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
Chapter Contents
INTRODUCTION
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 find today.
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 United States
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 outcrop belts.
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 UNITED STATES
Precambrian History
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.
Paleozoic History
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 continent.
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 this time.
Mesozoic History
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 Mountains.
Cenozoic History
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 Plains Province.
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 front.
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.
Quaternary History
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 ensued.