Download An Introduction to the Geology of the Southern Appalachians

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AN INTRODUCTION
TO THE GEOLOGY OF THE SOUTHERN APPALACHIANS
by George D. Swingle
Professor
of Geology, Department of Geology and Geography
University of Tennessee, Knoxville, Tenn.
Gatlinburg
is located at the eastern border of Tennessee
near the heart of the classic Southern Appalachians.
This
resort city, the gateway to the Great Smoky National
Park, is
situated
The Blue
in the foothills region of the Blue Ridge.
Ridge physiographic
province
is flanked on the west by the
Valley and Ridge and,on the east by the Piedmont.
Valley and Ridge, in Tennessee
Cumberland
Atlantic
Plateau,
Coastal
geologic
the boundaries
tifiable.
region has a characteristic
expression
which reflects
of the underlying
by the
rocks.
and rather
in part the
For the most part
between these regions are sharp and easily iden-
A west to east traverse
(line A-A' Figure 1), beginning
reveals a tableland
Plateau-Valley
is bounded eastward
Plain (Figure 1).
topographic
features
and adjacent states, is the
and the Piedmont
Each physiographic
distinctive
West of the
at the latitude of Gatlinburg
on the Cumberland
some 2000 feet in elevation.
and Ridge boundary
Plateau
At the
an abrupt escarpment,
whose
crest is 1000 feet above the general level of the Valley and
Ridge, is quite conspicious.
intervening
Abruptly,
The nearly parallel ridges and
valleys of the Valley and Ridge are striking.
at the southeast
margin of this province
Ridge front rises 1000 to almost 2000 feet.
foothills
in the vicinity of Gatlinburg
the Blue
The Blue Ridge
at elevations
of
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Fig. 1 ' Major physiographic
with
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elements ot the Southern AppalaChians
diagrammatic geologic cross section.
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generally
2000 to 3000 feet give way, less than 5 miles south-
eastward,
to the higher "and more rugged terrain of the Blue
Ridge crestal mountains.
Gatlinburg
Mt. LeConte 6 miles southeast of
is 6593 feet in elevation,
south of Gatlinburg
is at an elevation
Clingmans
Dome 10 miles
of 6642 feet.
These
peaks are among the highest in the Southern Appalachians
succeeded
only by Mount Mitchell
top is at 6684 feet.
ward from Gatlinburg
north of Asheville,
The mountainous
being
N. C. whose
Blue Ridge extends east-
some 80 miles where its
eastern border is
marked by a sharp descent of some 2000 feet to the floor of the
Piedmont.
The Piedmont plain slopes gradually eastward,
average elevation
the Atlantic
of 1000 feet at the foot of the Blue Ridge, to
Coastal Plain.
Unfortunately,
the geology of several provinces which con-
stitute the Southern Appalachians
as are the physiographic
more detailed
western
is not as readily understood
elements.
and complete
In general our knowledge
in the Plateau,
is
Valley and Ridge and
Blue Ridge than in the eastern Blue Ridge and Piedmont
regions.
This is due in part to the availability
topographic
presence
western
from an
maps, more abundant bedrock exposures
of fossiliferous
areas.
and less metamorphosed
of detailed
and the
rocks in the
Because of these factors the western
been studied more intensively
result our knowledge
areas have
than those to the east and as a
is more complete.
Geologic
The Appalachian
Mountain
element of the Southeastern
Setting
System, the dominant
tectonic
U. S. extends from the Cumberland
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Plateau eastward
miogeosynclinal
eastward
to the Atlantic
deposits
inferred
Paleozoic
"Appalachian
and deformational
revolution."
extending
However,
late
the sedimentary
from late Precambrian
Evidence for Precambrian
including
as in the sedimentary
planed
from the presumably
sequences.
the mountain
throughout
diastrophism
plutonism,
is found in the metamorphic
deformation
rocks
history of this region is quite lengthy, and
periods of deformation
morphism
of
features of this system are
to have resulted
also quite complex,
the Paleozoic.
Composed
to the west and eugeosynclinal
many of the structural
generally
Coastal Plain.
vulcanism
and later
and meta-
and plutonic rocks as well
After the late Paleozoic
system was extensively
eroded, pene-
(perhaps 3 times) and the present landforms carved by
differential
erosion.
Description
of Tectonic Units
Cumberland
Plateau
West of the Valley and Ridge this province
prised of relatively
flat-lying
Pennsylvanian
totaling
a mile or more in thickness.
features
are the Pine Mountain
distances westward.
A spectacular
sequence
and Alabama.
overthrusts
along
have moved short
faulted anticline
occurs near the middle of the region extending
Tennessee
and older sediments
The main structural
and Cumberland
which upper layers of the sedimentary
is mainly com-
(Sequatchie)
over 150 miles in
Its axis is nearly straight,
paralleling
the main structural
lineament of the Appalachians,
and is essen-
tially horizontal.
The fold is breached by erosion resulting
in
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a valley,
below
3 miles
or so wide, whose
the general
floor is 1000 feet or more
level of the Plateau.
Valley
In this province,
and Ridge
40 or more miles
is a generally
comformable
Pennsylvanian)
sediments
sequence
thickness
averages
3 or so miles.
