The (Brief) Geologic Story of the

Chapel Hill, Hillsborough and Durham Area

 

Introduction

The Chapel Hill, Hillsborough and Durham area is underlain by two basic rock types.

1) Older (approximately 630 to 615 million years old), very resistant, slightly metamorphosed crystalline rocks of volcanic (extruded out onto the earth’s surface) and intrusive (solidified from magma at depth in the Earth) origin.  These rocks are part of the Carolina terrane and form the “highlands” in the Chapel Hill, Hillsborough and Durham area.

2) Younger, less resistant, not metamorphosed sedimentary rocks (approximately 220 million years old).  These rocks are part of the Triassic basin and underlie the “lowlands” of the Chapel Hill, Hillsborough and Durham area.

The rocks of the region are best exposed along the areas many rivers and streams.  For example: 

The Little River:
Along the Little River, the older, very resistant rocks of volcanic and intrusive origin (Carolina terrane) are present from it’s headwaters in northern Orange and Durham Counties to just east of the Little River reservoir dam near the Treyburn development.  The younger, less resistant, sedimentary rocks (Triassic basin) are present east of Old Oxford Road to where the Little River flows into Falls Lake (Figure 1).  Little River Regional Park is located in the area dominated by the older volcanic and intrusive rocks within the Carolina terrane. 

The Eno River:
The older, very resistant rocks of volcanic and intrusive origin (Carolina terrane) are present along the Eno River from it’s headwaters in northern Orange County to just east of West Point on the Eno Park in Durham County.  The younger, less resistant, sedimentary rocks (Triassic basin) are present east of West Point on the Eno Park to where the Eno River flows into Falls Lake (Figure 1).  Eno River State Park is located in the area dominated by the older volcanic and intrusive rocks.

New Hope Creek, Bolin Creek and Morgan Creek:
New Hope Creek, Bolin Creek and Morgan Creek all have their headwaters within the older, very resistant rocks of volcanic and intrusive origin (Carolina terrane).  As these creeks flow toward the east, they flow out of the Carolina terrane and into the younger, less resistant sedimentary rocks of the Triassic basin.  The creeks flow out of the older Carolina terrane rocks and into the younger rocks of the Triassic basin: just upstream of Erwin Road on New Hope Creek, just east of Franklin Street on Bolin Creek and in the vicinity of Mason Farm Biological Reserve on Morgan Creek.

Figure 1:  Map showing areas underlain by the older volcanic and intrusive rock (the Carolina terrane) and younger sedimentary rocks (Triassic basin).

Simplified geology

The Geologic Story

 

The Birth of a Volcanic Island Arc

The older, very resistant rocks of volcanic and intrusive origin of the Carolina terrane (Figure 1), were once part of a volcanic island arc (called the Carolinia island arc by geologists).  This volcanic island arc formed over 630 million years ago off the coast of the ancient continent called Gondwana (Figure 2) hundreds of miles from ancient North America (Laurentia).  Gondwana included portions of the present day African, South American and Antarctica continents.  The island arc formed in an oceanic-oceanic crust convergence zone.  One oceanic plate was pushed under the other and was forced deep in the earth into the mantle.  During the subduction of the oceanic plate a portion of the mantle began to melt (Figure 3).  This melting formed magma (molten rock) that slowing rose to near the surface (within 1-5 miles) of the sea floor settling in magma chambers all along the length of the subduction zone.

Figure 2: Sketch of the Earth approximately 630 million years ago when the Carolinia volcanic island arc was active.Paleoreconstruction

Figure 3: Diagram of an oceanic - oceanic plate convergence zone showing subduction trench and volcanic island arc.

Ocean-Ocean convergence

 

Above the magma chamber the earth began to bulge and swell as underwater mountains began to emerge from the ocean floor along the length of the forming volcanic island arc (Figure 4). 

 

Figure 4: Cross-sectional sketch of a volcanic island.

volcanic island

Some of the magma from the deep seated magma chambers moved toward the surface of the ocean floor along cracks (faults and fractures) and erupted onto the sea floor.  These underwater eruptions discharged billions of tons of lava, ash and volcanic debris on the sea floor building enormous piles of volcanic debris many 1000’s of feet thick.  Some of the magma that remained deep in the Earth would have cooled very slowly forming individual mineral grains that are visible with the naked eye.  This slow cooling of the magma formed igneous intrusive rock types like granite, granodiorite and diorite.  Magma that migrated all the way to the surface of the Earth formed lava flows that cooled fast and did not form individual mineral grains visible with the naked eye.  This rapid cooling of the magma formed igneous rock types like dacitic, andesitic and basaltic lavas.

The piles of volcanic debris built up high enough that volcanic islands began to break the surface of the ocean.  More eruptions followed building the islands larger and larger.  As these volcanic islands emerged above the ocean surface, erosion began working on their destruction.  Existing volcanic and intrusive rocks were eroded and the resulting sediment was carried down the steep sides of the volcanic islands into the sea.  The eroding of the islands is evident in the presence of sedimentary rock types (mudstones, siltstones and conglomerate) within the older Carolina terrane rocks in the Chapel Hill, Hillsborough and Durham area.  At the same time that the volcanoes were eroding, they were still actively erupting ash and lava.  As such, the mudstones, siltstones and conglomerates are interlayered in places with volcanic tuffs and lava flows. 

 

Hydrothermal Alteration

This cycle of volcanism and erosion would have been repeated hundreds (if not thousands) of times during the life of the volcanic islands. Other blobs of magma would have intruded into buried ash and other volcanic deposits from previous eruptions. Rainwater and sea water would have percolated downward through the rocks to high-temperature regions surrounding hot magma. There, the water was heated, became less dense and rose back to the surface along fissures and cracks. When the heated water reached the surface of the Earth, hot springs, geysers and fumaroles likely formed (collectively called hydrothermal activity). This process is currently taking place in Yellowstone National Park (e.g. Old Faithful geyser). The hydrothermal activity extracted silica and other minerals and elements from some rock types or added silica to other rock types forming hydrothermally altered rocks. Evidence of hydrothermally altered rock is present in the pyrophyllite deposits associated with Occoneechee Mountain just south of Hillsborough. The altered ash was later metamorphosed into the pyrophyllite. Another common mineral that sometimes forms from hydrothermal alterations is quartz. Quartz is a very resistant mineral and since it is resistant, it can be found in many places in the Chapel Hill, Hillsborough and Durham area. After other rock types have eroded away the quartz is left behind.

Primitive Life in the Chapel Hill, Hillsborough and Durham Area

This time period in Earth’s history saw the rapid spread of life forms across the Earth’s seas. Some of the earliest organisms were soft-bodied worm like creatures. Some of these early life forms probably crawled around in the sediment and volcanic ash material that now forms the rocks of the Chapel Hill, Hillsborough and Durham area. In the 1970’s a geology graduate student found some interesting impressions in rock along the South Fork of the Little River (not far from the Little River Regional Park). James Wright (the graduate student) and Lynn Glover (his advisor) from Virginia Tech (Virginia Polytechnic Institute and State University) showed the impressions to Preston Cloud, an internationally known expert on early life forms. Cloud et al., (1976) later identified the impressions as Vermiforma antiqua. At the time, this fossil was scientifically very significant and was identified as the oldest yet known from North America. The slab of rock containing the impressions was removed to the U.S. Geological Survey in Reston, VA for display and examination. The slab is currently part of the Smithsonian Institution collection in Washington, DC. A small fragment of the slab with an impression can be seen on display at the North Carolina Museum of Natural Sciences in Raleigh. Since the publishing of the Cloud et al., (1976) paper, other fossils have been found in North America and the origin of the impressions from the Little River have been brought into question (Seilacher et al., 2000). Other experts still believe the Little River fossils to be true fossils and represent an important piece to the geologic history of the Chapel Hill, Hillsborough and Durham area and North Carolina.


The End of Volcanism in the Chapel Hill, Hillsborough and Durham Area and the Collision of the Volcanic Island Arc with another Island.

Volcanic activity appears to have decreased and ended approximately 610 million years ago in the Chapel Hill, Hillsborough and Durham area.  Another period of volcanism took place on the Carolinia volcanic arc but the evidence for this activity is mainly exposed near the Virginia/North Carolina State line.  Eventually the volcanic islands were eroded and slowly sank beneath the sea.  

Sometime between Ca. 579 – 554 million years ago (Pollock, 2007) the volcanic arc that carried the rocks of the Chapel Hill, Hillsborough and Durham area smashed into (interacted with) another island that caused the layers of tuffs and lavas, sediments and intrusive rocks to be folded and undergo low grade metamorphism (changed through heat and pressure).  The metamorphism turned the rocks into metatuffs, metamorphosed lavas, metagranodiorite, etc.  Known as the Virgilina deformation (Glover and Sinha, 1973), the rocks in the Little River area were folded into a set of large anticlines and synclines (Figure 5) and developed an almost vertical foliation (planar features from metamorphism) (Hibbard et al., 2000).  Evidence of the Virgilina Deformation is the presence of fin-shaped outcrops of rock in a few locations in the Chapel Hill, Hillsborough and Durham area.  The “fins” of rock are actually the original layering now turned on end.

Figure 5: Sketch of folded rock layers into anticlines and synclines

fold style CT

Renewed Volcanism to the West

Following the Virgilina Deformation a new chain of volcanoes began to form to the west of the present day Little River area.  The rocks of the Asheboro area and to the southwest into the Uwharrie Mountains and Morrow Mountain State Park area are part of an approximate 550 to 530 million year old volcanic island arc.  The rocks of the Uwharrie Mountains and Morrow Mountain State Park are younger but very similar to the rocks of Little River area.  These volcanoes, like the volcanoes of the Little River area went extinct and were eroded and sank beneath the ocean and were covered by sediments.  As this renewed volcanism was taking place in the Uwharrie Mountains, the volcanic island arc that the volcanoes were riding on was slowing traveling toward the Ancient North American coast.  By 450 million years ago the island arc was in the process of slamming into (accreting with) ancient North America.

Collision with Ancient North America

Approximately 450 million years ago, the volcanic island arc that included the rocks of the Chapel Hill, Hillsborough and Durham area and the Uwharrie Mountains slammed into Ancient North America forming a coast range set of mountains (Figure 6).  Geologists call the land mass that consisted of the extinct volcanic arc of the Chapel Hill, Hillsborough and Durham area, the Uwharrie Mountain area and other related areas the Carolina Zone (Hibbard et al., 2002).  This collision deformed the rock layers into large anticlines and synclines and metamorphosed the rocks.

Figure 6: Diagram showing volcanic island arc (Carolina Zone) colliding with ancient North America.

Carolina Zone collides

 

The Collision of Gondwana and the Formation of the Supercontinent Pangea

Out in the ancient Atlantic Ocean (called the Rheic Ocean by geologists) the continent of Gondwana was approaching (Figure 7).  Approximately 300 million years ago the African side of the ancient continent of Gondwana slammed into the Ancient North American continent (Laurentia) forming the 1000 mile long (from Newfoundland to Alabama) Appalachian Mountain chain and the supercontinent Pangea (Figure 8).  

Figure 7: Diagrams showing the volcanic island arcs of the Carolina Zone accreted to ancient North America and collision with the ancient Africa portion of Gondwana with ancient North America (Laurentia).

Africa collides

 

 

Figure 8: Sketch of the supercontinent Pangea.

Pangea and NC

The Rifting of Pangea and the Triassic Basin

When the Supercontinent Pangea began to split apart approximately 245 million years ago, a system of rift basins (similar to the modern day East African Rift system) were formed all along the east coast of North America (Figure 9).  Called the Newark Rift System, the splitting apart of Pangea formed the Atlantic Ocean and several inland fault bounded rift valleys. 

The faulting that occurred during the split-up of Pangea formed many large and small faults.  Often the faults were zones were silica rich fluids traveled.  With time the silica in the fault zones precipitated out and formed thick quartz lined fault zones or fractures.  Some of the white colored quartz pebbles, cobbles and boulders present in the Chapel Hill, Hillsborough and Durham area may have originated from quartz mineralization during brittle faulting.  (Note: rock composed of quartz is very common in the Chapel Hill, Hillsborough and Durham area.  Quartz mineralization from brittle faulting and, as mentioned in another section, hydrothermal alteration may also be responsible for the presence of quartz in an area.)

Land to either side of the rift basin began to erode rapidly filling the fault bounded lowlands with sand, silt and clay.  The deposits of sand, silt and clay later turned into the red to maroon colored sandstones, siltstones and mudstones common in the basin.

Figure 9: Sketch of rift basins along the Atlantic Ocean.

Rift basins in NC

 

Dikes and Sills Intrude the Triassic Basin and into the Volcanic and Intrusive Rocks

During the Jurassic (approximately 195-205 million years ago) dikes and sills of mafic composition intruded the sediments of the Triassic basin and surrounding crystalline rocks.  This rock is known as diabase and is more resistant than the surrounding sandstones and siltstones and often forms resistant ridges in the Triassic basin.  Penny’s Bend on the Eno River is underlain by diabase.  Diabase is composed of minerals that contain abundant iron and magnesium in comparison to the Triassic sediments.  Because of the abundance of iron and magnesium, unique plant communities sometime develop on top of areas underlain by diabase (e.g. The diabase glades).

