Plate Tectonic Framework
The elevation of the Earth's solid surface--including the floors of the oceans--provides an excellent means to introduce plate tectonics [Figure>>]. The Earth's outer shell is subdivided into a number of relatively rigid pieces (plates) that move over the surface of the Earth and with respect to one another. The edges of the plates are called "plate boundaries." The three types of plate boundaries [Figure>>] are: 1) divergent, in which two plates move away from another; 2) convergent, in which two plates move toward one another; and 3) transform, in which two plates slide past another. The movement of the plates at the plate boundaries produces earthquakes and volcanic activity. Divergent plate boundaries are typically found in the ocean basins and are marked by large undersea mountain ranges called mid-ocean ridges or seafloor spreading centers. The middle of the Atlantic Ocean contains such a mid-ocean ridge, which is clearly visible on the global topographic map [Figure>>]. At convergent plate boundaries, one plate may dive (subduct) beneath another plate, forming a trench (the deepest parts of the oceans on the topographic map); alternatively, two plates may collide to form spectacular mountain ranges such as the Alps and Himalayas. The San Andreas fault in California is an example of a transform fault, along which the North American and Pacific plates slide past another [Figure>>].
New Jersey is located on the North American plate, approximately midway between the Mid-Atlantic ridge (the boundary between the North American and Eurasian plates) and the convergent and transform boundaries along the western edge of the North American continent [Figure>>]. Because NJ is situated in the interior of a plate, NJ currently undergoes relatively little earthquake and no volcanic activity, especially compared to the U.S. west coast [Figure>>]. However, this was not the case in the geologic past, as is discussed in more detail below.
Although NJ is situated in the interior of a plate, it is located on a continental margin--the boundary between continental crust and oceanic crust [Figure>>]. This boundary does not coincide with the present-day coastline, but is located near the edge of the continental shelf. The NJ continental margin is a passive continental margin because it is located far from a plate boundary and is tectonically quiet. An active continental margin is located on or adjacent to a plate boundary and is tectonically active; an example would be the western North American continental margin [Figure>>].
Topography shows the elevation of the Earth's surface. The figure at right shows a relief map of the conterminous United States; this image was constructed using digital elevation data. Topographic features in and near New Jersey include the Coastal Plain, Appalachian Piedmont, Appalachian Valley and Ridge, and the Appalachian Plateau. The Coastal Plain has the lowest relief and is adjacent to the Atlantic Ocean (and Gulf of Mexico). The Piedmont generally has low relief--rolling hills and the like. The Valley and Ridge, like its name implies, consist of alternating linear or curvilinear valleys and ridges of moderate relief. This is the eroded remnants of the Appalachian mountain system, which, when it formed over 300 million years ago, probably looked very much like the present-day Himalayas [Figure>>]. The Appalachian Plateau has moderate to low relief, and consists of highly eroded layers of sedimentary rock shed off the Appalachian mountains to the east.
Satellite images of North America that have been processed to show relief also show the topography of North America [Figure>>]. By clicking on the various images, you will zoom in closer and closer to New Jersey.
To set the stage for an examination of NJ geology, we first need to consider the geology of the contiguous United States. On the geologic map at right [Figure>>], the different colors shows the areal extent of different rock units of different geological ages. Relatively complicated geology occurs in areas where numerous colors are juxtaposed in a small area; relatively simple geology occurs in areas where a single color or few colors extend over a large area. The western 1/3 and eastern 1/4 of the US are relatively complex geologically. The midwest is relatively simple geologically. By superimposing the geologic map of the US on the topographic relief map we examined earlier, we can see that there is often a close correlation between geology and topography [Figure>>]. For example, the Coastal Plain and Appalachian Plateau are relatively geologically simple, whereas the Piedmont and the Valley & Ridge are relatively complex [Figure>>].
Geology of New Jersey
We're now ready to look at the geology of NJ in greater detail. The first figure>> shows the NJ part of the US geologic/topographic map along with a delineation of the topographic/geologic provinces, called physiographic provinces. This geologic map shows that the Valley & Ridge as defined on this map has the highest topographic relief and most complex geology, whereas the Coastal Plain has the lowest relief. A more detailed geologic map allows us to refine the physiographic provinces of NJ (northwestern NJ is subdivided into the Valley & Ridge and Highlands provinces) and to discuss their geologic characteristics [Figure>>].
