The Hudson Valley fold belt is a west-verging fold thrust belt located in Upstate New York. This feature has been heavily studied by local geology students, as road cuts along Route 23 near the town of Catskill give a clear cross section of the entire fold belt. It is bounded by the Catskill mountains to the west and the Hudson river to the east. The belt itself is rather narrow, being on average less than 4 kilometers wide, and is often described in literature as a miniature valley and ridge province. It is difficult to define the westernmost limit of this structure, as west directed displacement can be found a few kilometers away from the westernmost fold in the form of cleavage duplexes. There is some deformed strata the same age as the belt east of the river, which suggests the belt was once wider than its present dimensions. It extends for roughly 80 kilometers between the cities of Kingston to the south and Albany to the north, where it meets the Mohawk River valley.
https://www.google.ca/maps/@42.2662968,-73.8730662,77860m/data=!3m1!1e3?hl=en
Figure 1. Google map of the region containing the Hudson Valley fold belt, which lies between the cities of Albany and Kingston.
Figure 2: Structures associated with the Hudson Valley fold belt in New Paltz, New York.
http://www.newpaltz.edu/geology/nysga.html
http://www.newpaltz.edu/geology/nysga.html
Figure 3: Sedimentary layers of the Hudson Valley fold belt (Marshak, 1986).
The stratigraphic units found in the fold belt represent sediment deposited by turbidity currents on the continental margin of Laurentia, which would later become North America. The lowest exposed unit is a flysch from the middle Ordovician, which was deformed into tight folds with east dipping axial planes before the younger units of the formation were deposited. This creates what is referred to as the Taconic unconformity, which formed as a result of the Taconic orogeny 440 million years ago. The next two groups are from the lower Devonian and are followed by middle Devonian limestone. Distinguishing rock units of the Helderberg group which follow the Taconic unconformity are the Manlius formation, a laminated micrite deposited in a tidal flat, the Coeymans formation, a lime grainstone from a beach environment, the Kalkberg and New Scotland formations, both lime wakestones, and the Becraft formation, which is also a lime grainstone. Following the Helderberg group is the Tristates group, with both groups originating from a shallow sea that advanced and retreated repeatedly during the lower Devonian.The youngest layers exhibiting deformation are the Bakoven shale and the Mount Marian formation. Based on the coloration of conodont fossils contained in the rocks they appear to have been exposed to a maximum temperature of 200 to 240°C, though it is not known if this occurred during or after the deformation. Most of these rock units are carbonates, with the only significant non-carbonate being the Esopus shale. The largest stratigraphic throw of the Hudson Valley belt is near Kingston, where the Esopus formation was thrusted over the Onondaga limestone.
The stratigraphic units found in the fold belt represent sediment deposited by turbidity currents on the continental margin of Laurentia, which would later become North America. The lowest exposed unit is a flysch from the middle Ordovician, which was deformed into tight folds with east dipping axial planes before the younger units of the formation were deposited. This creates what is referred to as the Taconic unconformity, which formed as a result of the Taconic orogeny 440 million years ago. The next two groups are from the lower Devonian and are followed by middle Devonian limestone. Distinguishing rock units of the Helderberg group which follow the Taconic unconformity are the Manlius formation, a laminated micrite deposited in a tidal flat, the Coeymans formation, a lime grainstone from a beach environment, the Kalkberg and New Scotland formations, both lime wakestones, and the Becraft formation, which is also a lime grainstone. Following the Helderberg group is the Tristates group, with both groups originating from a shallow sea that advanced and retreated repeatedly during the lower Devonian.The youngest layers exhibiting deformation are the Bakoven shale and the Mount Marian formation. Based on the coloration of conodont fossils contained in the rocks they appear to have been exposed to a maximum temperature of 200 to 240°C, though it is not known if this occurred during or after the deformation. Most of these rock units are carbonates, with the only significant non-carbonate being the Esopus shale. The largest stratigraphic throw of the Hudson Valley belt is near Kingston, where the Esopus formation was thrusted over the Onondaga limestone.
Figure 4: The Taconic angular unconformity.
http://hudsonvalleygeologist.blogspot.ca/2012/05/day-6-hudson-valley-fold-thrust-belt.html
http://hudsonvalleygeologist.blogspot.ca/2012/05/day-6-hudson-valley-fold-thrust-belt.html
Figure 5: Geological map of the region northwest of Catskill, New York along Route 23 that gives a clear west to east transect of the feature. (Marshak, 1989).
