Geologic History of the Southeastern United States and
Its Effects on Soils of the Region
M.J. Guccione and D.L. Zachary
University of Arkansas
Of the five factors of soil formation, the geologic history of a region
directly affects four factors and indirectly affects the remaining factor.
Parent material is the soil-forming factor most directly associated with
geology. Rocks and sediment present at the earth’s surface that serve as
parent material for soil are the result of the geologic history of a region.
Thus, to understand the distribution of the soils in a region, we must
understand the distribution of the rock and sediment. A second factor of
soil formation is time, and this is also determined by the geologic history
of the region. Though a soil can be no older than rock or sediment that
it is formed in, the actual duration of weathering is most commonly the
result of landscape evolution and is only locally determined by the age
of the rock or sediment. Topography is the third factor affected by the
geologic history, and this is also the result of landscape evolution. Topography
may also be affected by the physical properties of the rock or sediment
that control the angle of repose or the maximum slope of a material. The
fourth factor, climate, is affected by the latitudinal position of a region
on the earth’s surface and its position with respect to the rest of the
continent, the ocean basins, and large lakes. All of these features are
controlled by movement of the earth’s plates. In addition, the surrounding
topography also controls the local climate. The last factor of soil formation,
vegetation, is indirectly the result of the geologic history. Vegetation
is controlled by climate, topography, and parent material, all of which
are affected by the geologic history of an area.
The purpose of this introductory chapter is to examine this geologic
history of the southeastern United States as a framework to explain the
distribution of rock and sediment in the region and the evolution of the
landscape. These factors will provide the setting for the soils that we
GEOLOGIC TIME AND A CALENDAR
The geologic history of surface features in the southeastern United
States is long and diverse and can only be discussed within the framework
of a geologic time scale. The scale, illustrated in Fig. 1, separates Precambrian
rocks formed during the first 87% of geologic time from rocks and sediments
of the Phanerozoic Eon that formed during the last 13% of geologic time.
Events in geologic history during the Precambrian Eon are poorly known.
Most Precambrian rocks are covered today by a veneer of Phanerozoic sedimentary
rocks and are known from only a few areas in the southeastern United States.
Organisms with calcitic or aragonitic skeletons did not evolve until late
in the Precambrian and were not available for the development of a time
scale. With the evolution and expansion of skeletal and shelled organisms
at the end of the Precambrian, the basis for a time scale appeared.
Workers during the 19th and 20th centuries divided the sedimentary rock
succession into units based on the ranges of fossil taxa. The units are
called systems, and the intervals of time in which they formed are termed
periods. The development and refinement of radiometric dating systems during
the latter part of the 20th century allowed the time boundaries of geologic
systems to be closely constrained. The geologic time scale has been calibrated
using absolute dates.
Although forming only a small part of the earth’s history compared to
the Precambrian, Phanerozoic rocks are much more widely exposed, their
ages are well documented, and the geologic events that formed them are
far better understood..9 Water and Chemical Transport in Soils of the Southeastern
Fig. 1. Geologic time scale for the southeastern United States.
PLATE TECTONICS - AN OVERVIEW
Concepts of Plate Tectonics developed around the 1960s revolutionizing
the study of geologic processes in the southeastern United States as well
as in the rest of the world. Geophysical studies indicate that the outer
100 km of the earth, termed the lithosphere, is rigid and brittle. This
lithosphere is dynamic and is constantly in motion driven by slow-moving
currents in the underlying, somewhat plastic athenosphere. The lithosphere
is broken into a number of plate-like segments. These segments or plates
are in motion. Though the movement is not very fast (1 to 5 cm/year), it
can be considerable during geologic time.
Plate boundaries are especially active areas. The plates may split apart
or diverge, collide with each other or converge, or may move past each
other. Earthquakes and faulting are associated with all three types of
plate motion, and volcanic activity is associated with the first two types.
In the southeastern United States, the first two types of plate motion
have occurred during geologic history. The geographic positions of plate
boundaries change during geologic time; with old ones becoming inactive
and new ones appearing.
