October 4. Palisades of the Hudson

Last month I talked about the intrusive and extrusive igneous rocks associated with the Newark Grabens, the pull-apart basins associated with the initial opening of the Atlantic Ocean. They are found throughout the eastern United States coastal region, but some of the best exposures are in New York and New Jersey, where one of the best known is the Palisades of the Hudson River.

The basaltic igneous rocks that make the Palisades are surrounded by sedimentary rocks. The basalt is in the form of a sill, an igneous body injected pretty much along bedding planes in older sedimentary rocks. If the igneous rock cut across the beds, it would be called a dike. The Palisades Sill is about 1000 feet thick and it crops out for about 50 miles, from Staten Island north through New Jersey and along the Hudson River into New York. Technically it’s a diabase, which means the rock has the composition of basalt but it crystallized at intermediate depth. Gabbro is the name for rocks of similar composition that crystallized at great depth, where there was time for large crystals to grow, and basalt is often reserved for the same kind of magma that solidified very quickly, as in a lava flow on the surface, so its crystals are very small, even microscopic. Diabase is somewhere between them in terms of its texture, or crystal size.   

Palisades sill in Newark Graben (source: NPS)

You may recall that toward the end of last month I talked about the Palisade Disturbance, a time of ongoing faulting that tilted the rocks of the Newark Graben. This was probably a continuation of the extension that would ultimately break Africa and North America apart to form the Atlantic Ocean, but in late Triassic time the rocks were breaking into the basins and tilting the basin fill, but not completely rifting continental crust apart.

The Palisades Sill was injected into the sediments of the Newark Group, the basin fill materials, after the Palisades Disturbance had tilted them. How do we know? One line of evidence is the orientation of columnar joints, cooling features in the basalt, which suggests that the molten magma was injected at an angle about the same as the angle the rocks are tilted today. That is to say, the Newark Group sediments were already tilted when the Palisades Sill was forced between the tilted beds. The alternative would have been that the molten rock was injected into horizontal beds, then cooled, then the whole package was tilted. But that does not seem to have been the case.

The age of the Palisades Sill has commonly been given as about 200 million years, right about at the Triassic-Jurassic boundary, but more recent dates put it at 192 to 186 million years ago, in the early Jurassic. It’s pretty likely that it formed in multiple stages, repeated pulses of injected molten material, which is suggested by some compositional differences through it.

—Richard I. Gibson