J - Devonian Acadian Orogeny


  
The Devonian Acadian Orogeny and Catskill Clastic Wedge
 
Middle to Late Devonian; 380 - 350 mya


~A Synopsis of the Acadian Orogeny~
 
The Acadian orogeny was first recognized in the 19th century in the severely deformed and metamorphosed rocks around the Bay of Fundy, Nova Scotia, Canada. At its peak it formed a continuous chain of mountains along the east coast from southern Virginia north to Newfoundland, where it connected with another chain of mountains, the Caledonides, that continued on for thousands of miles more between Greenland and the Scandinavian countries . The Caledonian and Acadian orogenies created the supercontinent Laurussia, also known as the "Old Red Sandstone" continent ("Old Red" Map).
     
The Acadian orogeny occurred near the beginning of a major period of mountain building in earths' history lasting for 150 million years. Nearly every continent was affected. At the same time, sea level was rising and the Kaskaskia sea, entering from the Gulf of Mexico, spread north across North America placing most of it under water. Eventually it lapped onto the inland margins of the Acadian mountains in the east and the Antler mountains in the west (Nevada).
 
The Caledonian orogeny began before the Acadian and resulted from a collision between Baltica (western Europe, parts of Great Britain, Scandinavia, and Siberia) and the Greenland region of North America ("Old Red" Map). Greenland, although separated, was a part of North America until the Triassic when the Pangaea breakup/Atlantic rifting separated it; Stage L.
     
The Acadian orogeny occurred when the Armorica terrane, traveling along an east dipping subduction zone (modern direction), collided with North America. Armorica originally included portions of Great Britain and southern Europe, but during the Triassic rifting that opened the Atlantic it was torn in two; the portion remaining with North America is Avalon. Avalon added to the Mid-Atlantic all the land lying under the Coastal Plain province, east of Richmond and Washington, D.C. and out to the edge of the continental shelf (Province Map). We will use the term Avalon from this point on even though until the Triassic it was technically still Armorica.
     
Avalon did not collide with North America at an oblique angle from the southeast. The North American coast, although not as irregular as during the Taconic orogeny (Stage H) still had projecting irregularities that Avalon struck hardest. The result was a ragged, "hit and run" type of mountain building episode that began in the north and migrated south through time. The northern collision in the maritime provinces of Canada is recognized primarily by the severely deformed igneous and metamorphic rocks of Nova Scotia, with sediments spreading southward.
     
The second, Catskill, impact in New Jersey, southeastern New York and eastern Pennsylvania produced the large, deep Catskill foreland basin (Catskill Cross Section). The Catskill clastic wedge in east-central Pennsylvania is many thousands feet thick, but we can see the diminishing effects of the collision as the sediments thin steadily westward, and southwestward through western Virginia and eastern West Virginia toward Tennessee. 
     
The third and final impact, called the Price in the south, was smaller and took place in southern Virginia. It produced a small foreland subsidence and a moderate volume of sediment spreading west and north from the collision (Map).
 
As with the Taconic orogeny in the Mid-Atlantic,  the Acadian is evidenced from the sedimentary record and the thick Accumulation of clastic sediments that are spread throughout the western Valley and Ridge and eastern Allegheny plateau (Province Map). Eastern Virginia also shows the effects of the Acadian orogeny. There is evidence that a large volcano existed in the Richmond area, and a number of igneous plutons intruded into the eastern Piedmont during the Acadian orogeny (e.g. the Petersburg GranitePiedmont Belt Map). Metamorphism and folding also affected the Piedmont during the orogeny.

By the Mississippian, deposition of the Greenbriar Carbonates and filling of the foreland basin indicated the orogeny had come to an end.
    
While the Acadian orogeny was responsible for bringing the pieces of the current-day Mid-Atlantic region into place, it still didn't look like it would today. Instead, the Acadian mountains would have towered above the cities of Emporia, Richmond, Fredricksburg, Washington to the east and beyond. All of the rocks these cities now sit on were, in the Devonian, the roots of the Acadian mountains. To the west, running though the Valley and Ridge province and the Allegheny plateau, was the foreland basin, with rivers carrying the eroded sediment down to the coastline where much of it then flowed into the deep parts of the basin. Beyond that was the Kaskaskia epicontinental sea, stretching all the way from central Pennsylvania and West Virginia to Nevada (Environmental Map).  
     