REGIONAL GEOLOGIC FEATURES
OF THE SOUTHEASTERN UNITED STATES
The Gulf and Atlantic Coastal Plains dominate landscape features of
the southeastern United States. The Atlantic Coastal Plain extends southwestward
along the eastern seaboard of the United States and is, at its southern
end, continuous with the Gulf Coastal Plain that trends southwestward and
dominates a broad area north of the Gulf of Mexico. The plains slope gently
from interior highlands toward the Atlantic and Gulf coastlines with few
elevations higher than 100 m. The coastal plains are underlain by sedimentary
rocks of Cretaceous, Tertiary, and Quaternary age. Cretaceous rocks crop
out in a discontinuous belt adjacent to the highlands. They are composed
of terrigenous gravels and sandstone near the highlands and grade southward
and eastward into beds composed of limestone and marl. Sediments of Tertiary
and Quaternary age are present in successive outcrop belts in a coastward
direction. The youngest sediments of the Quaternary age also are exposed
along stream and river courses that flow across the Cretaceous and Tertiary
The Atlantic and Gulf Coastal Plains are bounded toward the interior
of the continent by a variety of highland features. The Atlantic Coastal
Plain passes westward into the Appalachian Piedmont and the Appalachian
Mountains. The Gulf Coastal Plain is bounded to the north by the Appalachian
Plateaus in Alabama, the Ozark Plateaus in southern Missouri and north-central
Arkansas, the Ouachita Mountains in central Arkansas, the Central Lowlands
in Oklahoma and north Texas, and the Great Plains in central Texas (Fig.
2). From the generally east-west trend of the Gulf Coastal Plain, a segment
protrudes north through eastern Louisiana and Arkansas and western Mississippi
and Tennessee. The Mississippi River flows southward down the axial region
of this protrusion to the Mississippi Delta and the Gulf of Mexico.
Three major mountain ranges are important landscape features of the
highland region. The Appalachian Mountains, composed of sedimentary units
of limestone, dolostone, shale, and sandstone, trend southwestward parallel
to and inland from the Atlantic coastline. The range is characterized by
high standing ridges of sedimentary rock that trend southwestward parallel
to the mountain trend. The Appalachian Range is composed of rocks of Paleozoic
age and passes southward beneath younger sedimentary strata. The Ouachita
Mountain range emerges from beneath sediments on the western side of the
Mississippi Embayment in central Arkansas and trends westward into Oklahoma.
In Oklahoma, the range curves to the south and is buried by Gulf Coastal
sediments in north Texas. The Arbuckle and Wichita Mountains form a linear
belt extending from the Ouachita trend in southeastern Oklahoma northwestward
to western Oklahoma. The more southerly Arbuckle Mountains are dominantly
composed of Paleozoic limestone and dolomite. Rhyolite lava flows of Late
Cambrian age and granite of Precambrian age form the Wichitas.
The Ozark Plateaus in Arkansas and the Interior Low Plateaus in Tennessee
occur inboard of the ranges. These plateaus are underlain by undeformed
sedimentary units of limestone and dolomite of early Paleozoic age and
sandstone and shale of late Paleozoic age. The elevation of the plateaus
rises to a maximum of 800 meters in northwest Arkansas.
The Central Lowlands of north Texas are underlain by sedimentary rocks
of Pennsylvanian and Permian age. A thin veneer of Triassic gravel overlies
the Permian. Rocks of the Central Lowlands pass beneath the Edwards Plateau
and Great Plains to the south and west. The Edwards Plateau extends westward
from the Gulf Coastal Plain in Texas. The Plateau is underlain mainly by
limestone of the Cretaceous Edwards Group. The surface rises to the west
and passes beneath Tertiary sediments of the Great Plains.
Fig.2. Geologic map of the southeastern United States.
GEOLOGIC HISTORY OF THE SOUTHEASTERN
Precambrian igneous rocks dating back to 1.4 billion years before present
occur in the Appalachian Piedmont, east of the main mountain trend in the
St. Francis Mountains of Missouri, in the Arbuckle and Wichita Mountains
of Oklahoma, and form the Plano Uplift of central Texas.
Exposures of Precambrian metamorphic rocks occur in the Van Horn Mountains
of west Texas and the Appalachian Piedmont. These exposures are part of
the igneous-metamorphic platform on which Paleozoic and younger sedimentary
rocks were deposited in the southeastern United States. Knowledge about
their history comes basically from these isolated exposures and from samples
taken from wells drilled through the sedimentary cover. The youngest of
these rocks appear to be about 1 billion years old based on radiometric
dates. The oldest Cambrian sedimentary rocks are only 570 million years
old. This suggests that a long period of weathering and erosion strongly
modified the Precambrian surface after emplacement of the intrusive igneous
rocks and eruption of volcanic rocks. A substantial amount of rock and
historical record was probably removed before the first sediments were
deposited in Cambrian seas.
A single tectonic event began in late Precambrian time and continues
to influence historical events. Continental divergence began along a north-trending
line along the axial region of the Mississippi Embayment. Separation and
continental stretching thinned the crust and subsidence occurred forming
an elongate basin that filled with late Precambrian and Cambrian sediments.
Divergence ceased before continental breakup occurred and the sediments
are known from deep wells. The spreading event left a zone of weakness
extending into the continental interior. The modern Mississippi and ancestral
versions of the Mississippi followed this zone throughout the Paleozoic,
and the Mississippi Embayment is at present a prominent extension of the
Gulf Coastal Plain into the continental interior.
Except for the Gulf and Atlantic Coastal Plains, the Edwards Plateau
and the High Plains, Paleozoic rocks underlie most land surfaces in the
southeastern United States. Exposures of these rocks in the mountain ranges
and on plateaus allow a much more complete record of historical events
to be recovered.
During the early Paleozoic periods the stable cratonic surface was transgressed
by broad shallow seas in which mainly carbonate sediments accumulated.
The carbonate succession is broken by thin but widespread units of quartz
sandstone. Occasional regression of the seas exposed the rocks to weathering
and erosion. Subsequent transgressions of these surfaces produced unconformities
that partially truncate successions in certain areas. The continental margins
were less stable. Rifting that began in Late Precambrian time eventually
forming the Mississippi Embayment was also active around the current southern
margin of the North American continent. Continental breakup occurred along
a line now occupied by the Ouachita Orogenic belt as divergence continued
into Cambrian time. Rifting also appears to have occurred along the Atlantic
seaboard along a line parallel to the modern Appalachians. The southeastern
United States was bounded to the east and south by oceanic bodies of water.
Sediment from land areas accumulated in these newly formed oceanic areas
marginal to the continent. Most of these deposits accumulated in deep water
as continental separation proceeded.
A reversal of the divergence occurred as Africa converged with the eastern
margin of North America and South America converged with the southern margin
of North America during late Paleozoic time. Continental collision on eastern
and southern margins during Mississippian and Pennsylvanian time created
the ancestral Ouachita and Appalachian Mountains as high-standing ranges
and as sources of sediment, eroded and transported into the interior of
the continent. Continued collision and suturing of the continental masses
created extensive deformation and uplift of the previously deposited Mississippian
and Pennsylvanian sediment forming the highly faulted and folded modern
Appalachian and Ouachita Mountain ranges. This collision event and the
weight of the resultant mountains depressed the crust and created a series
of basins immediately inland of the ranges. The Appalachian basin that
extends from Pennsylvania through western West Virginia and into Kentucky
and Tennessee, the Black Warrior basin of northern Alabama and Mississippi,
and the Arkoma basin of Arkansas and Oklahoma are part of this basin series.
The period of divergence during late Precambrian and early Cambrian
time also caused rifting along a west-trending line in southern Oklahoma.
As with the Mississippi Embayment, divergence ceased early in the Paleozoic
leaving a zone of weakness and subsidence. Thick successions of limestone
and dolomite accumulated in this elongated basin. The late Paleozoic collision
compressed and uplifted the carbonate rocks forming the Arbuckle Mountains.
Along the same trend further west, igneous basement rock was uplifted forming
the Wichita Mountains. The Anadarko basin, positioned north of the Arbuckle-Wichita
trend, subsided throughout the Paleozoic. Carbonate rocks accumulated forming
the greatest thickness of Paleozoic sedimentary rocks on the North American
The collision also caused uplift on the craton side of the ranges forming
the Ozark Uplift and St. Francis Mountains of Missouri and the Plano Uplift
of central Texas. In both areas, Precambrian igneous rocks are exposed
at the surface. During Permian time, as mountain building ceased, most
of the southeast United States was exposed either undergoing erosion or
receiving terrestrial deposits. Extensive beds of red, nonmarine sandstone
in north-central Texas and north-central Oklahoma were deposited during
Mountain building terminated at the end of the Paleozic and the continent
was largely exposed. Dry climatic conditions allowed marine and nonmarine
evaporite deposits to accumulate. These conditions persisted into Mesozoic
time and influenced sediment deposition in Oklahoma and north-central Texas.
Exposure and erosion on the eastern part of the southeast craton left little
record of Triassic terrestrial deposits.
The supercontinent, formed by the collision of Africa and South America
with North America, began to break up during Triassic time. Divergent plate
motion carried those two continents away from North America forming the
modern Atlantic Ocean and the ancestral Gulf of Mexico. This divergent
movement or rifting stretched the continental crust and created a strong
tensional stress field that caused large normal faults to form parallel
to the axis of rifting. Displacement along the faults was sufficient to
create elongate basins on the downthrown side and high-standing mountain,
composed of granitoid rocks from the lower crust on the upthrown side.
Feldspar eroded from the upthrown side formed thick but narrow deposits
of feldspathic sandstone. Elongate basins of Triassic age are exposed at
the surface on the east side of the modern Appalachians range. East-trending,
basins from the Triassic age are encountered in the subsurface on the south
side of the Ouachita Mountains.
The Appalachian Piedmont forms an elongate between the Appalachian Mountains
and the modern Atlantic Coastal Plain. The Piedmont exists as a surface
of low relief slightly higher than the adjacent coastal plain. It is composed
of an array of metamorphic and igneous rocks having diverse origins. Intrusions
of granite and granodiorite date from the end of the Paleozoic when deep-seated
melting occurred during continental collision. Metamorphism also occurred
during this time. However, Precambrian metamorphic and igneous rocks also
occur in terrains previously attached to Africa prior to the collision
and left behind as Traissic rifting proceeded. Mesozoic rifting proceeded
well into the the Jurassic. The spreading process moved formerly stranded
blocks of continental and transitional crust forming peninsular Florida
and the slightly offshore Bahama platform. Features such as the Monroe
Uplift in southeast Arkansas and the Sabine Uplift in northern Louisiana
may also have been stranded blocks.
As rifting ceased late in Jurassic time, widespread carbonate deposition
began on shelves and ramps around the ancestral Gulf of Mexico. Evaporite
deposition in the central part of the Gulf formed thick deposits of halite.
Cretaceous seas spread across the southeastern United States inundating
most of the land surfaces except the existing Appalachian and Ouachita
highlands and the Ozark Uplift. These seas overstepped and buried earlier
formed Jurassic deposits with Cretaceous sediments. The modern Gulf of
Mexico emerged as a depositional feature during this time. Extensive carbonate
shelves rimmed the deepening part of the basin. Terrigenous deposits became
important only near highland features like the Ouachita and Appalachian
Mountains. Here, early in Cretaceous time, gravel and sand deposits formed
an extensive apron around the southern flank of the Ouachita Mountains.
Even in this area, carbonate sedimentation eventually prevailed forming
deposits of marl and chalk of Late Cretaceous age that today extend southwestward
to southern Texas. The widespread Late Cretaceous seas began to regress
at the onset of the Laramide Orogeny and the formation of the modern Rocky
The Cenozoic history of the southeastern United States primarily involves
the development of the Atlantic and Gulf Coastal Plains and the Great Plains
of west Texas. West Texas and western Oklahoma received waves of terrigenous
sediment transported eastward from the highstanding Rocky Mountain front.
Deposition of the sediment as alluvial fans and sheets agraded the land
surface and formed the Rolling Plains and the High Plains of the Great
Flowing streams, like the Missouri River and the Platte River, transported
clay and sand to the central Gulf of Mexico. The Pacos and the Rio Grande
Rivers delivered sediment loads to the western Gulf from the Rocky Mountain
At the beginning of Tertiary time, marine waters extended northward
into the Mississippi Embayment almost to southern Illinois. The coastline
extended southwestward along the foothills of the Ouachita Mountains in
Arkansas and Oklahoma thence southward through central Texas. The Atlantic
coastline extended westward to the foothill of the Appalachian Mountains
and may have inundated much of the Appalachian Piedmont.
Large volumes of sediment were transported from the western Rocky Mountain
source to the Gulf of Mexico. Lesser amounts came through the Ohio River
via the Mississippi River. The deposition of the sediment in coastal and
near coastal environments caused the coastline to prograde gulfward ultimately
to its current position. The large columns of sediment were accommodated
in the basin through subsidence caused by large normal faults; downthrown
toward the basin and with traces parallel to the basin margin. Eocene and
Miocene depositional centers received thousands of feet of sediment as
the basin subsided.
The Atlantic Coastal plain grew oceanward as streams from the Appalachian
Mountains delivered their sediment loads to coastal areas. With a less
imposing source and a smaller drainage area, it received less sediment
than the Gulf. Progradation of the coastline was significant, however,
subsidence was not as rapid and therefore thicknesses are less. Fluctuations
of sea level caused the coastal area in both the Atlantic and Gulf plains
to migrate, but overall the movement was regressive toward the Gulf.
The coastlines of both the Gulf of Mexico and Atlantic Ocean reached
their present positions during the Pleistocene Epoch of the Quaternary
Period, an interval that extends from 10,000 years before the present (BP)
to the present. Sediment continues to construct extensive depositional
centers around the peripheral of the Gulf and Atlantic coastlines as well
as deeper water submarine fans off the coast.
The Mississippi Embayment continued to be active during the Quaternary.
Divergence and spreading ceased. However, the thin crust initially caused
by stretching and rifting, fails from time to time as the North American
continent drifts westward. When this happens, faulting occurs and energy
is released causing earthquakes. Holocene faults in 1811 and 1812 caused
three earthquakes within a period of two months with magnitudes exceeding
8 on the Richter Scale. Outside of the Mississippi Embayment, stability