Strata and Geologic History

Figure 6: Strata of rock formations in area of Shenandoah National Park. The oldest rock 1,100 million years ago (Ma) is the former base of the Grenville mountains that formed when the supercontinent Rodinia formed and includes Old Rag granite and Pedlar granitic gneiss. The youngest layer of carbonate rocks on the surface have been largely eroded from the area of the Blue Ridge Mountains and were not observed.

fig06 Figure 6: Strata of rock formations in area of Shenandoah National Park. The oldest rock 1,100 million years ago (Ma) is the former base of the Grenville mountains that formed when the supercontinent Rodinia formed and includes Old Rag granite and Pedlar granitic gneiss. The youngest layer of carbonate rocks on the surface have been largely eroded from the area of the Blue Ridge Mountains and were not observed. Figure 31: Time scale of geologic events forming the Blue Ridge Mountains in Shenandoah National Park.

fig31 Figure 31: Time scale of geologic events forming the Blue Ridge Mountains in Shenandoah National Park.

Figure 32: 600-700 Ma (late Precambrian) schematic shows largely eroded Grenville mountain range and rivers/streams carrying and depositing sediment layers that will become the Swift Run Formation overlaying the Pedlar Formation. The rivers drain into the widening Iapetus Ocean. As Rodinia land continued to rift and separate, the basement rock thinned, and magma was ejected via dikes onto the surface to start the formation of the Catoctin Formation.

fig32 Figure 32: 600-700 Ma (late Precambrian) schematic shows largely eroded Grenville mountain range and rivers/streams carrying and depositing sediment layers that will become the Swift Run Formation overlaying the Pedlar Formation. The rivers drain into the widening Iapetus Ocean. As Rodinia land continued to rift and separate, the basement rock thinned, and magma was ejected via dikes onto the surface to start the formation of the Catoctin Formation. Figure 33: 570-600 Ma (late Precambrian) schematic shows continued intermittent lava flows forming layers of the Catoctin Formation with 100’s to thousands of years with deposition of silica-rich sediment in forming sedimentary interbeds. Feeder dikes continue to feed basalt to the surface.

Figure 33: 570-600 Ma (late Precambrian) schematic shows continued intermittent lava flows forming layers of the Catoctin Formation with 100’s to thousands of years with deposition of silica-rich sediment in forming sedimentary interbeds. Feeder dikes continue to feed basalt to the surface.

fig33 Figure 33: 570-600 Ma (late Precambrian) schematic shows continued intermittent lava flows forming layers of the Catoctin Formation with 100’s to thousands of years with deposition of silica-rich sediment in forming sedimentary interbeds. Feeder dikes continue to feed basalt to the surface. Figure 34: During the Cambrian Period (490 to 540 Ma), volcanism and lava flows end, and there is subsistence of the land and westward advance of the Iapetus Ocean with increased presence of rivers, lagoons and shallow seas. Granite mountains continue to weather and erode with deposits of mud, sand and quartz pebbles deposited to become the Weverton Formation.

Figure 34: During the Cambrian Period (490 to 540 Ma), volcanism and lava flows end, and there is subsistence of the land and westward advance of the Iapetus Ocean with increased presence of rivers, lagoons and shallow seas. Granite mountains continue to weather and erode with deposits of mud, sand and quartz pebbles deposited to become the Weverton Formation.

fig34 Figure 34: During the Cambrian Period (490 to 540 Ma), volcanism and lava flows end, and there is subsistence of the land and westward advance of the Iapetus Ocean with increased presence of rivers, lagoons and shallow seas. Granite mountains continue to weather and erode with deposits of mud, sand and quartz pebbles deposited to become the Weverton Formation. Figure 35: Further into the Cambrian Period, marine sediments continued to be deposited as sandy muds and clays and became the metasandstones and phyllites of the Hampton formation. White sand from beaches and bar sands accumulated up to 600 to 1000 feet to become Erwin Formation quartzite.

Figure 35: Further into the Cambrian Period, marine sediments continued to be deposited as sandy muds and clays and became the metasandstones and phyllites of the Hampton formation. White sand from beaches and bar sands accumulated up to 600 to 1000 feet to become Erwin Formation quartzite.

fig35 Figure 35: Further into the Cambrian Period, marine sediments continued to be deposited as sandy muds and clays and became the metasandstones and phyllites of the Hampton formation. White sand from beaches and bar sands accumulated up to 600 to 1000 feet to become Erwin Formation quartzite.

Figure 36: In the Mid- to Late-Cambrian Period and proceeding into the Ordivician Period (445 to 485 Ma), the ocean moved further West and lime-precipitating marine organisms laid down carbonate layers that reached depths of 12,000 feet. At the time before closing of the Iapetus Ocean and formation of Pangea, carbonate, sedimentary, and volcanic layers above what is now the Pedlar formation of gneiss may have reached up to 2.5 miles thick. Geothermal forces alone would have resulted in metamorphic changes in the deep layers of rock.

fig36 Figure 36: In the Mid- to Late-Cambrian Period and proceeding into the Ordivician Period (445 to 485 Ma), the ocean moved further West and lime-precipitating marine organisms laid down carbonate layers that reached depths of 12,000 feet. At the time before closing of the Iapetus Ocean and formation of Pangea, carbonate, sedimentary, and volcanic layers above what is now the Pedlar formation of gneiss may have reached up to 2.5 miles thick. Geothermal forces alone would have resulted in metamorphic changes in the deep layers of rock.

Figure 37: As the Iapetus Ocean closed from 450 to 300 Ma, enormous diastrophic forces came to bear in the region of Shenandoah National Park resulting in compression, extension and shearing forces that result in folding, faulting, and warping of strata with metamorphism within deeper layers. Weaker forces occurred during the Taconic (500-440 Ma) and Acadian (375-325 Ma) orogenies when island arcs collided with ancient North America (Laurentia). The Alleghanian orogeny (320-250 Ma) had a much more profound effect as Gondwana (ancient Africa) and Laurentia (ancient North America) collided giving rise to the supercontinent Pangea and the Appalacian mountains which were as high as the current Himalayas. Diastrophic forces resulted in strata being force from the East to the West as an overturned thrust fault. Weathering and erosion have worn down the strata to what is currently seen.

fig37 Figure 37: As the Iapetus Ocean closed from 450 to 300 Ma, enormous diastrophic forces came to bear in the region of Shenandoah National Park resulting in compression, extension and shearing forces that result in folding, faulting, and warping of strata with metamorphism within deeper layers. Weaker forces occurred during the Taconic (500-440 Ma) and Acadian (375-325 Ma) orogenies when island arcs collided with ancient North America (Laurentia). The Alleghanian orogeny (320-250 Ma) had a much more profound effect as Gondwana (ancient Africa) and Laurentia (ancient North America) collided giving rise to the supercontinent Pangea and the Appalacian mountains which were as high as the current Himalayas. Diastrophic forces resulted in strata being force from the East to the West as an overturned thrust fault. Weathering and erosion have worn down the strata to what is currently seen.