The Acadian orogeny is a long-lasting mountain
building event which began in the Middle Devonian
, reaching a climax in the early Late Devonian
[Ryder, R.T., Swezey, C.S., Crangle, R.D., Jr., and Trippi, M.T., 2008, Geologic cross section E-E' through the central Appalachian Basin from the Findlay Arch, Wood County, Ohio, to the Valley and Ridge Province, Pendleton County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-2985, 2 sheets with 48-page pamphlet. https://pubs.er.usgs.gov/publication/sim2985]
It was active for approximately 50 million years, beginning roughly around 375 million years ago, with deformational, plutonic
, and metamorphic
events extending into the Early Mississippian
The Acadian orogeny is the third of the four orogenies that created the Appalachian orogen
and subsequent basin. The preceding orogenies consisted of the Potomac and Taconic orogeny
, which followed a rift/drift stage in the Late Neoproterozoic
The Acadian orogeny involved the collision of a series of Avalonia
n continental fragments with the Laurasia
n continent. Geographically, the Acadian orogeny extended from the Canadian Maritime provinces migrating in a southwesterly direction toward Alabama.
However, the Northern Appalachian region, from New England northeastward into Gaspé
region of Canada, was the most greatly affected region by the collision.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny
, with metamorphic events in southwestern Texas
and northern Mexico
, and with the Antler orogeny
of the Great Basin
reconstruction showing the Appalachian Basin area during the Middle Devonian
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
The paleolatitude of Gondwana during the Middle to Late Devonian resided around intermediate latitudes of about 50°S.
The collision creating the Acadian orogeny resulted in the closing of the southern Iapetus ocean
and the formation of a high mountain belt.
After the Acadian collision took place, Gondwana began to retreat from Laurentia with the newly accreted Avalonia
s left behind. As Gondwana moved away, a new ocean opened up, the Rheic Ocean
, during the Middle to Late Devonian, and subsequently its closure would result in the formation of the Alleghanian orogeny
Laurentia is the North American paleocontinent
, which also includes present day Northwest Ireland
, the North Slope of Alaska
, and the Chukotsk
peninsula of NE Russia
During the Ordovician-Devonian time, Laurentia remained at the same paleolatitude, slightly south of the equator in the southern hemisphere, with relatively the same paleolongitude.
Major defining tectonic
events include the Neoproterozoic rift sequence from the breakup of Grenville
basement rocks, thermal subsidence
related to the Early Cambrian
to Middle Ordovician drift sequence during the opening of the Iapetus ocean
, the Appalachian accretionary events to the eastern continental margin, and the resulting foreland-basin
and clastic wedge
Avalonian terranes that constitute Avalonia
are the following modern-day regions: northern France, Belgium (the Ardennes
, 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 SE 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 orogenic 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 (North America
) and Avalon terranes.
One or more of the Avalonian terranes accreted with the eastern margin of Laurentia, most likely beginning in the late Early Devonian.
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
These phases were called tectophases, and represented the sequence of collisions that occurred from the Avalonian terranes accreting to Laurentia. As a result of these tectophases, delta
s developed on the adjacent parts of the stable craton, eastern margin of Laurentia.
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.
300px|The Appalachian Orogeny, a result of three separate continental collisions. USGS
As mentioned previously, the Acadian Orogeny formed from the oblique convergence or major transcurrent movement along a large strike-slip fault
moving in a southeasterly direction. Avalonian terranes
accreted to the eastern margin of Laurentia
during the Late to Middle Devonian.
This was a continent-continent collision
with Baltica/Avalonia obliquely subducting below the eastern Laurentia margin, which closed the southern Iapetus ocean and created a high mountain belt
The collision initiated a sequence of events where the older rocks on Laurentia and Avalonia were subjected deformation
, and uplift
that occurred over a large area of eastern Laurentia (Naylor).
During the course of the orogeny
, new faults formed, while older faults were reactivated.
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 (Bradley).
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.
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.
Because major clastic wedge
s and basinal deposits are distributed in a southwestward progression, it is assumed they originated from areas near promontories
, areas along the continental margin where deformation is concentrated. As mention above, it was postulated that there are three, possibly four tectophases that represent increased convergence or possible collision between the continental margin promontories and Avalon terranes.
The earliest tectophase was located at the St. Lawrence Promontory in northern New England and in the Canadian Maritime
The St. Lawrence tectophase was active during the Early to Middle Devonian with intense transpressional
deformation which formed a basin in the Gaspé Peninsula
, northern New Brunswick, and northern New England.
Clastic wedges were present in this area, but the evidence for them has been mostly destroyed by succeeding tectonism.
The second tectophase, during the Middle Devonian, represents the collision with the New York promontory. The southward migration of deformation reflected the third tectophase, which marks the collision of Avalon terranes with the Virginia promontory in the Middle to Late Devonian time. The effects of the New York and Virginia promontories together produced the Catskill Delta
As the migration of deformation continued southward along the fault zone, during the Early Mississippian time, the final collision occurred with the Alabama promontory .
Ettensohn later refers to the fourth tectophase as the Mississippian tectophase of the Acadian Orogeny, since it demonstrated an unusually long duration (Mississippian-Early Pennsylvanian).
Subsequently, the Pocono & equivalent clastic wedges essentially filled the epicontinental sea
. The deposition of Middle Mississippian carbonates
marks the end of the Acadian Orogeny and Catskill Delta complex.
The Acadian Foreland Basin
s are a product of tectonic deformational loading, or crustal thickening
along the orogen, a consequence of overthrusting and fold
ing. 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.
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.
Subsidence follows bulge movement and uplift, and is produced on the craton
ic side of the orogen due to regional isostatic
adjustment to the load by the lithosphere.
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.
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 depocenter
The Acadian 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 (Pocono
) 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.
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 is deposited in a four-stage pattern that is observed in each tectophase. The formation of the foreland basin through rapid subsidence
initiated a transgressive sequences that deposited basinal black shales. After the black shales are deposited, the migration of deformation continues southward, and regression dominates, 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.
*Central Pangean Mountains
*Dictionary of Geological Terms, 3rd. Edition,1984, Robert L. Bates and Julia A. Jackson, Eds., prepared by The American Geological Institute
*International Tectonic Dictionary, 1967, Memoir 7, ''Am. Assoc. Petroleum Geologists'', p. 114
Category:Orogenies of North America
Category:Paleozoic North America
Category:Geology of Quebec
Category:Geology of Newfoundland and Labrador
Category:Geology of New York (state)
Category:Devonian Newfoundland and Labrador
Category:Devonian geology of New York (state)
Category:Carboniferous Newfoundland and Labrador
Category:Carboniferous geology of New York (state)