Watchung Mountains - Geology

Geology

200 million years ago, magma intruded into the Newark Basin, then an active rift basin associated with the breakup of the supercontinent Pangaea. The magma was initially contained within the sedimentary strata of the basin, forming large intrusions like the Palisades Sill, but it ultimately broke out to the surface through large, episodic eruptions. The Watchung Mountains were originally formed from these eruptions, consisting of three separate flood basalts that may have filled nearly the entire Newark Basin. Each time the basin filled with basalt, which cooled into blocky trap rock, a period of limited volcanic activity followed, allowing sediment to be deposited on top of the previously erupted layer of basalt. In this way, the Newark Basin became layered with alternating strata of Watchung basalt and Jurassic sedimentary rock.

Throughout the early Jurassic, the Newark Basin underwent extensive dipping and folding. The western side of the basin plunged deeper into the crust, tilting the basin’s strata to an angle of between 5 and 25 degrees. Localized deformation of the western edge of the basin along the Ramapo Fault System formed alternating synclines and anticlines that warped the layers of basalt and sedimentary rock.

Erosion began to attack the basin as rifting failed and deposition of new sediments ceased. Over millions of years, erosion ate downward through the tilted rock of the basin, eventually encountering the basalt layers, which are significantly more erosion resistant than the surrounding sedimentary rock. The result of this has been that the exposed edges of the eroding basalt layers have managed to persist longer than the exposed edges of the sedimentary layers, causing them to project prominently above the surrounding surface terrain as high ridges.

Today, the flood basalts are preserved in the synclines adjacent to the Ramapo Fault system. It is in these synclines that the basalt layers are thick and warped into downward dipping trap rock sheets, descending below the current erosional surface of the basin. Notably, the synclines preserve not only the basalt layers, but also some overlying Jurassic sedimentary rock. The largest syncline in the basin, the Watchung syncline, contains the greater portion of the Watchung flood basalts as they appear today. The projecting, eroding edges of the flood basalts preserved in the syncline form the three ridges of the Watchung Mountains. Jurassic sedimentary rock layers between and above the ridges form the Feltville, Towaco and Boonton formations. Elsewhere in the Newark Basin, smaller synclines preserve the Watchung Outliers, additional fragments of the flood basalts and associated overlying sediments that have survived into the modern era.

Because the majority of the Watchung Mountains are composed of extrusive igneous trap rock, they display characteristic columnar jointing and stacked lava flows. These features are readily noted along the eastern faces of the ridges, which often present mural precipices, or vertical escarpments. Similar features can also be seen in the Palisades Sill, although these were formed within the Earth's crust. Additionally, the Watchungs feature not only blocky aa lava, but also ropey and billowing pahoehoe flows.

The magma which generated the Watchungs and the Palisades also formed the intrusive igneous Sourland Mountain in Central New Jersey, as well as a series of smaller outlying volcanic ridges in the region. Cushetunk Mountain, a ring-shaped volcanic mountain between Sourland Mountain and the Watchungs, is of the same geologic lineage.

The Metacomet Mountains in the Connecticut River Basin, another aborted rift valley, came into existence around the same time as the Watchungs, also through extrusive eruptions. While non-contiguous, the two ranges may be considered geologic cousins, having formed under similar circumstances during the rifting of Pangaea. The same erosive and tectonic forces which elevated the Watchungs also served to raise the Metacomets.