PALISADES SILL & Contiguous Sediments

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FIELD TRIP STOP – PALISADES SILL, NEW JERSEY (Triassic Sill with Magmatic Differentiation injected within sediments)

LOCATION:  From Manhattan, cross the George Washington Bridge. Get off the first exit and proceed south to Edgewater, N.J. More specific directions can be found in Schuberth (1968).

 

GEOLOGIC FEATURES: Triassic Diabase Sill; Magmatic Differentiation of Olivine;; Varied crystal sizes in igneous body due to cooling rates; Hornfels and Contact Metamorphism; Triassic Sediments; River Channel Deposits.

 

DESCRIPTION: During the Triassic Period, magma rose towards the surface about 200 mya to be injected between host beds of Triassic sediments. Heating at the contacts between the magma and the cooler sediments caused both baking of the sedimentary rock and rapid cooling of the magma at the margins of the sill producing a fine (aphanitic) igneous texture. Longer periods of cooling near the center of the sill resulted in the formation of progressively larger crystals (phaneritic texture) towards the center of the sill. Early crystallization of Olivine from the melt (Bowen’s Reaction Series) resulted in the initial settling of that heavy mineral towards the bottom of the sill where the Olivine would come to rest on the top of the already chilled igneous margin found near the lower contact. Due to the instability of Olivine at the Earth’s surface, relatively rapid weathering of the unstable Olivine upon uplift and exposure resulted in a distinctive deeply eroded “Olivine Rotten Zone.”

STUDENT QUESTIONS:

(1) How does a Sill differ from a Lava flow?

(2) Describe the process by which large igneous crystals (coarse-grained or phaneritic texture) are formed in an igneous body versus the process forming exceedingly small crystals (fine-grained or aphanitic texture.

(3) How does regional metamorphism differ from contact metamorphism?

(4) Why is a mineral formed early in the cooling history of a magma more unstable at surface conditions as compared to minerals formed much later in the magma’s cooling history?

(5) What is the process of Magmatic Differentiation?

(6) CHALLENGE: How can the Bowen’s Reaction Series result in several different magmas (and rocks) of different compositions to be produced from an initial single homogeneous magma?

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:

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Figure 1 - Looking west from Manhattan across the Hudson River, the George Washington Bridge connects New York City to the near vertical wall of the Palisades, New Jersey. The Palisades, a Diabase Sill, extends nearly 40 miles mainly along the western border of the Hudson River. The Palisades Sill dips westward between 10-15 degrees, eventually steepening to become a dike.  Triassic sediments above the Sill have been eroded away in the photo above but are found in increasing thicknesses as one continues westward.

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Figure 2 - The lower part of the "Olivine Rotten Zone."  Notice the increased weathering towards the bottom of the photo where the concentration of unstable Olivine is the highest.

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Figure 3 - A more distant view of the above photo.  The "Olivine Rotten Zone" lies above the fine-grained chilled lower contact of the Palisades Diabase.

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Figure 4 - In places, the "Olivine Rotten Zone" has been reinforced with brick or concrete to prevent erosion.  In places, swallows have made holes in the the most unconsolidated parts of this zone.

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Figure 5 - Typical sedimentary redbeds of the Stockton Formation found beneath the Sill and outcropping along the trail found west of the Hudson River.  This rock formation is composed of arkose sandstones and red shales indicative of streams and swamps. The Stockton lies unconformably on top of the metamorphic rocks of the New York City Group.

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Figure 6 - A stream channel in the Stockton Formation.

Figure 7 - Stream channel deposits in the Stockton Formation.  Note the scattered cross-bedding within the channel.

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Figure 8 - Upper contact of the Palisades Sill's fine-grained Chilled Zone with the Contact Metamorphosed layered strata of Stockton (Lockatong?) Formation.

Figure 9 - View of the upper contact of the Palisades Sill with the overlying Trassic Sediments as exposed along Interstate-80.