Destruction of a Mountain Chain and the Formation of the Coastal Plain

As Pangea continued to split apart, erosion immediately began wearing down the mountain highlands.  From the beginning of the Triassic to the end of the Cretaceous periods, the great mountain range was eroded down in approximately 170 million years.  The mountains were essentially broken down into sand, silt and clay and transported along streams and rivers to the east and were deposited on the newly forming coast of North Carolina.  The sediment was deposited in layers starting in the Jurassic period and continues today.  These sediments make up the Coastal Plain portion of the State.  The Coastal Plain is an east-dipping wedge of sediment that is only a few feet thick just east of Raleigh but thickens to more than 10,000 feet below Cape Hatteras.

By the end of the Cretaceous period, the great highlands created by the formation of Pangea had eroded down to a peneplain of gently rolling hills.  (A peneplain is a gently undulating land surface which has been produced by long lasting erosion by streams and rivers.) 

Cenozoic Uplift of the Piedmont and Formation of Modern Floodplain Deposits

During the Cenozoic Era (66 million years ago to present), the Piedmont continued to erode, however at a slower pace.  Gradually, due to isostatic forces in the earths crust, the Piedmont was uplifted.  (Isostatic forces are similar to the buoyancy of a boat in water, the heavier the cargo the lower the boat sits in the water or conversely the lighter the cargo the higher the boat sits in the water.  The continental plates essentially float on top of the mantle.  As rock is removed by erosion the earth’s crust will react by uplifting a proportional amount similar to a boat that will “sit high in the water” when its cargo is removed.) 

Millions of years of erosion had leveled the landscape to a nearly flat plain with gently meandering streams and rivers.  As Cenozoic uplift began, the streams and rivers became entrenched in their floodplains.  This entrencement of the rivers caused the rivers to down-cut and become incised in their channels as the land surface slowly uplifted.  The Little River, Eno River, New Hope Creek, Bolin Creek and Morgan Creek are incised drainages with steep banks in many places.  Erosion is continuing today along the rivers and streams.  The sand, silt and clay deposits present in the river bed of the rivers and streams are slowly making a long journey toward the Atlantic Ocean.  During flood events, as the water level rise, sand, silt and clay particles are transported over the river banks and are deposited.  These deposits are know and Alluvium – flood plain deposits.  Alluvium is the youngest geologic unit within the Chapel Hill, Hillsborough and Durham area.  Someday the sand, silt and clay material that compose the alluvium will be deposited at the mouth of the Neuse or Cape Fear Rivers near the coast and if buried deep enough will become sandstone, siltstone and mudstone.  During the next mountain building event these rocks may be pushed up into mountains and be eroded away, starting the whole process over.

References:

Cloud, P., Wright, J., and Glover, L., 1976, Traces of animal life from 620-million-year-old rocks in North Carolina: American Scientist, v. 64, p. 396-406.

Hibbard, J., Stoddard, E.F., Secor, D., Jr., and Dennis, A., 2002, The Carolina Zone: Overview of Neoproterozoic to early Paleozoic peri-Gondwanan terranes along the eastern flank of the southern Appalachians: Earth Science Reviews, v. 57, n. 3/4, p. 299-339.

Glover. L., Sinha, A., 1973, The Virgilina deformation, a late Precambrian to Early Cambrian (?) orogenic event in the central Piedmont of Virginia and North Carolina, American Journal of Science, 273-A, 234-251.

Newton, M.C., 1983, A late Precambrian resurgent cauldron in the Carolina slate belt of North Carolina, U.S.A., M.S. thesis, Virginia Polytechnic Institute and State University, 89 p.

Pollock, J. C., 2007, The Neoproterozoic-Early Paleozoic tectonic evolution of the peri-Gondwanan margin of the Appalachian orogen: an integrated geochronological, geochemical and isotopic study from North Carolina and Newfoundland. Unpublished PhD dissertation, North Carolina State University, 194 p.

Seilacher, A., Meschede, M., Bolton, E.W., and Luginsland, H., 2000, Pecambrian “fossil” Verimorma is a tectograph: Geology, v. 28, p. 235-238.