Structural Feild trip Write Up


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Structural Feild trip Write Up

  1. 1. Field Trip to the Hudson Valley Fold Belt<br />Gloria Gill<br />104451458<br />Structural Geology<br />April 2009<br />Introduction:<br />The following includes my interpretations and analysis of the geologic structures that I studied during our field trip to the Hudson Valley Fold Belt. The Hudson River lies to the East of the Hudson Valley Fold belt, and the Catskill Mountains lie to the West. During the Acadian Orogeny, between 330 and 360 million years ago, Avalon (New England area) collided with Laurentia creating the Catskill Mountains. This entire area was then folded again, exposing the folded Silurian decollement and the underlying Taconic Unconformity. These events lead to a variety of both large and small-scale structures that help piece together the history of this area. <br />Stop A1:<br />This outcrop exposes the folded Austin Glen Formation, with weak cleavage in a long exposure along the river. The cleavage is most prominent in the fine grain beds. These beds were deposited over a longer period of deposition than the coarse grain beds that have grated bedding which indicates catastrophic deposition. This area has prominent pencil cleavage, caused by two fissile plain directions that are at a high angle to each other. For example one cleavage direction may be due to bedding plains and the other perpendicular to bedding due to shortening. <br />You can find some great representations of Angular Cleavage Refraction in this outcrop. See Figure 1. Cleavage refraction occurs because it is nature’s way to prevent slip between the layers when one layer wants to shorten more than the other due to its composition. Cleavage refraction is preferred over slip between the layers because it cost less energy. In this example we see slaty fined grained cleavage and massive coarse-grained cleavage in alternating layers. In <br />order to have layers shorten equally in the horizontal direction, the slaty cleavage also shortens in a vertical direction and therefore is orientated closer to bedding. <br />Figure 1. Angular Cleavage Refraction: the top is the slaty fine-grained cleavage and the bottom is the massive coarse-grained cleavage. In order to have layers shorten equally in the horizontal direction, the slaty cleavage also shortens in a vertical direction and therefore is orientated closer to bedding. <br />Further down the road there is a prominent fold, see Figure 2. This change in bedding orientation is called an overturned anticline. The strike and dip for the top right side is (026˚, 52˚ W) and the bottom left is (314˚, 65˚ N). I plotted these limbs on Stereonet #1. The point of intersection between the two plains represents the axis of the fold belt, which is plunging steeply toward the North. I also measured the rake of the slickenlines to be 42˚ in the left plain. This turns out to be 91˚ from the fold axis, which indicates layer parallel slip and therefore further evidence for folding in this area. If you look closely at this fold structure you see that the bottom left limb of the anticline transitions into a syncline. The strike and dip for the left limb of the syncline is (017˚, 42˚ NW) and the right side is also (314˚, 65˚ N) because it is shared with the anticline. Stereonet #2 is a representation of this syncline, and it shows that the fold axis is also plunging to the North.<br />Figure 2. Overturned Anticline at stop A1. The strike and dip for the top right side is (026˚, 52˚ W) and the bottom left is (314˚, 65˚ N).<br />Stop A2:<br />In Johnson-Iorio Park there is a large road cut in the Austin Glen Formation. Here we see folding and some large and interesting joint surfaces such as the plumose structure in Figure 3. A plumose structure is a featherlike series of hackles radiating from an origin axis. Such hackles are indicator of a joint surface that was produced when the rock layer was broken in tension. This area also has the characteristic pencil cleavage.<br />Figure 3. Plumose Structure: a featherlike series of hackles radiating from an origin axis. Such hackles are indicator of a joint surface that was produced when the rock layer was broken in tension.<br />The chaotic deformation at the south end of the park contains a prominent fold that most likely formed before the sediments were fully lithified. This fold is verging towards the West. This indicates westward transport, with the foreland being toward the west and the hinderland toward the east.<br />Also evident in this road cut is a huge syncline structure. The fold axis of this syncline is plunging roughly parallel to the Hudson River, meaning that the shortening direction was East- West. Within the convex side of syncline there are tension gashes in the massive granular rock filled by veins that formed perpendicular to bedding. Due to shortening on the concave side of this structure there is out of syncline thrusting, which are faults that form in the syncline to make room form the brittle rock that is unable to fix it’s space problem by ductile behavior. Across the road, Figure 4 shows a picture of the layer parallel veins that form due to layer parallel slip. When this vein formed, water followed thru the cracks and pressure was released when the layers slipped, causing calcium carbonate to precipitate out of the water and form these light colored veins. <br />Figure 4: Layer parallel veins that formed due to layer parallel slip.