Table Of ContentSerpentine Geoecology of the Appalachian and Ouachitan Orogen
E.B. Alexander
Soils and Geoecology
The Appalachian orogen is a belt of deformed terranes stretching southwest from Newfoundland
to Alabama, and terranes of the Ouachitan orogen are exposed discontinuously west of Alabama
in Arkansas, Oklahoma, and Texas (Fig. 1). Some terranes of these orogens were thrust over the
Grenville Province of the North American continent and many others were attached to the
continent during the Paleozoic Era. The serpentine, or ultramafic rocks, of eastern North America
are in these accreted terranes that were added to the continent more than one-quarter of a billion
years (Ga) ago.
Figure 1. Tectonically altered terranes of the Appalachian and Ouachitan Orogens.
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Ultramafic rocks, which are commonly called serpentine among ecologists, are sparse in the
Appalachian and Ouachitan orogenic belts, but they have distinctive soils and vegetation. The
serpentine plants are of special interest to botanists, because many of the plants are rare species
or are present only in serpentine habitats. These habitats are seldom placed in a geoecological
perspective. Huggett (1995) has presented the concepts and principles of geoecology.
Subsequently, Alexander et al. (2007) have addressed serpentine geoecology and characterized
serpentine geoecosystems of western North America. This article is presented to characterize the
serpentine geoecosystems of the Appalachian and Ouachitan orogens along the eastern and
southeastern margins of the continent. It is provided to aid in the linkage of serpentine habitats to
the geology and soils that make them distinctive. The geological and biological aspects are
considered to be one integrated system in geoecology. A presentation of the geological
framework and other abiotic aspects of geoecosystems is followed by a more integrated
presentation of the geological and biological aspects that emphasizes soils and their vegetative
cover.
1. Geologic setting
Various Proterozoic continents were amalgamated into a supercontinent called Rhodinia about
one billion years ago and exotic terranes were accreted to it during the Grenville orogeny. After
about one-quarter of a billion years, Rhodinia began to split apart (Fig. 2). By about 0.6 Ga ago
Rhodinia had separated into several large fragments– mainly Laurentia (ancient North America),
Siberia, Baltica, and Gondwana, a supercontinent.
The Iapetus Ocean that formed between Laurentia and Gondwana following the breakup of
Rhodinia began to close during the early Paleozoic. Most of the oceanic crust and volcanic arcs
from the Iapetus Ocean were subducted as it closed, but some oceanic crust and sediments were
thrust over rocks of the Grenville Province that occupied the margin of Laurentia, and some
magmatic arc and oceanic fragments were attached to the outer edge of the continent. During this
first Paleozoic mountain building event, called the Taconic orogeny, clastic sedimentary rocks
and limestone resting over the Grenville Province were deformed by folding and faulting. These
sedimentary rocks have been further deformed in subsequent Paleozoic orogenies and now
comprise the Valley and Ridge Mountains of the inner Appalachian Mountain Range from
Pennsylvania to Alabama and the less prominent Humber Zone from Newfoundland southwest to
New York. Oceanic crust and magmatic arc terranes accreted to the continent during the Taconic
orogeny comprise the Dunage domain in the northern Appalachian and the Piedmont domain in
the southern Appalachian.
As the Iapetus Ocean closed early in Paleozoic, the Rheic Ocean opened between Gondwana
and terranes that had formed along the margin of the Gondwana supercontinent. As the Rheic
Ocean expanded, these peri-Gondwana terranes began to collide with North America–colliding
first were terranes of the Ganderia domain in the northern Appalachian and terranes of the
Carolinia domain in the southern Appalachian, followed by terranes of the Avalonia domain in
the northern Appalachian. Both of these Mid-Paleozoic collision events have been assigned to
one prolonged event, the Acadian orogen, but some authorities have proposed two shorter events,
which they call the Salinian and Acadian orogenies (Eyles and Miall 2007).
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Figure 2. Geologic events pertinent to the development of the Appalachian and Ouachitan areas.
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During the late Paleozoic, Gondwana collided with Laurentia along the southeastern margin
of North America. This event is known as the Alleghany orogeny, which may be linked to the
Variscan (or Hercynian) orogeny in Europe, and the Ouachita orogeny from Arkansas through
Oklahoma to Texas.
1.2 Occurrence of ultramafic rocks
The ultramafic rocks are in the terranes east of the Valley and Ridge Mountains in the southern
Appalachians and southeast of the corresponding Humber Zone in the northern Appalachians.