The Acadian orogeny is not as complex in its foreland basins and mountain structure as the Taconic (Stage H), but its rocks are more widely and obviously exposed such that we know much about the environment, climate, and life of the time. In the mid-Atlantic region, the orogeny consisted of two pulses resulting in two clastic wedges, which are described below.
 
~Catskill and Pocono Clastic Wedges~

The Catskill Clastic Wedge    (Map)
 
As far back as 1913 Joseph Barrell drew a map clearly showing the Acadian mountains in New Jersey and southeastern New York, and the clastic wedge spreading westward across New York, Pennsylvania and south toward Maryland, West Virginia, and Virginia. Barrell called this clastic wedge the Catskill delta. The "Catskill delta", however, contains no true "deltas" in the sense we use the term today for, say, the Mississippi delta, or the Nile delta (J1). For this reason the term Catskill clastic wedge is more accurate.
     
The Catskill foreland Basin is best developed and deepest in eastern Pennsylvania and adjoining New York, but shallows relatively quickly toward the west and southwest. In western Pennsylvania and West Virginia, the foreland Basin has reached normal Cratonic depths (several tens of feet down to perhaps a hundred feet or so, depending on the level of the Kaskaskia sea), and it is no longer considered an actual foreland basin. Evidence of shallowing toward the southwest is indicated in the thickness of the existing sediments. Near Winchester the clastic wedge is about 8500 feet thick. By White Sulfur Springs, West Virginia it has thinned to about 5000 feet, and at Big Stone Gap, far southwest Virginia, it is only 1000 feet thick. Off into Tennessee its thickness is measured in 10's of feet. 
     
Outcrops of the clastic wedge are found throughout the western Valley and Ridge, but develop more completely along the Allegheny Front in West Virginia at the edge of the Allegheny plateau province. The eastern most outcrops are found in Fort Valley within the Massanutten mountain. Between these rocks and the Avalon terrane to the east there is no stratigraphic record. The history of the foreland Basin is found in the Sequence of Formations that make it up. They are shown in the (Stratigraphic Section), and in this reconstruction of the Foreland Basin

 
History of the Catskill Foreland Basin
With the onset of the orogeny, the foreland Basin subsided rapidly. We see this looking at the stratigraphic record. The Oriskany white, Quartz Sandstone (right, and Stratigraphic Column) lying directly under the clastic wedge indicates tectonic stability. Directly on top of that, however, are the Needmore and Millboro Formations, which are deep water black Shales. It is as if the floor just dropped out from under the Mid-Atlantic region like a trap door (J 2).
Of course, the foreland subsidence accompanies the rise of the Acadian mountains, and these two coupled Events indicate the geologic speed of the collision. As the Avalon terrane, the hinterland, rode over eastern North America along a Suture Zone to create the mountain, it depresses the continent down with its weight in the process of sliding upward.

The basin then began to fill with sediments eroded from the mountain. The Catskill clastic wedge prograded (J 3) out into the basin; that is, filled in with sediment from east to west. The environmental systems near the mountain were short, braided rivers quickly flowing down to a narrow coastline, and then dumping large volumes of sediment directly into the Basin as underwater avalanches (turbidity currents). The turbidity currents built a submarine fan (Environmental Map) preserved in the Brallier Formation
     
The photograph to the right is of a few of the brallier Turbidite deposits (called Bouma Sequences); each sand/Shale couplet represents a single turbidity current. There are many thousands more. The turbidity currents only flow so far, however, so distally, away from the lower edges of the fan, are the black Shales of the Needmore/Millboro Formations deposited in the deepest parts of the basin. You can see this in the Cross Section where the sloping surface of the Brallier Formation changes down slope into the deepest Basin shaleProgressively, flood by flood, the sediment filled the basin, building (prograding ) the clastic wedge out toward the North West, and stacking the environments (Formations) on top of each other. 
 
The meandering rivers (Hampshire FormationCatskills in Pa and NY) grew longer and longer, and become more sluggish as they brought their sediment load down to the coast. In the left picture, for example, the blocky sandstones are the river channel deposits, and the silts and muds under them are flood plain deposits. Each subsequent flood deposited silts/muds and through many floods the rivers migrated across the flood plain depositing the channel sands as they went. The deposits accumulated until they reach 2000 feet thick, and many tens of miles wide. Likewise, the shelf environment (Foreknobs Formation), which began very narrow, grew wider and wider as the clastic wedge prograded. By the time the clastic wedge reached central Pennsylvania and West Virginia, it had filled the foreland basin. After this, the sediment spilled out into the Kaskaskia sea, onto the Craton but by this time the supply of sediment was diminishing rapidly as the eroding mountains approached sea level (Environmental Map).
 