<br />Stop A3<br />This stop is home to a very interesting geologic puzzle as shown in Figure 5. The dilemma is that the structure looks to have ripple marks but it was deposited in a moderately deep-water depositional environment. Ripple marks usually only form in a near shore, shallow environment, so then what could be the cause of this structure? It is possible that the deep water had very high energetic movement/turbulence and therefore was able to produce such marks. Or perhaps this structure is due to compositional mineral property such as concordial fracture. I personally believe in the latter more strongly since the rock is massively granular and seems to have weak cleavage planes that could cause the fracture pattern.<br />Figure 5. Mysterious Ripple Marks <br />Stop K1<br />In this Kingston area you can examine the folding and thrusting of the Acadian Orogeny. The exposure on the North side of the Road is clearly an anticline, with distinct bedding layers. The orientation of the fold axis of this anticline is consistent with stop A1 in the sense that shortening is in the east-west direction. You can see clearly the Esopus, Carliste Center, Schoharie ad Onadaga Limestone formations transition from bottom up. As you walk from the Anticline axis toward the syncline you can see the black cherty bed characteristic of the Carliste Center formation, as well as the white banding characteristic of the Schoharie formation. As seen in Figure 6, these beds are topographically higher in altitude because they are more erosionally resistant than the Esopus, which is weaker, and therefore has lower topography. Topography is largely dependant on the formations are that brought to the mean erosion level and their resistance to erosion. This formation also shows evidence of cleavage refraction. However, unlike the cleavage refraction found in stop A1, this is gradually rather than angular. This is indicated by the smooth transition between cleavage layers forming a wave or S-like appearance. <br />Figure 6. Outside edge of syncline showing topography differences between the Esopus, the Carliste Center and Schoharie Formations.<br />Stop K2<br />Figure 7 is a picture taken in Kingston, shows a large fold (anticline) in the upper Becraft, Alsen and Port Ewen Formations, therefore the lithology gets siltier upward toward the Port Ewen. Due to this lithology, there is evidence of solution cleavage in the Port Ewen formation; this cleavage is orientated perpendicular to the sigma one direction. Sigma one is plunging <45˚ to the West. In contrast to this pressure solution cleavage, the pressure solution stylolites were formed due to loading stresses rather than tectonic stressed. These stylolites are formed early in the digenesis process due to vertical overloading that causes calcium carbonate to dissolve away leaving behind the darker insoluble impurities. Therefore stylolites form horizontally, parallel to bedding. The fact that they are no longer horizontal indicates that they formed before the tectonic event. <br />Figure 7. Anticline in Kingston, PA. South side of the road. Formations from bottom up are as follows: upper Becraft, Alsen and Port Ewen.<br />The road cuts almost normal to the fold axis, allowing us to see two side of the anticline. The center high point of the Becraft is definitely higher on the south side of the road by approximately 4.2 meters. I also paced out the length of the Becraft at ground level on both sides of the road and found that the south side was longer by about 16 meters. These two measurements indicate that the fold axis is plunging to the Northwest. In order to get a more precise measurement of the trend and plunge of the fold axis, we measured the strike and dip of both limbs of the anticline. The average mean strike and dip for the west limb was (344˚, 17˚ W) and for the east limb (320˚, 17˚ E). Stereonet 3 shows that according to these measurements the trend and plunge of the fold axis is (333˚, 04˚). However, this measurement is only as accurate as our strike and dip measurements that were very difficult to precisely achieve. Therefore in order to double check, we paced out the distance between the center of the Becraft on the south side of the road to the center of the Becraft on the north side of the road and found the distance to be 39.2 meters. We also took the trend of this path and found it to be 175˚. Using this distance measurement, the height difference and simple trigonometry we calculated the plunge to be 06˚. This calculation can also be written as (355˚, 06˚) which is not too far off from our stereonet findings. <br />Vegetation growth and slickenfibers orientated East-West indicate layer parallel slip between layers. This makes sense because the west transport created tension in the Port Ewen and shorting in the Becraft, forcing the layers to slip relative to each other. Veins perpendicular to bedding in the convex side of the brittle and ridged Becraft formed in a similar fashion to the tensional veins found in the Austin Glenn formation, discussed earlier.