1. Regional Tectonic History of Northern Virginia
The objective of this one-day field trip is to examine the field relations that allow us to characterize the major physiographic provinces of northern Virginia and to interpret the tectonic history of this area. We will visit outcrops in the Coastal Plain, Piedmont (including a Mesozoic basin), and Blue Ridge provinces (fig. 1), for the purpose of comparing and contrasting their geology.
We will discuss tectonic events in terms of the Wilson Cycle of ocean-basin opening (rifting) and closing (mountain building), the final product being the mountain belt. The Appalachian Mountains are an excellent example of the repetitive nature of the Wilson Cycle (fig. 2). The tectonic events we will discuss include (oldest to youngest) Middle Proterozoic (Grenville) mountain building, Late Proterozoic (Proto-Atlantic or Iapetus) rifting, Paleozoic Appalachian (Taconic, Acadian, Alleghanian) mountain building, and Mesozoic (Atlantic) rifting.
The dynamic nature of the Earth is a consequence of plate tectonics. These plates are made of rigid continental and oceanic lithosphere. This lithosphere overlies an asthenosphere that is in constant motion. The lithospheric plates experience tensional, compressional, and shearing forces that lead to processes such as rifting, collisional tectonics, and transform faulting. In the process, new ocean floor may be created as well as chains of volcanic islands, areas of earthquake activity, and new mountain ranges. The continents themselves may shift their position as plates move. All of this affects the shape of the land and the distribution of rocks, minerals, fossils, climate, and natural resources. Northern Virginia has experienced several major tectonic episodes that are now recorded in the local rock record and may be visited at many places. The sites to be visited on this trip are selected on the basis of their significance relative to the tectonic history. These sites also illustrate the field relations that allow the recognition of the relative timing of these events.Geologic History
Over one billion years ago in the Mesoproterozoic, narrow strips of land (microcontinents, volcanic arcs, and suspect terranes) collided with and compressed the eastern edge of the then developing North American continent. The rocks from these events are part of the Grenville province of the Precambrian Canadian Shield of North America. This collision between proto-North America and other continents led to the formation of a supercontinent called Rodinia. The formation of Rodinia also resulted in the formation of a mountain range: the Grenville Mountains, now the Grenville province of the Canadian Shield. This mountain-building event is called the Grenville orogeny. Locally, these rocks are found in the Blue Ridge province and are considered to be the oldest rocks in northern Virginia. They are exposed at the surface only after much weathering and erosion of overlying younger rock. Magmatic processes that accompanied this orogeny produced molten rock that was injected into the crust. These igneous rocks, along with the sediments that were eroded from the eastern margin of the Precambrian shield before this collision, were deformed and metamorphosed. We will see metaigneous rocks of Grenville age below the unconformity at Stop 9.
Following the Grenville orogeny, tensional forces associated with changes in mantle convection and (or) other processes led to a breakup of the Rodinian supercontinent. This latest Precambrian (Neoproterozoic) and Early Cambrian rifting led to the opening of the Proto-Atlantic or Iapetus Ocean that predates the Atlantic Ocean. Basaltic rocks that formed during this rifting (and were subsequently metamorphosed during the Paleozoic) are now part of the Catoctin Formation and will be visited at Stop 8. During rifting, fragments of the Grenville continental crust were broken off and became islands, later to be reunited with North America by subsequent closure of the ocean. The Goochland terrane of the Piedmont province may be one of these Grenville fragments (Spears and others, this volume).
As the Iapetus Ocean continued to widen, the rift margin became passive. Sediment continued to be deposited on the eastern edge of the continent, and a broad clastic shelf developed (Chilhowee Group). As the Grenville Mountains were eroded during the Cambrian, the source of clastics diminished and deposition of limestones predominated into the Early Ordovician. Locally, these limestones can be seen in the Frederick Valley and the Shenandoah Valley but will not be seen on this trip. Information about this part of the tectonic history of Virginia can be found in Fichter and Diecchio (1993) and in the additional resources listed at the end of this field guide. During the early Paleozoic the east coast of North America was aligned parallel to the equator that passed through the central part of the United States, roughly coincident with the present-day longitude of Kansas.
During the Middle Ordovician, the plate movements reversed, Iapetus began to close, and once again, continental plates began to converge. As Gondwana approached North America, a subduction zone formed and a volcanic arc complex developed off the east coast of North America. Continental fragments like the Goochland terrane (Spears and others, this volume), that had previously detached from North America, were now caught along with the volcanic arc between North America and Gondwana. Over time, the volcanic islands and continental fragments were thrust back onto North America, eventually to become today's Piedmont terrane. The carbonate and clastic rocks that were deposited on the eastern edge of North America during the Cambrian and Ordovician were now caught between the Piedmont and the eastern edge of North America, and subsequently were compressed. This entire compressional episode, like the earlier Grenville orogeny, resulted in thrust faults, folds, felsic intrusions (like the Occoquan Granite of Stop 3), volcanics (like the Chopawamsic Formation of Stop 2), and metamorphism (evident at Stops 2–5). All of this activity was part of a mountain-building episode known as the Taconic orogeny. This was the first phase in the building of the Appalachian Mountains.
