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EarthCache

Four Hundred Million Year-Old Mud Cracks

A cache by njgeology
Hidden : 11/15/2006
In New Jersey, United States
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The rocks exposed at this Earthcache highlight two environmental changes that occurred during the geologic history of northwestern New Jersey. You'll also find deformed 400 million year-old mud cracks that tells us the story of continental collision and the building of a mountain range.


LOCATION: Haney’s Mill, Delaware Water Gap National Recreation Area, New Jersey

ACCESSING THE TRAIL: Park in the small dirt lot just south of the bridge over Flat Brook. You can see the Earthcache location from the lot. Note: If you have a compass, bring it with you. This is a popular fly-fishing spot; please be respectful of your fellow outdoorsfolk.



A ROCKY RECORD OF ENVIRONMENTAL CHANGE

The presence of limestone at this Earthcache at Haney’s Mill, not far from the sandstones and shales on neighboring Kittatinny Mountain, is evidence of environmental changes that occurred in this region over millions of years.

If you look up and to the west, you’ll see the crest and western flank of Kittatinny Mountain. The rocks you see on Kittatinny Mountain are older than the limestones in the outcrop in front of you. The gray, white and red rocks on Kittatinny Mountain, called sandstones and shales, formed from sediments that were deposited in rivers and shallow marine environments. The light gray rocks where you are standing, called limestones, formed under different conditions, in the quieter waters of lagoons and intertidal zones.

The gray sandstones are middle Silurian in age, approximately 430 million years old. They formed from quartz-rich sediments left behind by fast-moving rivers that once flowed through the Taconic Mountains. The Taconic Mountains were part of a mountain range that once existed to the east and north of this site, but have since eroded away.

During the late Silurian Period, the mountains continued to erode. In the west, sea level rose and the shorelines slowly encroached across western New Jersey. By this time, the topography was very gentle and the rivers flowed more slowly (perhaps like southern New Jersey today) but with an eroding mountain range to the east. The overlying (and therefore younger) red sedimentary rocks called shales formed in this type of gently-moving river and shallow coastal environment.

Over several million years, conditions changed, and the environment became more tranquil. Slow-moving rivers carried less sand and clay to the coast, and the coastal waters became clear. Slowly the type of sediment that formed in the marine waters changed from river-supplied sand and clay to limestone formed by marine critters. Limestone generally needs clear and calm waters to develop. The limestone rocks here at Haney’s Mill shows evidence of this environmental change.

FOUR HUNDRED MILLION YEAR-OLD MUD CRACKS

The limestones are thinly-bedded to laminated (that is, even more thinly-bedded), almost like the pages of a book. They are part of what we call the Bossardville Limestone that is late Silurian age, approximately 418 million years old. These limestones formed in areas like a back lagoon or what we call the intertidal zone, meaning between the tides. An intertidal zone is under water during high tide but exposed during low tide.



One bed at this site shows what happened when this type of lime sediment underwent repeated drying and re-wetting. The lime sediments behaved much like mud behaves in a dried-up lakebed or on many construction sites. After a rain, mud will shrink as it dries, and cracks will form. These cracks will meet and intersect, forming polygons of uncracked fine-grained clay material. The mud-cracked polygons can be many-sided but tend to form rounded shapes without any preferred orientation. In this case, the sediment was not clay but rather very fine-grained lime material.




Some of the layers spent more time under water and less time above water, while a few beds experienced more dry periods and periodic flooding events. Here you can see a thick mud-cracked bed between two thinly-bedded to laminated layers without mud cracks. This shows that these three layers developed under different environmental conditions. The uncracked top and bottom layers were mostly wet; the mud-cracked middle layer was mostly dry. Over time these sediments were buried and solidified into rock.

These cracks are much thicker than normal cracks because the process kept repeating. Here the cracked bed is close to 36 inches thick. Each time the water covered this area it laid down new sediment which then dried out and cracked after the tide went out. In this way deeply-cracked sediment is formed; scientists call these “prism cracks”. The term “prism cracks” is used because the thick columns of mud-cracked sediment resemble prisms.

SAYS WHO??

Geologists learn how these types of deposits develop by studying how they form in modern environments such as the Caribbean or Florida. Study of prism cracks in modern environments revealed that algae and bacteria play a part to help trap the new sediment. The sediment is a lime mud that has been well-mixed by the burrowing of different organisms.


WHAT ELSE CAN WE LEARN FROM THE MUD CRACKS?

The second important aspect of this outcrop also has to do with the mud cracks. As described above, mud cracks form roughly rounded polygons. But the polygons here are not rounded but generally oval. Therefore, these previously-rounded mud crack polygons have been deformed; they have been squeezed into ovals. The important thing to notice is that all the ovals are aligned, with the long axis of each oval pointing in the same direction. (The “long axis” is the direction that the long edge if the oval is pointing.) What could have squeezed (or “compressed”) these features and in what direction was the compression applied?

As we look at these and other surrounding rock layers we see further evidence of pressure. The layers here are no longer horizontal, as most intertidal zones are now. These beds now tilt down towards the northwest. In addition, layers of rock up on Kittatinny Mountain show signs of folding similar to what happens if you push opposite sides of a piece of paper towards each other. All of this deformation resulted from the collision of continents on different tectonic plates. The tectonic plate collision that compressed the rocks here occurred when all of the continents moved together and formed the supercontinent named Pangaea, around 265 million years ago. When the North American plate collided with the African plate, closing the ancestral Atlantic Ocean basin in the process, a mountain range formed where the plates join. What’s left of this old mountain range is the current Appalachian Mountains.

These deformed polygons are evidence of a compressional event. We can use the orientation of the long axis and the short axis of the ovals to determine how the land was compressed at this location. The long axis shows the direction of low pressure. The short axis shows the direction in which pressure was applied. These two directions are 90 degrees apart.

To claim this cache: Answer the following questions, and post your answers in your log. Tell us how many people were in your group. (You don't have to wait for a confirmation from us to claim the cache. We trust you!)

1. What is the compass direction of the long axis of the deformed mud cracks? (If you have a compass, tell me the compass bearing. If not, just estimate the direction.) 2. Therefore, in which direction was the pressure applied here when the African and North American plates collided? Post a photograph of yourself or your GPS receiver beside the mud-cracked outcrop.

REFERENCES

Dimicco, R.V., and Hardie, L.A., 1994, Sedimentary structures and early diagenetic features of shallow marine carbonate deposits, Society of Sedimentary Geology, SEPM Atlas Series no. 1, 265p.

Fischer, A.G., 1964, The Lofer Cyclothems of the Alpine Triassic, in Merriam, D.F., editor, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, p.107-149.




This Earthcache is brought to you by the
NEW JERSEY GEOLOGICAL SURVEY
an agency of the New Jersey Department of Environmental Protection.



Visit us at www.njgeology.org




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Current Time:
Last Updated: on 9/24/2014 4:07:00 PM Pacific Daylight Time (11:07 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum