Transcript Geologic History of Chapel Hill

GEOLOGY OF THE CHAPEL HILL AREA
Shown in Geologic Cross-sections
Adapted from a 1990 paper by
John Dennison
W. Burleigh Harris
Paul Fullagar
Richard Diecchio
FIGURE ONE
The 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.
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
FIGURE THREE
Continuing 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 Elementary
Culbreth Middle School
Small outdoor park behind Hanes Art Center
American Stone Quarry (5 miles west of Carrboro on NC 54)
FIGURE FOUR
By 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.
There was a long gap in the record of rocks
reserved near Chapel Hill. The next mountainbuilding 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).
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.
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
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.
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.
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.
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.
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.
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.