Imbricate
dipping,
repeat
of the sequence
thrusts,
These faults
are of large stratigraphic
for hundreds
of miles
Appalachians.
province
coarsen
and thicken
generally
carbonate
southeastward
sequence
Ridge rocks although
strongly
crushed,
cleavage
is present
eastern
portions
observed
mainly
the province.
portion
of the
sediments
which
contrast
to the northwest.
are relatively
southeast
grain of the
clastic
in marked
folded tightly
in places
to the
The Valley and
and locally
unmetamorphosed.
Weak fracture
in select rock types near the middle
of the region
to
throw and some extend
in the southeastern
have in their thrust blocks
(Cambrian
but which probably
across
along the structural
The faults
in Tennessee,
of Paleozoic
of varying
segments
across
and
but slaty .cleavage has not been
in the rocks of this province.
Blue Ridge
This province
east trending
northwest
east.
can be conveniently
segments,
and the second,
The crystalline
latitude
and other
one dominantly
of Gatlinburg,
dominantly
segment,
into two north-
sedimentary
crystalline,
in the
to the south-
about 30 miles wide
is composed
types of gneisses
divided
chiefly
and schists
at the
of mica, hornblende
plus granitic
and
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ultramafic
intrusives.
cambrian
These rocks are considered
and at least in part constitute
upon which the sediments
deposited.
rocks).
Lower Cambrian
extends
sedimentary
is Precambrian
Two sequences
the basement
complex
of the eastern Blue Ridge were
The dominantly
near Gatlinburg,
mainly Pre-
portion of the Blue Ridge,
(with but minor early Cambrian
of chiefly clastic rocks are present,
Chilhowee
the
Group, about a mile thick, which
for several hundred miles along the eastern Blue Ridge,
and the underlying
Ocoee sediments which are areally quite
restricted.
The Ocoee clastics
and possibly
twice this amount.
overlying
Chilhowee,
locally schists,
different
are at least five miles thick
They are, in contrast with the
metamorphosed
to slates, phyllites,
and have obviously
environment
The structural
been deposited
than the overlying
Paleozoic
in quite a
rocks.
features of this portion of the Blue Ridge
are world famous, this being the classic Appalachian
tremendous
klippen.
overthrusts
as evidenced
The Grandfather
Gatlinburg,
is evidence
Blue Ridge.
75 miles n9rtheast
of the 30 mile westward
of
transport of the
the limestone-floored
Cades, Wears and others) are windows
Ridge thrust sheet and indicate westward
approximating
area of
by several fenster areas and
Mountain Window,
Near Gatlinburg
(Tuckaleechee,
and
horizontal
cove
areas
in the Blue
movements
10 miles.
Piedmont
This province,
deciphered
over 150 miles across, is the least
of the Southern Appalachians.
of its rocks is generally
The surficial
distri-
known but their ages, stratigraphic,
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structural
and metamorphic
The Piedmont
which
relationships
are poorly understood.
is commonly divided into somewhat
generally
Appalachians.
parallel
the northeast
From the Atlantic
Blue Ridge the more conspicious
(I) The Carolina
strike of the
Coastal Plain westward
slate belt, some 50 miles wide and
area are characteristically
strongly
siltstones .and slates.
is unknown but is probably
altered.
Precambrian
dynamically
The rocks of this
felsic and mafic volcanics
calated with partly tuffaceous
are but slightly
to the
of these belts are:
having a strike length of over 400 miles.
thickness
linear belts
over 2 miles.
metamorphosed
interTheir
These rocks
but Dear plutons are
Their age is unknown but they are presumably
or early Paleozoic.
(2) The Charlotte
plutonic belt west of the slates is
about 30 miles across and typified by rocks of approximately
granitic
composition.
Paleozoic
age, possibly
These intrusives
spanning
are believed
to be of
several periods of that era.
(3) The Kings Mountain belt~ a narrow dejective
Late Precambrian
erate
or Early Paleozoic
and schist, separates
quartzite,
zone of
marble, conglom-
the slate belt and the Inner
Piedmont.
(4) The Inner or Western
dominated
morphism
zone.
Piedmont,
by mica gneisses and schists.
of the Appalachians
50 miles wide, is
Here the regional meta-
peaks in the sillimanite-garnet
The age of the rocks of this area was long believed
Precambrian
but the possibility
of a Paleozoic
age is now
to be
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entertained.
significant
Granitic rocks are also present here as well as
ultramafic
plugs and dikes.
(5) The Brevard belt separates
Blue Ridge.
This belt, a narrow zone of phyllite,
locally limestone,
metamorphosed
the Inner Piedmont
is characterized
and
schist and
by its rocks being less
than those in adjacent belts.
Some workers
sur-
mise that this belt is a major fault zone; others believe it is
synclinal.
Selected General References
Guides to southeastern
America.
Hadley,
geology:
1955, Geological
Society of
Jarvis B., and Goldsmith, Richard, 1963, Geology of the
eastern Great Smoky Mountains,North Carolina and
Tennessee:
U. S. Geological Survey Prof. Paper, 349-B.
King, Philip B., 1950, Tectonic framework of southeastern United
States:
Am. Assoc. of Petroleum Geologists, Bull. Vol.
34, No.4.
King, Philip B, 1964, Geology of the Central Great Smoky Mountains, Tennessee:
U. S. Geological Survey Prof. Paper
349-C.
Rodgers, John, 1953, Geologic map of East Tennessee with explanatory text: Tennessee Div. of Geol. Bull. 58.
Stuckey, Jasper L. and Conrad, Stephen G., 1958, Geologic map of
North Carolina:
Explanatory text for geologic map of
North Carolina:
North Carolina Division of Mineral
Resources, Bull. 71.