Palisades Sill - Geology

Geology

The end of the Triassic Period saw large-scale rifting during the break-up of Pangaea. What is now eastern North America began to separate from what is now north-western Africa, creating the young Atlantic Ocean. Magma was generated through decompression melting, and a portion of it was intruded into the sandstones and arkoses of the Stockton Formation within the Newark Basin, one of the Eastern North America Rift Basins. The magma would eventually solidify and, after millions of years, the overlying rocks would be uplifted and eroded, exposing the Palisades Sill as we know it today.

The composition of the sill is that of diabase, although its mineral assemblage is not uniform throughout the thickness of the body. The mineralogy of the sill consists principally of plagioclase feldspar, several varieties of pyroxenes, and olivine, with minor biotite, titanite, zircon and oxides.

Most researchers report that, generally speaking, the sill becomes progressively differentiated as one moves away from either the upper or lower contact. The "sandwich horizon" is the term given to the central region where both cooling fronts met; it is here where the diabase is the most differentiated.

The most intriguing geological feature of the sill is a 10-meter-thick olivine-rich zone roughly 10 meters (30 ft) from the lower contact. The modal percent of olivine goes from 0-2% within the main body of the sill to up to 28% within this layer. It is the origin of this layer, and subsequently, the sill as a whole, that has generated much of the attention, as well as the varying origin theories proposed for the intrusion (see below).

There is an average stratigraphic thickness of 300 meters (~1000 feet), with the famous cliffs rising 100 meters (300 ft) on average above sea level. The intrusion dips between 10 and 15 degrees westward for most of its length. It has been determined through stratigraphic studies that the sill was intruded at a depth of approximately 3-5 kilometers. These studies also concluded that the sill was emplaced in a position nearly identical to its current one (10-to-15-degree dip); this is further confirmed by the still-vertical orientation of the columnar jointing.

It has been proposed that the Watchung basalt flows of the Watchung Mountains are extrusive eruptions of the same magma that created the Palisades Sill. Magnetic and gravity measurements have indicated the presence of a large subsurface dike between the Palisades intrusion and the Ladentown basalt, an extrusive body of Watchung basalt north of Suffern, New York. More recently, the various Watchung flows have been correlated to geochemically distinct layers within the Palisades sill, bolstering the theory that eruptions of the Palisades magma were responsible for the episodic flood basalts of the Newark Basin.