Three categories of faults are found in the belt: bedding parallel faults, cross strata faults, and strike-slip and normal faults. Bedding parallel faults are exactly what their name implies: their slip surfaces are parallel to the bedding of both the hanging wall and the footwall strata. No breccia or gouge is present, so these faults must have moved by crack seal extension instead of sliding. Instead, calcite fibers and spar often appear on these fault surfaces. The cross strata faults are also what their name suggests. These faults cut across the bedding planes and displace stratigraphic markers, and are also associated with calcite fibers. Many of these faults form ramp structures typical of thin-skinned deformation. It is thought that many of these faults formed in the later stages of deformation, because they seem to follow the folds that formed early on. The normal faults are represented on the map above as dashed lines and are oblique to the other belts of the fold belt. Their age and mode of formation is not known. On all faults, the hanging wall rocks moved westward relative to the footwall rocks.
There are two scales of folds in the Hudson Valley fold belt: megascopic and mesoscopic. There are ten main megascopic folds in this formation, with amplitudes between 50 and 120 meters and wavelengths of 200 to 800 meters. The synclines of these folds are wider and more symmetrical than the anticlines, and the northwestern limbs are either steep or overturned. The hinges of this series of folds are not always parallel. These folds were formed by the movement of rock over fault bends, regional flexure, and the formation of ramps. The mesoscopic folds (represented on the map) are far smaller, with amplitudes and wavelengths ranging from 1 centimeter to 10 meters. The exact type of fold appears to depend on the rock unit in which they are found, forming chevron folds in the Kalkberg formation and crenulations in the Esopus shale.
North of Kingston, the structures trend north/south to N10°E, while south of the town they change to N30°E. Because of the changes in their structural trend, structures southwest of Kingston are not considered to be part of the belt. This is an example of an orocline, a secondary bend where the curve is caused by the reorientation of preexisting structures. In this case, the older Hudson Valley fold belt structures were reoriented by the Appalachian fold thrust belt to the south.
Three categories of faults are found in the belt: bedding parallel faults, cross strata faults, and strike-slip and normal faults. Bedding parallel faults are exactly what their name implies: their slip surfaces are parallel to the bedding of both the hanging wall and the footwall strata. No breccia or gouge is present, so these faults must have moved by crack seal extension instead of sliding. Instead, calcite fibers and spar often appear on these fault surfaces. The cross strata faults are also what their name suggests. These faults cut across the bedding planes and displace stratigraphic markers, and are also associated with calcite fibers. Many of these faults form ramp structures typical of thin-skinned deformation. It is thought that many of these faults formed in the later stages of deformation, because they seem to follow the folds that formed early on. The normal faults are represented on the map above as dashed lines and are oblique to the other belts of the fold belt. Their age and mode of formation is not known. On all faults, the hanging wall rocks moved westward relative to the footwall rocks.
There are two scales of folds in the Hudson Valley fold belt: megascopic and mesoscopic. There are ten main megascopic folds in this formation, with amplitudes between 50 and 120 meters and wavelengths of 200 to 800 meters. The synclines of these folds are wider and more symmetrical than the anticlines, and the northwestern limbs are either steep or overturned. The hinges of this series of folds are not always parallel. These folds were formed by the movement of rock over fault bends, regional flexure, and the formation of ramps. The mesoscopic folds (represented on the map) are far smaller, with amplitudes and wavelengths ranging from 1 centimeter to 10 meters. The exact type of fold appears to depend on the rock unit in which they are found, forming chevron folds in the Kalkberg formation and crenulations in the Esopus shale.
North of Kingston, the structures trend north/south to N10°E, while south of the town they change to N30°E. Because of the changes in their structural trend, structures southwest of Kingston are not considered to be part of the belt. This is an example of an orocline, a secondary bend where the curve is caused by the reorientation of preexisting structures. In this case, the older Hudson Valley fold belt structures were reoriented by the Appalachian fold thrust belt to the south.
