beds, and minor argillaceous limestone. The strata was deposited in a four-stage pattern that is observed in each tectophase. The formation of the foreland basin through rapid subsidence initiated transgressive sequences that deposited basinal black shales. After the black shales were deposited, the migration of deformation continued southward, and regression dominated, particularly on the east side of the basin. As collision intensified, subsidence in the foreland basin declined, and sedimentation was replaced by an influx of calcareous silty shales and carbonates. These deposits reflect small transgressive-regressive cycles in a delta-front and delta platform environments. The third stage is represented by regional uplift, which accompanied the collision of an Avalon terrane with a promontory, and subsequently, developed a regional disconformity. The fourth and final stage is represented by tectonic quiescence with a widespread carbonate deposition in a slowly transgressing sea. ==See also==