From Pennsylvania south to Alabama these zones, or domains, are from west to east, the Blue
Ridge Mountains, Inner Piedmont and Outer Piedmont. In Canada down through New England
they are, from west to east, the Dunnage, Gander, amd Avalon domains (Fig. 3). The Atlantic
coastal plain, which is exposed east of the southern Appalachian, and the Meguma terrane that is
exposed in Nova Scotia, have no ultramafic rocks in them.
Most of the ultramafic rocks originated in oceanic environments and have gone through
multiple stages of deformation and alteration. Ophiolites are clearly identifiable only in
Newfoundland, Quebec, and western Maine. Many small bodies of ultramafic rocks occur in
fragments of oceanic crust in the northern Appalachian and from the Blue Ridge Mountains of
the southern Appalachian eastward. across the Piedmont.
Although the ultramafic rocks in ophiolites, which are fragments of ocean crust, are
presumed to be the predominate ones in the Appalachians, some are in differentiated magma
chambers and sills. Because the northern and southern Appalachians are quite different, separate
sections follow for each.
1.2 Northern Appalachian ultramafic rocks
Ultramafic rocks of the northern Appalachians are most concentrated in small bodies along, or
near, faults that represent the western margin of the Dunnage domain. These faults are
concentrated along the Baie Verte-Brompton and Taconian Lines (Williams and St-Juliens 1982,
Hatch 1982, Fig. 4) from Newfoundland through southeastern Quebec, Vermont, and western
Massachusetts and Connecticut. The outer margin of the Humber Zone is along these lines. Some
oceanic and volcanic arc rocks were thrust over those of the Humber zone when the Dunnage
domain impinged upon Laurentia during the Taconic orogeny. Arrival of the Gander domain
during the early Acadian (or Salinian) caused more deformation of the Dunnage domian, and the
Avalon domain arrive during the late Acadian orogeny.
Ophiolite is readily discernable where the Bay of Islands complex has been obducted
westward from the Baie Verte-Brompton line in Newfoundland and Quebec, but deformation and
alteration make ophiolites more difficult to identify south of the Thetford Mines, Asbestos, and
Mount Oxford ophiolites in Quebec. The general sizes of ultramafic bodies diminish southward
along the Baie Verte-Brompton-Taconic Line and in western New England the rocks are
characteristically almost completely serpentinized with some talc schist and talc-magnesite or
magnesite aureoles. The origins of terranes with the ophiolites of Boil Mountain and the Jim
Pond formation in western Maine (Boudette 1982) are controversal (Penet and Tremblay 1995,
Kusky et al. 1996, Moench and Aleinikoff 2003, Gerbi et al. 2006).
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Figure 3. Northern Appalachian Humber, Dunnage, Gander, Avalon, and Meguma domains.
Figure 4.. Serpentine localities in southeastern Canada and Maine.
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Small bodies of ultramafic rock are present in the Dunnage and Gander Zones along the
Connecticut Valley–Gaspé trough and on the Bronson anticlinorum in central New England and
adjacent Canada (Lyons et al. 1982). These small bodies are not known to be of geoecological
significance. Outcrops of serpentinite that are large enough to be geoecologically significant
occur on Deer and Little Deer Isles in Penobscot Bay (Wing 1951).
The Bay of Island ophiolite on Newfoundland is one of the most complete that is known,
apart from the Semail ophiolite of Oman on the Arabian Peninsula. From top to bottom marine
sediments, pillow basalt, sheeted dikes, gabbro, and deformed peridotite are all present. Rocks on
which the ophiolite came to rest are thermally altered (Malpas 1979). Large areas of ultramafic
rocks are exposed at Hare Bay and in the Bay of Islands Range in northern and western
Newfoundland. Many smaller exposures of ultramafic bodies are scattered through the Dunnage
and Gander Zones of Newfoundland southeast of the Baie Verte-Brompton line.
1.4 Southern Appalachian ultramafic rocks
Ultramafic rocks of the southern Appalachian Orogen south of the relatively large Baltimore
mafic complex in Maryland and southeastern Pennsylvania (Fig. 5), are most concentrated in the
Blue Ridge Mountains and associated foothills and highlands (Fig 6). In western North Carolina
and northern Georgia there are many small and lenticular bodies of dunite conforming to the
regional rock foliation
(Misra and Keller
1978).
Serpentinization is
evident around the
margins of these
bodies, and soapstone
is common.