The Valley & Ridge province consists of early Paleozoic-age (Cambrian, Ordovician, Silurian, Devonian) sedimentary rocks. The more erosionally resistant sandstones and conglomerates form topographic ridges; the less resistant shales and limestones form topographic valleys. Geologic structures include folds, reverse faults (thrust faults), and rock cleavage (foliation). These structures formed during one or more phases of the Appalachian orogeny, resulting from plate subduction and collision. The Cambrian and lower Ordovician rocks accumulated on a passive continental margin (much like exists today) [Figure>>]. This passive margin became an active margin in Ordovician time, and produced the first set of Appalachian mountains. This phase of mountain-building is known as the Taconic orogeny. During and after the Taconic orogeny, upper Ordovician, Silurian, and lower Devonian sediments accumulated in a foreland basin, a large depression produced by the weight of the Taconic mountains. These foreland-basin rocks were then deformed in the Devonian Acadian orogeny and Carboniferous-Permian Alleghanian orogeny. The Alleghanian orogeny involved the collision of North America and Africa and heralded the final assembly of the supercontinent of Pangea.
The Highlands province is similar to the Valley & Ridge in terms of topography and geology. In fact, the only difference is that Precambrian metamorphic rocks are present in the Highlands [Figure>>]. These rocks are approximately 1 billion years old, include gneiss and marble, and were deformed in the Grenville orogeny which involved the collision of plates in Precambrian time. These metamorphic rocks were then re-deformed during the same mountain-building phases that affected the Valley & Ridge province.
The Piedmont province consists overwhelmingly of Triassic-Jurassic sedimentary and igneous rocks. A narrow sliver of Precambrian rocks is exposed in southern Mercer county. The Triassic-Jurassic rocks accumulated in a large sedimentary basin known as the Newark rift basin. This basin formed during the breakup of the supercontinent of Pangea. Geologic structures include normal faults, folds, and joints. The igneous rocks--extrusive basalt and intrusive diabase--are more resistant to erosion and form topographic highs. The Watchung Mountains are underlain by basalt; the Palisades and Rocky Hill are underlain by diabase.
After North America and Africa separated in Middle Jurassic time (~175 million years ago), NJ was once again situated on a passive continental margin, which gradually subsided due to (1) cooling of rocks that were heated during rifting and breakup and (2) the loading of sediment deposited along the continental margin. The sediments that accumulated on this passive continental margin were strongly affected by changes in sea level. Sea level was particularly high in Cretaceous time, and the sediments of the Coastal Plain province accumulated during and after this time [Figure>>]. In fact, the boundary between the Coastal Plain and Piedmont provinces corresponds approximately to the location of the coastline in Cretaceous time. Coastal plain sediments consist of sands and clays deposited in a marine or coastal environment. These layers of sediment are inclined very gently to the southeast and do not generally contain any faults or folds. Because these layers are not resistant to erosion, topographic relief in the Coastal Plain is minor.
During the relatively recently ice ages (the last ice sheet began to melt away ~20,000 years ago), sea level was lower than at present, because large volumes of water ordinarily contained in the oceans was trapped in the glacial ice. The ice-age shoreline was located near the edge of the continental shelf [Figure>>]. As the ice sheets melted, sea level rose to its present-day level, and the shoreline migrated northwestward in response to the rising sea level. The northern part of New Jersey was covered by glacial ice during the last major ice age. The thick black line on the geologic map marks the southern limit of this ice sheet [Figure>>].
Tectonic History of NJ and eastern North America
We come full circle in this introduction to NJ geology by reviewing the plate tectonic history of NJ and eastern North America [Figure>>]. The major themes are (A) plate collision during the Grenville orogeny (1 billion years ago), which resulted in the formation of a supercontinent; (B) rifting, supercontinental fragmentation, and opening of the proto-Atlantic Ocean (Iapetus Ocean) around 500 million years ago; (C) subduction, closing of the Iapetus, plate collision, and assembly of Pangea, which formed the Appalachian mountains in at least three distinct phases of mountain-building (Taconic, Acadian, Alleghanian); this process ended between 300 and 250 million years ago; (D) rifting preceding the fragmentation of Pangea, which was responsible for producing the Newark basin (225-175 million years ago); and (E) opening of the Atlantic Ocean (at 175 million years ago) and sedimentation on the passive continental margin (strongly influenced by global sea-level fluctuations), the exposed part of which is the coastal plain.
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