<br />Stop K3:<br />Next we head around the corner to an outcrop on the West side of route 32 and find complicated fault geometries. We are sure that these are faults because the gouge is badly broken up. Slickenfibers in the crevasses of the fault indicate that we are looking in the strike direction of the anticlines observed in stops K1 and K2. The orientation of the faults in this Manlius formation suggests that these are lateral thrust ramps. In figure 8, sigma one is in and out of the picture. <br />Figure 8. West side of route 32, Lateral trust ramps. Sigma one is in and out of the page. <br />Stop K4:<br />Steep, folded beds of Becraft and Alsen characterize the last stop in Kingston. Based on running my hand along the Slickenfibers in the fault zone on the Southeast side of the road, the Northwest side of the fault moved up. Across the street, the Slickenfibers indicate the same sense of slip. A small vertical shear zone (Figure 9) further down the road has cleavage plains running obliquely across it. Sigma one was perpendicular to these cleavage plains, indicating a slip direction that correlates with Northwest side up. <br />Figure 9.A small vertical shear zone with cleavage plains running obliquely across it, giving the orientation of sigma one.<br />Stop C1a<br />Here there is a very nice exposure of the Taconic Unconformity, the Roundout Formation, the late Silurian formation and early Devonian formation. The formations from NW to SE are Kalkberg, Coeyman, Manlius, Roundout and Austin Glen. In between the Ordovician strata, deformed by the Taconic Orogeny and the early Silurian beds, deformed later by the deformation of the Hudson Valley Fold Belt during the Mid-Silurian, lies the Taconic Unconformity. This unconformity in Figure 10, represents a 60 million year gap in time! Slickenlines that run along the unconformity are a small-scale indicator of top down movement. The folds that bend toward the unconformity further support this westward movement. This makes sense since general motion at previous sites was westward. This slip was originally horizontal and happened while the Roundout was active during the Acadian Orogeny. It slipped along the unconformity because it was energetically favorable. The average strike and dip above the unconformity was (038˚, 46˚ W) and (024˚, 69˚ W) above the unconformity. Stereonet 4, shows that the trend and plunge of the fold axis to be (002˚, 32˚). I also plotted the poles of the plains in order to help piece together the orientation of the Austin Glen before the Acadian Orogeny. The orders of events are as follows: First the Austin Glen was deposited; Next, the Taconic Orogeny changed the orientation of the Austin Glen. This first Orogeny did not affect the Roundout because it was not yet deposited; The third step is more deposition and erosion and of coarse deposition of the Roundout; This is followed by the second Orogeny called the Acadian Orogeny in which the Roundout acted as a decollement. This Orogeny changed the orientation of both the Austin Glen and Roundout to the same degree. Therefore working backwards we can calculate the original orientation of the Austin Glen and the degree of rotation of the Taconic Orogeny, assume the simplest rotation actually happened. See work on Stereograph 4. Further down the road bedding dips toward the Northwest, revealing that this is a large syncline structure. <br />Figure 10. Taconic Unconformity, with steeply dipping Austin Glen below it (right) and Silurian Roundout and Devonian Manlius above it (left).<br />Stop C1b<br />This outcrop is best understood in the context of stop C1a, because this is the other limb of the syncline mentioned above. As you walk through the outcrop from East to West, you encounter these formations in the following order: Kalkberg, Manlius, Roundout and then the Kalkberg again! See Figure 11. There is also evidence some secondary slip. The Kalkberg appears twice because it as been doubled over. Thrusting caused the Manlius (older rock) to be thrusted over the Kalkberg (younger rock). This is partly due to the Roundout acting as a major detachment maker during the Acadian Orogeny. As the critical wedge grows and collects material toward the foreland, it must also get higher to maintain the critical taper, causing out of sequence thrusting. At the same time, the detachment grows and skips to a weaker higher level merging toward the surface. When the shallowest detachment reaches rock it folds and faults whatever is in its path. This bulldozer model of the events that took place during the Acadian Orogeny explains the deformation here. <br />Figure 11. An outcrop in the Cat Skills, from East to West, (Right to left) you encounter these formations in the following order at ground level: Kalkberg, Manlius, Roundout and then the Kalkberg again!<br />Stop C1c<br />Going west, you find micritic Manlius and dark fossiliferous Coeymen followed by the Kalkberg which contains faulting and slickenfibers. There is heavy folding in this area accompanied by vein fill, and layer parallel slip. Shear zones in the Kalkberg have inclined cleavage that indicates that slip was to the west. The fold that appears on both sides of the road is more prominent on the North side of the road. There is not as much shortening and it appears obliquely on the South side of the road. This is due to the ability of folds to die out along strike. A good analogy is a bunched up tablecloth. Folds in the cloth do not have to run the full length. However, there must be compensation for this, perhaps another fold.