Alteration of dunite
and harzburgite in the
lenticular bodies
increases toward the
northeast and ultra-
mafic rocks in the
Abemarle-Nelson belt
of Virginia are mainly
amphibole-chlorite
schist, serpentinite,
soapstone, and altered
peridotite. The
Abemarle-Nelson is a
belt <1 km wide with
several narrow bands
Figure 5. Serpentine localities in the northern Piedmont. Rock outcrop
<2 km to >20 km
locations and designations after Larabee (1966).
long.
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Figure 6. Serpentine of southern Appalachian localities. Rock outcrop locations and designations
after Larabee (1966). The Brevard is a fault zone along the southeastern margin of the Blue
Ridge Mountains.
There are no ultramafic rocks in the Valley and Ridge physiographic division. The Blue
Ridge Mountains polygon in Figure 7 is a physiographic division, rather than a petrologic one,
and rocks in much of it in Tennessee and adjacent parts of Georgia and North Carolina are like
those of the Valley and Ridge.
Small lenses of altered ultramafic rocks occur throughout the Inner Piedmont (Misra and
Keller 1978). The relatively large Hammett Grove suite along the edge of the King Mountain belt
may represent an ophiolite in which the ultramafic rocks have been altered to soapstone and
serpentinite (Mittwede 1989). It is exposed for about 11 km along strike, with a common width
of 200 to 300 m wide, but up to 1 km wide (Butler 1989). Iredell soils (Hapludalfs with “mixed”
mineralogy) were mapped on metapyroxenite and metagabbro of the Hammet Grove suite (Jones
1962, Mittwede 1989).
Ultramafic rocks of the Charlotte and King Mountain belts are mainly minor constituents of
mafic intrusive complexes that are generally located near faults (Butler 1989). Several ultramafic
bodies up to 20 km (discontinuous) long and 2 km wide are scattered across the Outer Piedmont,
where they are less common than on the Inner Piedmont.
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The largest exposures of ultramafic rocks are in the Baltimore mafic complex. Ultramafic
rocks of the Baltimore mafic complex are in a basal unit of serpentinized dunite (forsterite,
olivine with little iron) and chromitite and in an overlying gabbroic unit approximately 5 km
thick that contains many cumulus peridotite (olivine is in chysolite of the peridotite) layers
(Hanan and Sinha 1989). This mafic complex is tectonically placed against rocks of the
Wissahickon formation that is in the Glenarm series of early Paleozoic metasedimentary rocks.
The Glenarm series rests unconformably on Proterozoic basement of Baltimore gneiss (Hanan
and Sinha 1989). Rocks of the Baltimore mafic complex are mainly in three large blocks from
southeastern Pennsylvania to Baltimore in Maryland. Also, lenses and pods of the ultramafic
Soldiers Delight belt occur in the Wissahickon formation.
1.5 Ouachita orogen
Late Proterozoic rifting opened an ocean along the edge of Laurentia that is now the southeastern
margin of the North American craton. Marine sediments were deposited along this edge of the
craton until the late Paleozoic Era. Then sediments were thrust over the margin of the craton
during the Alleghany orogeny. Small rootless pods of ultramafic rocks are scattered through the
Paron Nappe of the Benton uplift in the Ouachita Mountains.
Effects of Quachita orogenic activity are evident in the Arbuckle and Wichita Mountains of
Oklahoma and the Llano and Marathon uplifts in Texas (Fig. 1). Ultramafic rocks are reported
from the Llano uplift, but in Grenville basement, rather than in Paleozoic strata (Garrison 1981).
The main ultramafic body is interpreted to be a serpentinized block of ocean crust emplaced as an
olistostrome fragment in volcanoclastic arc-flank deposits. It is about 6 km long and 0.5 to 2 km
wide. The serpentinite is lizardite with accessory magnetite; relict minerals are sparse.
2. Physiography
The ups and downs of the Appalachian orogen were recorded by Paleozoic sediments. When the
Appalachians were high, sediments were spread across the Appalachian Plateau west of the
Appalachians and across contiguous or adjacent areas in Europe and Africa. When the
Appalachians were low, sediments were deposited in basins within the area of the orogen and in
the Iapetus Ocean. Toward the end of the Paleozoic Era, rifting produced basins that were then
filled with Triassic sediments and volcanics. The Connecticut Valley in central New England and
the Bay of Fundy between New Brunswick and Nova Scotia are remnants of two of these basins.
Mountains of the Appalachian are generally highest near the northwest margin of the orogen
and decrease in elevation toward the Atlantic coastal plain. The White Mountains in New
England are exceptions; they are the highest in the northern Appalachians. The southeastern
margin of the Appalachian orogen is obscured by the Atlantic coastal plain, which hides the
allochthonous Suwanee domain.
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2.1 Northern Appalachian physiography
On the northwest margin of the Appalachian orogen, the Long Range Mountains in
Newfoundland and the Notre Dame Mountains of Gaspésie rise abruptly above the St. Lawrence
lowland. From that margin, the topographic highs are represented by one or more planes that
slope southeast to the Atlantic coastal plain, which is offshore of Newfoundland and Nova
Scotia. Some geologists have suggested that the summits that represent these planes are remnants
of ancient peneplains.
South from the Notre Dame Mountains, the western margin of the northern Appalachians is
bound by low valleys. These are the Champlain Valley west of the Green Mountains and the
Hudson Valley west of the Taconic Mountains. The highest summits of New England are in the
White Mountains of New Hampshire (Mount Washington, 1889 m, Fig. 3) and Maine (Mount
Katahdin, 1580 m), rather than in the Green and Taconic Mountains (Denny 1982). A massive
delta west of the Taconic Mountains, the Catskill delta, was produced from Appalachian
sediments after Acadian uplift. Obviously, the Hudson Valley did not exist west of the Taconic
Mountains for some time after the middle Paleozoic Acadian orogeny.
Major valleys are oriented north-south parallel to the general trend of geological structure in
New England, and around toward the northeast in Maine and southeastern Canada. Some rivers
flow across these trends–most notably, the St. John River that flows across highlands in New
Brunswick and adjacent Maine and has a tributary that cuts through the Notre Dame Mountains
in Quebec to form a valley that is continuous from the St Lawrence River Valley southeastward
to lowland adjacent to the Atlantic Ocean.
All of the northern Appalachians south to Long Island were covered by ice during the last
major glaciation. There may have been some unglaciated nunataks on Newfoundland and the
Gaspé Peninsula. For sometime after the Laurentide continental glacier had melted, icecaps
persisted on Newfoundland and the Gaspé Peninsula. Ice cover slowly disappeared from the
Gaspé Peninsula between 10 and 13.5 ka ago. Permafrost is still present above about 1200 m on
Mont Jacques-Cartier (1270 m), but the active layer above the permafrost is about 6 m deep
(Gray and Brown 1979).
2.2 Southern Appalachian physiography
The highest parts of the southern Appalachians are in the Blue Ridge Mountains in North
Carolina (Mitchell Mountain, 2037 m) and highs diminish both southward and northward (Hack
1982). The highest mountains in northern Virginia, Maryland, and Pennsylvania are in the Valley
and Ridge, rather than in the Blue Ridge Mountains. As in the northern Appalachians, the
topographic highs are represented by one or more planes that slope southeast to the Atlantic
coastal plain. In the southern Appalachian, however, there are two abrupt drops in the northwest-
southeast topographic trend. One is at the transition from the Blue Ridge Mountains and
associated foothills and highlands at the Brevard zone (Fig. 6) to the much lower, undulating to
hilly Piedmont, and the other is at the Fall Line from the Piedmont to the Post-Paleozoic
sedimentary cover on the Atlantic coastal plain.
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Figure 7. Physiographic divisions of the southern Appalachian Mountains.
2.3 Ouachita Orogen
The Ouachita, Arbuckle, and Wichita Mountains have high relief, but relatively low altitude. The
highest mountains in these areas are Magazine Mountain (839 m) in the Ouachita Mountains and
Mt. Marcy (713 m) in the Wichita Mountains. Relief is low in the Llano uplift area.
3. Climate
Climates of the Appalachian Mountains are continental, because prevailing west winds minimize
the ameliorating effects of the Atlantic Ocean. The mean annual temperature ranges from -5o to
20o Celsius (Fig. 8A). The frost-free season is about 3 or 4 months in the north to 7 months in the
south and 7 or 8 months in the area of the Ouachita orogen. Mean annual precipitation is mostly
in the 75 to 150 cm range, but up to about 200 cm at the higher elevations in the southern
Appalachian Mountains (Fig. 8B). It decreases westward to about 60 cm in the Llano area and
less in the Marathon area.
Summer temperatures are cool in Newfoundland to hot at the lower elevations in the southern
Appalachian and in western areas. The winter precipitation is mostly snow in the northern
Appalachian and mostly rain at the lower elevations in the southern Appalachian and in western
areas. Assuming that the rate of evapotranspiration is about 2 to 3 cm/month for each degree
Description:Ultramafic rocks of the southern Appalachian Orogen south of the relatively large Baltimore mafic complex in Maryland and southeastern Pennsylvania (Fig.