We will not visit the Valley and Ridge province on this trip; however we will briefly discuss its geology to complete the story. Further information can be found in Fichter and Diecchio (1993) and in the additional resources listed at the end of this field guide. The results of these events can be seen in sediments deposited west of the Blue Ridge, in the Valley and Ridge province, and in igneous and metamorphic rocks and deformation in the Piedmont. Of note, two additional mountain-building periods, the Acadian orogeny and the Alleghanian orogeny, occurred during the Paleozoic Era. These orogenies are associated with continued collision and the resulting folds, faults, intrusions, and metamorphism as occurred during the earlier Taconic orogeny. While all this mountain-building activity was taking place, sediments were accumulating west of the Blue Ridge, in the Appalachian basin, which developed on the eastern edge of North America in part as a result of loading by thrust sheets and sediment. These sediments were themselves folded and faulted during the orogenies. At the end of the Paleozoic Era, the final result of the full collision of North America and Gondwana was the formation of the Appalachian Mountains on the supercontinent of Pangea.
Pangea existed during most of the Permian and Triassic. During the Triassic, the plates once again began to pull apart. Rifting began to break up Pangea. One system of rifts opened up along the extent of today's east coast, to become the Atlantic Ocean. This rift system included intracontinental rift basins known as the Mesozoic basins. Here in northern Virginia, one such basin is known as the Culpeper basin (Stops 5–7).
As rifting progressed, erosion of the higher land on either side of the Mesozoic basins produced a variety of sediments that accumulated in fluvial, deltaic, and lacustrine environments (Stops 5–7). The rifting also produced mafic igneous intrusions and volcanism (mafic dikes of Stop 3 and diabase sill of Stop 6) in the basins. Locally the igneous rocks are more resistant to erosion; thus as the sedimentary rocks have been eroded many of these igneous rocks, even intrusions formed at depth, are expressed today as topographic highs.
The Mesozoic basins eventually became inactive as the continental margin became passive. However, the Atlantic continued to open along the Mid-Atlantic Ridge and continues even today. During the Mesozoic, sediments also began to be deposited on the new continental margin, continued to be deposited through the Tertiary, and are still being deposited today. These sediments are now part of the Coastal Plain province and the continental shelf. We will see Coastal Plain deposits at Stops 1 and 2.Acknowledgments
This field guide has been reviewed and improved by Stephen W. Kline of Arkansas Tech University.References Cited
Aleinikoff, J.N., Horton, J.W., Jr., Drake, A.A., Jr., and Fanning, C.M., 2002, Shrimp and conventional U-Pb ages of Ordovician granites and tonalities in the central Appalachian Piedmont; Implications for Paleozoic tectonic events: American Journal of Science, v. 302, p. 50–75.
Drake, A.A., Jr., Froelich, A.J., Weems, R.E., and Lee, K.Y., 1994, Geologic map of the Manassas quadrangle, Fairfax and Prince William Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ–1732, scale 1:24,000.
Fichter, L.S., 1993, The geologic evolution of Virginia: National Association of Geology Teachers Short Course, Notebook of Illustrations.
Fichter, L.S., and Diecchio, R.J., 1993, Evidence for the progressive closure of the Proto-Atlantic Ocean in the Valley and Ridge province of northern Virginia and eastern West Virginia: National Association of Geology Teachers, Eastern Section Meeting Field Trip Guidebook, Harrisonburg, Va., p. 27–49.
Kline, S.W., Lyttle, P.T., and Schindler, J.S., 1991, Late Proterozoic sedimentation and tectonics in northern Virginia, in Schultz, Art, and Compton-Gooding, Ellen, eds., Geologic evolution of the Eastern United States, Field Trip Guidebook, NE-SE GSA, 1991: Virginia Museum of Natural History Guidebook 2, p. 263–294.
Lee, K.Y., and Froelich, A.J., 1989, Triassic-Jurassic stratigraphy of the Culpeper and Barboursville basins, Virginia and Maryland: U.S. Geological Survey Professional Paper 1472, 52 p.
Mixon, R.B., Southwick, D.L., and Read, J.C., 1972, Geologic map of the Quantico quadrangle, Prince William and Stafford Counties, Virginia, and Charles County, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ–1044, scale 1:24,000.
Seiders, V.M., and Mixon, R.B., 1981, Geologic map of the Occoquan quadrangle and part of the Fort Belvoir quadrangle, Prince William and Fairfax Counties, Virginia: U.S. Geological Survey Miscellaneous Investigations Series Map I–1175, scale 1:24,000.
Southworth, Scott, Burton, W.C., Schindler, J.S., and Froelich, A.J., 2000, Digital geologic map of Loudoun County: U.S. Geological Survey Open-File Report 99–150, 1 CD-ROM.Additional Resources
Cecil, K.K., Whisonant R.C., and Sethi, P.S., 2000, Teacher's guide for the geology of Virginia: Charlottesville, Virginia Division of Mineral Resources, 132 p.
Baedke, S.J., and Fichter, L.S., 1999–2000, The geological evolution of Virginia and the mid-Atlantic region: available online at http://csmres.jmu.edu/geollab/vageol/vahist/
Roberts, Chad, and Bailey, C.M., 1997–2003, The geology of Virginia: available online at http://www.wm.edu/geology/virginia/
Sethi, P.S., Whisonant, R.C., and Cecil, K.K., 1999, Geology of Virginia, CD-ROM 1, Introduction and geologic background: Charlottesville, Virginia Division of Mineral Resources, 1 CD-ROM.
Sethi, P.S., Whisonant, R.C., Cecil, K.K., and Newbill, P.L., 2000, Geology of Virginia, CD-ROM 2, Coastal Plain: Charlottesville, Virginia Division of Mineral Resources, 1 CD-ROM.
Sethi, P.S., Whisonant, R.C., Cecil, K.K., and Newbill, P.L., 2000, Geology of Virginia, CD-ROM 3, Piedmont and Blue Ridge: Charlottesville, Virginia Division of Mineral Resources, 2 CD-ROMs.
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