GEOLOGY OF THE CHAPEL HILL AREA Shown in Geologic Cross-sections Adapted from a 1990 paper by

1. GEOLOGY OF THE CHAPEL HILL AREA Shown in Geologic Cross-sections Adapted from a 1990 paper by John DennisonW. Burleigh HarrisPaul FullagarRichard Diecchio

2. FIGURE ONEThe earliest rocks in the Chapel Hill area are about 700 million years old (m.y.), according to data by radioactive isotopes. They were formed from volcanic ash and lava flows which built up a series of small islands rising above the muddy sea floor. Probably a portion of the ancient ocean floor was sliding beneath a continent, similar to the present day setting of the Aleutian Islands in Alaska. These volcanoes were located east of present day North America on the other side of the ancient Iapetus Ocean.

3. FIGURE TWO By the late Precambrian (550 m.y.) the sediments and volcanic layers deposited on the Iapetus Ocean floor had been crumpled and folded. Such a process is occurring today on the floor of the Pacific ocean near the Aleutian Islands

4. FIGURE THREEContinuing igneous activity caused intrusions into these folded sediments and volcanics. This molten magma cooled to form solidified intrusive igneous rocks consisting of some gabbro, and much more diorite and granite, with smaller dikes of aplite (feldspar and quartz) and mineral veins of epidote and quartz. Much of the gold-bearing rocks in NC are associated with the igneous activity portrayed in fig. 1-3 These rock formations can be found at: Frank Porter Graham ElementaryCulbreth Middle SchoolSmall outdoor park behind Hanes Art CenterAmerican Stone Quarry (5 miles west of Carrboro on NC 54)

5. FIGURE FOURBy Ordovician time (500 m.y.) the spreading of the Iapetus ocean ceased, and it began to close as the marginal continents drifted back toward each other. (Present-day analogy: Africa moving toward the Mediterranean Sea compressing rocks over southern edge of Europe – causing folded and faulted rocks of Alps and Cyprus). This Ordovician compression formed a mountain range on the eastern edge of North America. The older volcanic sediments were further crumpled and somewhat metamorphosed to form “slate” in the Piedmont. This mountain-building event is called the Taconic orogeny. Chapel Hill was situated 20 degrees south latitude but Slate Belt rocks of NC had become a part of the North American continent.

6. There was a long gap in the record of rocks reserved near Chapel Hill. The next mountain-building event, the Acadian orogeny (370 m.y.) affected parts of the Piedmont west of Chapel Hill. After this, deposits of oil, coal and natural gas formed in the tropical seaways and swamps (in present day TN, VA & WV)Ending this long gap, a new episode of mountain building activity thrust the Blue Ridge masses over the eastern edges of TN & VA, and folded and faulted the sedimentary rocks for 70 miles west of the Brevard Fault. (the Valley and Ridge province).

7. Erosion through the Blue Ridge thrust near Grandfather Mountain has created a “geologic window.,” which exposes the underlying rock layers. This latest orogeny is called the Appalachian Orogeny (270 m.y.) At this time Chapel Hill was positioned just north of the equator. Igneous intrusions during this time formed the granite used to construct the NC state capitol building in Raleigh. The compression rammed western Africa into North America closing the Iapetus Ocean for good. A giant continent called Pangea became welded together by metamorphism of the smaller continental pieces.

8. FIGURE FIVE The gigantic continent of Pangea began to crack apart during the late Triassic period (195 m.y.) A large fault basin sunk into the Piedmont as this rifting occurred. Renewed erosion of the Piedmont land surface supplied sand and mud to streams which built alluvial fans into the basin (similar to Death Valley or Dead Sea basin in Israel) Generally semi-arid at this time but conditions varied as climate cycles changed. During wet periods, coal swamps occurred in the Sanford area of the basin

9. FIGURE 5 contd.During moderately arid times, the sedimentary fill became sun-cracked and brick red. During extreme dry times, agate-colored chert formed in small playa lakes near the present Governors Inn. At this time, three-toed dinosaurs roamed the basin floor, and beautiful impressions of plant leaves developed in some of the muds which were later excavated to manufacture brick.

10. FIGURE SIX At the beginning of the Jurassic period (180 m.y.) the last episode of basin subsidence occurred and rifting of eastern North America became so severe that fractures tapped magma tens of miles deep into the Earth. Basaltic dikes intruded into these fractures (low hill on which Ephesus Baptist Church is built). There are dozens of these Triassic-Jurassic fault basins scattered along the eastern edge of North America. The final stage of the breaking open of the continent is comparable to the present rift valley activity in eastern Africa.

11. Another gap in the rock record at Chapel Hill About 160 m.y. the cracks became so large that the African continent began to drift away from North America, and the ancestral Atlantic Ocean was born. Such a narrow spreading center is similar to the Red Sea or Gulf of California today. The present Persian Gulf is an example of a wider stage of development. Spreading concentrated along the Mid-Atlantic Ridge in the center of the ocean, with volcanic islands developing from taller eruptive mounds (like modern Iceland or the Azores). By Cretaceous time (100 m.y.) the Atlantic Ocean had split the continental masses nearly pole-to-pole. Even though seafloor spreading is about the rate of fingernail growth, since the Jurassic opening of the seaway, this has been enough to move the continents 3000 miles apart.

12. Streams draining the Blue Ridge portion of the Appalachian Mountains and the Piedmont flowed southeastward across NC and built Coastal Plain river deposits onto the eastern edge of the Atlantic ocean. Some geologists think the coastal plain sedimentary cover once extended inland to about Burlington, but if so, subsequent erosion washed that cover from the Chapel Hill area. The farthest inland extent of Coastal Plain sediments today is eastern and southern Wake County and near Sanford. These sediments contain fossil logs, sharks teeth, sea shells and even peat to the east. The Outer banks are reworked Coastal Plain sediments which were shaped into the present islands by rising sea level after melting of ice sheets situated in more polar latitudes. NC was not glaciated during the coldest part of the last glacial episode (19,000 years ago) even in the mountains. However the cold stripped the high peaks of all their vegetation, and today’s spruce-fir-balsam flora of the high mountains was displaced to lower elevations of the Piedmont during the ice ages. Continued melting of glacial remnants in cold latitudes has caused the sea level to rise about a foot in the last century, accompanied by a landward shift of the barrier islands.

13. FIGURE SEVEN Slight uplift of the Piedmont has encouraged stream erosion to cut into the upland, forming incised valleys (Umstead Road). Streams are etching out a lowland in the Triassic shaly strata along downstream portions of Bolin and New Hope Creeks (UNC and Chapel Hill Country Club golf courses). There is an escarpment between the harder volcanic rocks of the Slate belt and the Triassic Basin (East Franklin St. hill, Weaver Dairy Rd. hill). The location of Chapel Hill in an upland area, with an absence of large rivers, seemed to contribute to the difficulty of developing an adequate water supply for a rapidly growing area. The problem may have been solved by OWASA using spent rock quarries.

14. The granite surface of the uplands disintegrates as feldspar weathers into clay minerals. The left-over, loose, resistant tan-colored material is quartz and is sold as “Chapel Hill gravel” (it is all over our courtyard at CHHS). A dam has flooded part of the Triassic basin creating Jordan lake, a water supply for Cary. Areas underlain by Triassic shales contain the clay mineral smectitie, which swells and contracts with changing soil moisture conditions of drought and wet episodes. Buildings situated in the Triassic basin may have shifting foundations, poorly sliding windows, and cracked bricks. The soils drain poorly and new structures outside city limits have difficulty getting approved due tofailing “perk tests” for septic tank approval. Wells drilled in the basin generally have low yields.