<br />Stop C2<br />This very long outcrop has many classic fold and thrust belt structures. The Rip van Winkle anticline is verging to the west. It shortens more on the south side of the road where fault has accommodated more folding. This structure is a fault propagation fold. This means that as the fault propagates the tip of the fault has zero slip and acts like a pivot. See Figure 12. There is also evidence of layer parallel slip in the relative brittle beds. Just to the west of the Rip van Winkle anticline is the Town & Country Syncline. On the North side of the road there is complex folding where shale is injected into a gap along a fault between more brittle limestone. Here, the Becraft possible behaved ductile to accommodate the folding. This is indicated by the lack of fracture and the change in thickness of the beds. However on the South side of the road you see a v-shaped pop-up formation in the Becraft. This is called out of syncline thrusting which solves the space problem of folding brittle rock. <br />Figure 12. Rip Van Winkle Anticline: Fault propagation fold.<br />As you walk down to the West you come to the central anticline. Here we see cross cutting veins in the Manlius, see figure 13, that represent a combination of layer parallel slip and tension due to flexure. There are prominent sigmodial veins the south side out crop. Figure 14 shows that these veins indicate the direction of shear. These veins cease to exist in the Kalkberg because it is less brittle than the Manlius, therefore there aren’t any tension cracks. Instead the Kalkberg has solution cleavage. There is a prominent triangular zone of heavily cleaved Kalkberg on the North side of the road. This solution cleavage is cause by compression due to a passive roof thrust as illustrated in Figure 15. Here the fault ramps up to a shallower depth thrusting the anticline over the footwall and shoving the Kalkberg into its self. However this is not evident on the South side of the road. Furthermore the sides of the road do not match, on the North side, the highest point has Kalkberg, over Coleman, over Manlius but on the South side Kalkberg is at road level beneath New Scotland. Therefore there must have been a lateral ramp where the road is, connecting the faults on the two sides of the road. <br />Figure 13.Cross cutting veins that represent a combination of layer parallel slip and tension due to flexure.<br />Figure 14.Sigmodial Veins, show direction of shear. <br />Figure 15.Illustration of the passive roof thrust at stop C2<br />Stop C3<br />This is the Mills Falls anticline that shows cleavage fanning. Since the Esopus formation is 80 km thick it is hard to see bedding well. However the orientation of vegetation growth can indicate the stratigraphy, since plants will grow in crevasses that collect dust. Fanned cleavage indicates that the cleavage happen prior to folding. There also is a change in the slop of the topography due to the weakness and erodability of the Esopus.<br />Stop C4<br />This is an example of messy anastamosing spaced cleavage in the Schoharie formation. This type of cleavage can easily be mistaken for bedding. See figure 16.<br />Figure 16.Anastamosing spaced cleavage in the Schoharie formation. This type of cleavage can easily be mistaken for bedding.<br />Stop C5<br />This abandoned road cut exposes the deformed Esopus above the undeformed Glenerie. Here the Esopus is deformed and the Glenerie untouched because the Esopus acts as a decollement maker. See Figure 17. The Glenerie and the basal Esopus have distinct bedding whereas the rest of the Esopus above has very intense solution cleavage and folding. This accommodates for the large amount of shortening that took place in the Esopus. The Esopus is a good decollement maker because it has the ability to fill in spaces created by folds. The contrast in strain above and below this detachment explains why the Glenerie shows much less shortening. The fact that there is little slip on the detachment surface indicates that this area must be close to the pin line. The vergence of the folds is to the west, indicating the sense of movement here, a shown in Figure 18.<br />Figure 17:Outcrop exposes the deformed Esopus above the undeformed Glenerie. Here the Esopus is deformed and the Glenerie untouched because the Esopus acts as a decollement maker.<br />Figure 18.Fold in the Esopus showing vergence to the West, indicating the sense of movement.<br />Conclusion:<br />This field trip was an extremely rewarding experience that enabled me to apply the knowledge that I learned in Structural Geology to real life questions. All of the sites that we visited correlated and added to the evidence that a westward Orogeny caused the deformation in this area. I do believe that I will from now on be a quite dangerous driver as I rubberneck to look at road cuts. However, I do feel it will be worth it since I can now show off my skills in interpreting the geologic history of that particular area. <br />