Figure 6: Thrust fault duplex along the Hudson Valley fold belt exposed in a quarry. Curved horses can be seen in the photo. The floor thrust is known as the Rondout detachment.
http://web.cortland.edu/gleasong/fdcamp.html
The Hudson Valley fold thrust belt is a good example of thin-skinned deformation, which occurs at a convergent margin where thrust faults appear only in surface rock layers and not in the deeper basement rocks as they do in thick skinned deformation. The deformation history of this feature is believed to be as follows: layer parallel shortening happened before or concurrent with the initial folding and thrusting of the rocks. Ramps began to form through the more competent layers, while detachments formed in the weaker ones. While this was occurring, non-layer parallel shortening occurred as bending and interlayer slip. As the thrusting continued, the rocks were exposed to both pure and simple shear strain. Finally, frictional coupling caused the faults to lock, and instead the strain caused bulk shortening of the rocks. Overall, the strain in this feature expressed itself through faulting, folding, and bedding parallel slip.
Shortening in the Hudson Valley fold belt occurred above two major detachment faults: the Austin Glen detachment and the Rondout detachment. The Austin Glen detachment is 500 meters below the surface and is not exposed anywhere along the fold belt. The Rondout detachment is stratigraphically higher, and in places where it is exposed it shows west verging mesoscopic folding. It crops out at or right above the post Taconic unconformity, and forms the floor thrust of a duplex. Horses stacked in duplexes are associated with the cross-strata faults, and occur in a variety of sizes.
http://web.cortland.edu/gleasong/fdcamp.html
The Hudson Valley fold thrust belt is a good example of thin-skinned deformation, which occurs at a convergent margin where thrust faults appear only in surface rock layers and not in the deeper basement rocks as they do in thick skinned deformation. The deformation history of this feature is believed to be as follows: layer parallel shortening happened before or concurrent with the initial folding and thrusting of the rocks. Ramps began to form through the more competent layers, while detachments formed in the weaker ones. While this was occurring, non-layer parallel shortening occurred as bending and interlayer slip. As the thrusting continued, the rocks were exposed to both pure and simple shear strain. Finally, frictional coupling caused the faults to lock, and instead the strain caused bulk shortening of the rocks. Overall, the strain in this feature expressed itself through faulting, folding, and bedding parallel slip.
Shortening in the Hudson Valley fold belt occurred above two major detachment faults: the Austin Glen detachment and the Rondout detachment. The Austin Glen detachment is 500 meters below the surface and is not exposed anywhere along the fold belt. The Rondout detachment is stratigraphically higher, and in places where it is exposed it shows west verging mesoscopic folding. It crops out at or right above the post Taconic unconformity, and forms the floor thrust of a duplex. Horses stacked in duplexes are associated with the cross-strata faults, and occur in a variety of sizes.
Figure 7. Three possible relationships between the Rondout detachment (white) and the Austin Glen detachment (grey). (Marshak, 1986).
In model A, internal shortening occurred in the Austin Glen formation, which lead to the folding of the overlying Rondout detachment. Here, faults do not extend below the Rondout detachment. Model B shows the Rondout detachment as a roof thrust and the Austin Glen detachment as a floor thrust of a duplex system. In model C, ramp faults of the system run all the way through both detachments. In this case, the Rondout detachment is not the basal detachment.
In model A, internal shortening occurred in the Austin Glen formation, which lead to the folding of the overlying Rondout detachment. Here, faults do not extend below the Rondout detachment. Model B shows the Rondout detachment as a roof thrust and the Austin Glen detachment as a floor thrust of a duplex system. In model C, ramp faults of the system run all the way through both detachments. In this case, the Rondout detachment is not the basal detachment.
Video: explanation of the Acadian orogeny. https://www.youtube.com/watch?v=zMtuXPJnhOI
Video explaining the Alleghanian orogeny. https://www.youtube.com/watch?v=fJZy_BCKrIU
Fold belts can appear in a variety of tectonic settings, so not many conclusions can be drawn about the environment of formation. The westward vergence of the HVB could relate it to the westward movement of part of New England relative to the rest of North America. Several theories exist as to how and when this region deformed. Its location between the Acadian foreland basin and the New England Acadian orogeny suggests that it may have formed in the middle Devonian as part of the Acadian orogeny. This mountain building event is associated with the creation of the Laurussian supercontinent and occurred over the Middle to Late Devonian, 380 to 350 million years ago. The Avalon microcontinent traveled along an east-dipping subduction zone and collided with what would later become North America at an oblique angle, forming a mountain chain that stretched from southern Virginia to Newfoundland. Another theory is that it formed as part of the Alleghanian orogeny 320 to 250 million years ago due to its continuity with structures in the central Appalachian mountains. In this orogeny, northwest Africa (at that time part of the Gondwanaland supercontinent) collided with eastern North America and closed the ocean between them, forming Pangaea. By this time, the mountains formed in the Acadian orogeny had been significantly weathered down. In this case movement would have been due either the presence of a transform system that existed on the eastern margin of North America at the time, or the collision between Africa and North America.