Following the filling in of the foreland basin, there is one more small stage of mountain building associated with this orgeny; the Price/Pocono, which formed the Price/Pocono Clastic Wedge

In southwest Virginia most of the thousands of feet of thickness of the Catskill clastic wedge to the north (Needmore, Brallier, Chemung, Greenland Gap Formations) is represented by a black Shale sometimes only 3 meters thick, the Big Stone Gap Member of the Chattanooga Formation. It is the final deposition of the Catskill clastic wedge. However, directly on top of the black Shale are Shoreline and river deposits of the Price/Pocono Formation (Price to the south, Pocono to the north). This influx marks the beginning of the third and last pulse of the Acadian orogeny. The pulse was small and short lived since the sediments of the Price/Pocono clastic wedge are not very widespread or thick (ranging from 300-1700 feet thick, vs. about 6500 for the Catskill clastic wedge.) 

The relationship of the Price/Pocono wedge to the Catskill clastic wedge is an important one. In northwest Virginia and West Virginia, for example, much of the Pocono Formation was deposited on shallow tidal marine shelves or in near shore deposits (beaches, estuaries, tidal flats, and marshes - represented by Coal beds). The picture to the left is of a tidal shelf sand bar near Raleigh Springs, Va. More importantly, these gray sandstones and gravels lie directly on top of the red, terrestrial, river deposits of the Hampshire Formation. This is significant because for the last 20-25 million years of the orogeny, a vast volume of sediment had flowed from the eroding Acadian mountains, prograding westward and filling in the foreland basin, but evidence from the deposits show that the Pocono shoreline was transgressing eastward, drowning and burying the Hampshire river sediments. 
     
The fact that this transition from Hampshire to Pocono occurred indicates that the Catskill mountains were eroded enough that sediment flow had slowed to a fraction of its former rate and a small rise in Relative Sea Level pushed the shore line eastward toward the base of the former mountains.
 
With this last pulse, this last clastic wedge, the Acadian orogeny concluded.

~North American Climates in the Devonian~
 
During the Silurian, eastern North America lay in the tropical deserts. But by the Devonian Virginia had drifted northward and into the trade wind belt between the equator and about 20o south of the equator ("Old Red" Map). This resulted in a change in climate from desert to a humid equatorial belt with steady winds and abundant rainfall.The trade winds blew out of the east, across the Rheic ocean, toward Virginia and along the way they picked up moisture. However, as the winds hit North America and the Acadian Mountains, the winds were forced to rise. As they rose, the moisture laden winds cooled, formed clouds, and dumped large amounts of (orographic) rain on the eastern flank of the Acadian mountains. The eastern Acadian mountains at this time then underwent torrential rainfalls and continuous mists (Cross Section). The orographic rain under current day scenarios would fall somewhere above Richmond and the Coastal Plain (Cross Section), however,conditions were different during the Acadian.  Most of the moisture carried by the trade winds was dumped on the eastern flank of the mountains so that little remained in the clouds as they crossed the mountain crest. As the winds descended and warmed up, they became drier, such that the western flank of the Acadian mountain was in a rain shadow. It may even have been desert-like part of the year, except along the river banks of the Hampshire meandering river where vegetation could tap into the deep reserves of underground water. It is also quite likely that pulses of mountain building raised the mountains so high that no rain could cross over the top for a time. 
     
Furthermore, because the winds were trade winds they tended to blow steadily from east to west. That is, in the Devonian Catskill coastline they blew steadily from the land toward the sea. Such offshore directed winds do not create many onshore directed waves and just offshore from the clastic wedge, in the epicontinental sea, we quickly enter the equatorial region, and the doldrums, therefore, wave action was likely minimal. These Devonian climatic conditions had significant impacts on the environments in developing the clastic wedge and the epicontinental sea. 

Each event in the geologic evolution of the Mid-Atlantic is uniquely different. We have discussed two different orogenies now, the Taconic and the Acadian, and even for all their unique differences they do have similarities. For example, subsiding foreland basins that fill with clastic wedges, and Carbonates or Quartz Sandstones below and above the clastic wedge.  At very large scales there are Wilson Cycles. Let us continue on, pausing next for a brief Mississippian Calm before the next orogeny. 
 
Contributed by Lynn Fichter  
Tuesday, August 05, 2014
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