Field Trip to the Hudson Valley Fold Belt

Gloria Gill104451458

Structural GeologyApril 2009

Introduction:

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.

Stop A1:

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.

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

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.

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

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.

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.

Stop A2:

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.

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).

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.

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.

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.

Stop A3

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.

Figure 4: Layer parallel veins that formed due to layer parallel slip.

Stop K1

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

Figure 5. Mysterious Ripple Marks

is indicated by the smooth transition between cleavage layers forming a wave or S-

like appearance.

Stop K2

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

Figure 6. Outside edge of syncline showing topography differences between the Esopus, the Carliste Center and Schoharie Formations.

form horizontally, parallel to bedding. The fact that they are no longer horizontal

indicates that they formed before the tectonic event.

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

Figure 7. Anticline in Kingston, PA. South side of the road. Formations from bottom up are as follows: upper Becraft, Alsen and Port Ewen.

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.

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.

Stop K3:

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.

Stop K4:

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.

Figure 8. West side of route 32, Lateral trust ramps. Sigma one is in and out of the page.

Figure 9.A small vertical shear zone with cleavage plains running obliquely across it, giving the orientation of sigma one.

Stop C1a

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.

Stop C1b

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

Figure 10. Taconic Unconformity, with steeply dipping Austin Glen below it (right) and Silurian Roundout and Devonian Manlius above it (left).

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.

Stop C1c

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

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!

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.

Stop C2

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.

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.

Figure 12. Rip Van Winkle Anticline: Fault propagation fold.

Figure 13.Cross cutting veins that represent a combination of layer parallel slip and tension due to flexure.

Figure 14.Sigmodial Veins, show direction of shear.

Figure 15.Illustration of the passive roof thrust at stop C2

Stop C3

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.

Stop C4

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.

Stop C5

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

Figure 16.Anastamosing spaced cleavage in the Schoharie formation. This type of cleavage

can easily be mistaken for bedding.

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.

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.

Conclusion:

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.

Figure 18.Fold in the Esopus showing vergence to the West, indicating the sense of movement.