View of supercontinent Pangaea. Map courtesy of Institute of Geophysics at The University of Texas Austin.
Early on, as the Basin was subsiding, massive unsorted sediments from the surrounding highlands were pouring into the lowland and later consolidated to form conglomerates, sandstones and mudrocks (shale etc.) known today as the Newark Supergroup. These sedimentary rocks are usually red or orange due to the oxidation of iron oxide minerals that were in the original sediments. There are also several lacustrine formations that represent large lake systems within the Basin.
The tensions associated with the pull-apart basins allowed continued faulting and tilting contemporaneous with the eruption of flood basalts and the emplacement of subsurface dikes and sills due to decompression melting of rocks of the earth’s mantle as they travel upward along thermal plumes.
The best-known igneous structure associated with the basins is the Palisades Sill that trends along the Hudson River for about 50 miles north of New York City. The layered rocks form massive cliffs along the River and are a well-known landform. The Sill is composed of diorite, a magmatic rock with larger crystals than the extrusive basalt and was the first igneous rock to appear in the Basin, perhaps around 200 Ma in the earliest Jurassic.
Still in the early Jurassic, volcanic rocks broke to the surface in the form of eruptions and flood basalts and formed the parent rock of the Watchung Mountains in the northeastern part of New Jersey. Olson (1980) described these episodes as three separate flood basalts that may have filled the entire basin with each eruption. After each major volcanic episode, the basin continued sinking and was again filled with sedimentary rocks with the end result being alternating layers of basalt and red sedimentary rocks. With aborted rifting of the basin, deposition and volcanism ceased and erosion became dominant.
Highlands of the Watchung Mountains with Paterson, New Jersey in the far background. Public Domain photo.
Perhaps more than any other group of minerals found in the Watchung Mountains, the best known are the zeolites (microporous aluminosilicate minerals) and their companions. One cannot attend a rock/ mineral show without noting Watchung specimens of analcime, stilbite, apopolite, chabazite, datolite, heulandite, stilbite and especially of pectolite (sodium calcium silicate) and prehnite (calcium aluminum silicate): both silicates often appear with zeolites. It appears that at Burger’s Quarry the extrusive magma cooled in a lake and the basalt forming pillow lava. Many of the fine zeolite specimens crystalized in the spaces between the pillows.
I continue to search dusty drawers in small rock shops for additional Cahn specimens or mineral labels. If any reader has a Cahn or Wulff label for thaumasite, please contact me.
Olsen, P. E., 1980, Triassic and Jurassic Formations of the Newark Basin in Manspeizer, W., ed., Field studies of New Jersey geology and guide to field trips: New York State Geological Association, 52nd Annual Meeting, Newark, New Jersey, Rutgers University.