Works Cited:
Burmeister, Kurtis, Marshak, Steven. 2006. Along-strike changes in fold-thrust belt architecture: Examples from the Hudson Valley, New York. Geological Society of America: Field Guide 8.
http://www.friendsofwilliamslake.org/pdfs/burmeister.pdf
Fichter, Lynn S. "The Devonian Acadian Orogeny And Catskill Clastic Wedge." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.
Fichter, Lynn S. "The Late Paleozoic Alleghanian Orogeny." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.
Harris, John, Van der Pluijm, Ben. 1997. "Relative timing of calcite twinning strain and fold-thrust belt development; Hudson Valley fold-thrust belt, New York, U.S.A." Journal of Structural Geology, 20 (1): 21-31.
http://www.sciencedirect.com/science/article/pii/S019181419700093X#
Marshak, Steven, Engelder, Terry. 1984. "Development of cleavage in limestone of a fold-thrust belt in eastern New York." Journal of Structural Geology, 7: 345-359
http://www.sciencedirect.com/science/article/pii/0191814185900409
Marshak, Steven. 1986. "Structure and tectonics of the Hudson Valley fold-thrust belt, eastern New York State." Geological Society of America Bulletin, 354-368.
http://gsabulletin.gsapubs.org/content/97/3/354.abstract
Marshak, Steven, Tabor, John. 1989. "Structure of the Kingston orocline in the Appalachian fold-thrust belt, New York." Geological Society of America Bulletin, 683-703
http://gsabulletin.gsapubs.org/content/101/5/683.short
Majerczyk, Chris. 2011. Geology of the Roberts Hill Area in the Hudson Valley Fold-Thrust Belt, Greene County, Eastern New York. Submitted Thesis.
https://www.ideals.illinois.edu/bitstream/handle/2142/29626/majerczyk_chris.pdf?sequence=1
Works Cited:
Burmeister, Kurtis, Marshak, Steven. 2006. Along-strike changes in fold-thrust belt architecture: Examples from the Hudson Valley, New York. Geological Society of America: Field Guide 8.
http://www.friendsofwilliamslake.org/pdfs/burmeister.pdf
Fichter, Lynn S. "The Devonian Acadian Orogeny And Catskill Clastic Wedge." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.
Fichter, Lynn S. "The Late Paleozoic Alleghanian Orogeny." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.
Harris, John, Van der Pluijm, Ben. 1997. "Relative timing of calcite twinning strain and fold-thrust belt development; Hudson Valley fold-thrust belt, New York, U.S.A." Journal of Structural Geology, 20 (1): 21-31.
http://www.sciencedirect.com/science/article/pii/S019181419700093X#
Marshak, Steven, Engelder, Terry. 1984. "Development of cleavage in limestone of a fold-thrust belt in eastern New York." Journal of Structural Geology, 7: 345-359
http://www.sciencedirect.com/science/article/pii/0191814185900409
Marshak, Steven. 1986. "Structure and tectonics of the Hudson Valley fold-thrust belt, eastern New York State." Geological Society of America Bulletin, 354-368.
http://gsabulletin.gsapubs.org/content/97/3/354.abstract
Marshak, Steven, Tabor, John. 1989. "Structure of the Kingston orocline in the Appalachian fold-thrust belt, New York." Geological Society of America Bulletin, 683-703
http://gsabulletin.gsapubs.org/content/101/5/683.short
Majerczyk, Chris. 2011. Geology of the Roberts Hill Area in the Hudson Valley Fold-Thrust Belt, Greene County, Eastern New York. Submitted Thesis.
https://www.ideals.illinois.edu/bitstream/handle/2142/29626/majerczyk_chris.pdf?sequence=1