Granton Sill Intrusion into Sedimentary

GRANTON SILL and Contiguous Sediments

FIELD TRIP STOP – GRANTON SILL INTRUSION INTO TRIASSIC SEDIMENTARY BEDS

LOCATION: Found in North Bergen, New Jersey near 80th St. and Tonnelle Ave. More specific directions can be found in Schuberth (1968).

GEOLOGIC FEATURES: Triassic Granton Diabase Sill; Triassic Lockatong Formation; Triassic Sediments; possible Xenolith; Cyclothem, Decompression Melting.

DESCRIPTION: At this exposure the Granton Sill is seen to be injected into and between beds of the Lockatong Formation. . The Granton Sill has basically the same diabase composition as the Palisades which is found a few miles to the east. The Lockatong is comprised mainly of black shales, red, purple, gray or black argillites, and thinly bedded sansdstones with occasional limestones. Environments generally represented quiet water lakes, ponds, or swamps with numerous fresh water fossils. Sediments are thought to have been deposited in cycles ("cyclothems") representing alternating wet and dry climates, each cycle lasting about 20,000 years. Origin of the Granton Sill's magma (as well as the Palisades Sill and Watchung Basalts of the Newark Basin) is commonly thought to be related to rifting events (and decompression melting) associated with the breakup of Pangaea.

STUDENT QUESTIONS:

(1) What is suggested by the diabase of the Granton Sill and Palisades Sill having the same composition?

(2) What is the cause of the black color in some of the shales of the Lockatong Formation.

(3) What is a Xenolith and how is it formed?

(4) What is the difference between a shale and an argillite?

(5) CHALLENGE: Discuss a possible large-scale phenomenon that might account for periodic depositional sequences such as the cycles within the Lockatong.

(6) CHALLENGE: :Explain the relationship of rifting to the generation of magma by "decompression melting."

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 - Outcrop of cyclic sedimentation within the Lockatong Formation illustrating alternations of black carbonaceous shales and lighter colored sediment.

IGNEOUS SILL

SEDIMENTARY LAYERS

Figure 2 - Notice the forceable intrusion of the dark homogeneous sill from the left (west) into and above the layered sediments of the Lockatong Fm. to the east. Sediments above the sill have been eroded away. As with the Palisades and Stockton Fm. beds are inclined to the west at about 15 degrees.

DIABASE SILL

XENOLITH?

Figure 3 - Here, an angular piece of the layered Lockatong is surrounded above and below by the diabase (right half of photo). If the piece were found to be completely torn off and incorporated within the diabase, it would be considered a Xenolith.