The Geology of Delaware

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Geologic History of the Delaware Piedmont

Fig A. Cross section of eastern North America as it may have looked 543 million years ago, active volcano is offshore.

The Delaware Piedmont is but a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny. This event was triggered by the formation of a subduction zone off the coast of the ancient North American continent that slid oceanic crust on the ancient North American plate beneath oceanic crust on the overriding plate, produced magma, and fueled an arc-shaped chain of volcanoes. This volcanic arc existed in the late Precambrian-early Paleozoic along most of the eastern margin of the ancient North American continent (Fig A). In Delaware, there is some evidence in the Wilmington Complex to suggest that the overriding oceanic plate included a small island cored by continental crust.

Figure B

As convergence continued, most of the sediments deposited on the subducting plate were scraped off to form a thick pile of deformed and metamorphosed rocks. In Delaware this accreted pile of sediments became the Wissahickon Formation. The many amphibolite layers in the Wissahickon suggest that these sediments may have been mixed with ash falls, basalt flows from the volcanoes, or slivers of underlying oceanic crust that were broken off during scraping (Fig B).

Eventually, continued convergence dragged the ancient North American continent into the subduction zone where it collided with the volcanic arc and pushed up a gigantic mountain range (Fig C). The creation of this range signified the end of the Taconic orogeny along the Appalachians. Today the once lofty mountains have eroded away leaving their roots exposed in the rolling hills of Delaware’s Piedmont. The intense metamorphism that occurred when the root zone was deeply buried in the base of the mountain range has obscured most of the rocks’ original features; however, careful study has recognized a series of rock units that represents the ancient continental margin.

Figure C

The amphibolites and “blue rocks” of the Wilmington Complex were formerly a volcanic island that existed seaward of the ancient North American continent about 500 million years ago. The gneisses of the Wissahickon Formation represent sediments deposited in a deep ocean basin between the volcanic island and the continental shelf. The pure white crystalline marble of the Cockeysville Marble is the metamorphosed equivalent of a carbonate bank or reef that formed just off the ancient shoreline. The impure quartzites of the Setters Formation were certainly dirty beach sands, and the Baltimore Gneiss that forms the basement under the Setters and Cockeysville formations is billion-year-old rock, assumed to be a remnant of the ancient North American continent (Figs A, B, and C).

Figure D

The rocks in the Delaware hills are still eroding (Fig D); their surfaces are fractured, broken, and covered with moss and lichen. As the rocks disintegrate, small pieces wash into the creeks and rivers to begin a journey that may take them to the Atlantic Ocean where they will be buried on the continental margin. Millions of years from now subduction may begin again off Delaware’s shore, and these sediments will be caught in another cycle of mountain building and erosion.