RAMAPO BORDER FAULT and associated FANGLOMERATE

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FIELD TRIP STOP – FAULT PLANE BETWEEN PRECAMBRIAN CRYSTALLINE ROCKS AND TRIASSIC FANGLOMERATES  

 

LOCATION:  On the west side of US-202 in Suffern, NY is the Ramapo Border Fault. About 2.5 miles east of the fault on US-202 is the exposure of the Fanglomerate. More specific directions can be found in Schuberth (1968).

 

GEOLOGIC FEATURES: Normal Fault; Fault Plane; Slickensides; Fanglomerate; Non-conformity; Crystalline PreCambrian rock (Ramapo Mts.); Triassic Redbeds (Brunswick Fm).

 

DESCRIPTION: These exposures show the Ramapo Border Fault separating the PreCambrian Gneiss and Granite of the New Jersey Highlands to the west from the Triassic Fanglomerates of the Brunswick Formation to the east.

     The fault is the western limit of the New Jersey Lowlands (Triassic Basin). The exposed southeast dipping fault plane is striated and polished (Slickensides) from movement of the downthrown block. 

      Conglomerates are composed of large pieces of Lower Paleozoic sedimentary rock that once covered the PreCambrian crystalline rocks to the west in a non-conformity.  (Inliers of these Lower Paleozoic are found within in-folded and in-faulted sections of the PreCambrian crystalline rocks and thus have not been stripped away by erosion.)  Triassic sediments get finer as you proceed east confirming the western source of sediment. The fault is thought to have been active during the deposition of about 10,000 feet of Triassic Brunswick sediments.

 

STUDENT QUESTIONS:

(1) How does a Normal Fault differ from a Reverse Fault?

(2) Why do Triassic sediments get finer as you move away from the sediment source.

(3) How is a Conglomerate defined?

(4) What is an Alluvial Fan and why do you suppose the word “Fanglomerate” was coined?

(5) Are the larger sediments of the Triassic strata rounded or angular? What does your answer say about the energy of the depositional environment?

(6) CHALLENGE: Describe the global geologic events behind the formation of the thick wedge of sediments within the Triassic Basin.

 

SELECTED  REFERENCES:

-Schuberth, Christopher J. 1968, The Geology of New York City and Environs. The Natural History Press, Garden City, New York. 304 pp.

PHOTOS:

Figure 1 - The Ramapo Border Fault. Note the polished fault scarp and slickensides (striations or grooves) in the PreCambrian gneiss and granite due to the frictional grinding of the downdropped block of Triassic sediments (the road surface). 

Figure 2 - Close-up of the slickensides and naturally polished surface of the fault scarp. Field trip leader, Christopher Schuberth is at the right of the photo.

Figure 3 - Fanglomerate composed of Lower Paleozoic sedimentay rock fragments originally derived from strata non-comformably overlying the PreCambrian highlands west of the fault.

Figure 4 - Close-up of coarse and rounded Fanglomerate of the Brunswick Formation.

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