GeoScienceWorld
Volume

The Appalachian-Ouachita Orogen in the United States

Edited by Robert D. Hatcher Jr., William A. Thomas and George W. Viele

Abstract

Includes 14 chapters on the Appalachian orogen, 15 of the Ouachita orogen, and a chapter on the connection between them beneath the eastern Gulf Coastal Plain. The Appalachian chapters synthesize the geologic development of the orogen by tectonostratigraphic intervals (pre-orogenic, Taconic, Acadian, Alleghanian, and post-Alleghanian), and also treat Paleozoic paleontologic control, regional geophysics, thermal history of the crystalline terranes, parts of the orogen buried beneath the Atlantic and eastern Gulf coastal plains, regional geomorphology, mineral and energy resources; an integration chapter also is included. The Ouachita chapters cover physical stratigraphy and biostratigraphy of the Paleozoic rocks, structural geology, a synthesis of the subsurface geology beneath the western Gulf Coastal Plain, a review of the mineral and energy resources, regional geophysics, and a tectonic synthesis. Twelve excellent plates provide four-color geologic maps, structural cross sections, tectonic syntheses, and geophysical maps; a black-and-white synthesis of Appalachian mineral deposits, and a reflection seismic cross section.

  1. Page 1
    Abstract

    The Appalachians are a deeply eroded Paleozoic mountain chain (Fig. 1) characterized by Philip King (1970, p. 437) as the most elegant mountain chain on Earth. He recognized that the orderly linearity of the chain is deceiving, and that the Appalachians are full of guile, arousing geologic controversies as acrimonious as any in our science.

  2. Page 7
    Abstract

    This chapter deals with the evolution of the various parts of the Appalachian orogen in the United States prior to the onset of the formative deformational episodes of the orogen in the early Paleozoic. The general discussion that follows is intended to add perspective to the more detailed discussions within each section.

  3. Page 101
    Abstract

    Until very recently, most geologists were conditioned to seek the effects of three major events–the Taconian, the Acadian, and the Alleghanian–within the Appalachian orogen. Things are not that simple, however, as the importance of older deformations is increasingly being recognized. Although this chapter is concerned primarily with the Taconic orogen (sensu stricto), two older deformational events are considered herein. These events are the Blountian and Penobscottian orogenies. The Penobscottian event has been recognized for some time (Neuman, 1967; Hall, 1969, 1970), but its importance in Appalachian geology has only recently become apparent by work in northern Maine (Osberg, 1983; Boone and others, 1984) and the Potomac Valley of Virginia and Maryland (Drake and Lyttle, 1981; Drake, 1987). In Maine, the Penobscottian can only be dated as pre-late Ibexian (pre-Arenigian), whereas in the Potomac Valley it is thought to be of late Middle Cambrian to early Late Cambrian (Dresbachian) age. Neither syn- nor post-orogenic sediments are recognized that could have resulted from the Penobscottian deformation. On the contrary, the Blountian event is recognized because of its syn-and post-orogenic sediment wedge, but deformational features related to the event have not as yet been recognized in the Blountian hinterland, although isotopic dating within the Blue Ridge is permissive of deformation at this time. The Blountian orogeny has been recognized for many years (Kay, 1942), and Rodgers (1953) has termed it the Blountian phase of the Taconicorogeny. In my opinion, it was a separate tectonic event that was completed prior to the Taconic (sensu stricto), as its uppermost molasse is overlain by distal Taconian syn- and post-orogenic deposits. It goes without saying that the effects of the Penobscottian and Blountian orogenies are difficult to recognize and separate from those of the Taconic orogeny. For this reason, the effects of the earlier events will be discussed with those of the Taconic where they are believed to be present.

  4. Page 179
    Abstract

    An orogenic cycle (Wilson, 1963) in simplest form may be of long duration, and it includes rifting, subduction, and final closure by which an ocean basin is initiated and ultimately destroyed. In real orogens the cycle must be more complicated, being punctuated, possibly at several intervals, by such things as cessation of subduction, collision of microplates, or by obduction. The Acadian orogeny probably represents such a punctuation in an orogenic cycle that lasted through Paleozoic time. Where the Acadian orogeny is recognized in the Appalachians, it was in some places preceded by earlier punctuations (Penobscottian, Taconian), and was followed by a final collisional event (Alleghanian). In the context of the Acadian orogeny, only those geologic features that have a causal relationship to the Acadian will be considered in this chapter.

  5. Page 233
    Abstract

    The Alleghanian orogeny is the most pervasive event to affect the central and southern Appalachians. This is the event referred to in the older literature as the “Appalachian revolution,” and is the mountain-building event most associate with the Appalachian chain. The Alleghanian orogen includes a foreland belt of folds and thrust faults that propagated into sedimentary rocks of the North American craton in the western part of the southern and central Appalachians, and to the east, an internally complex belt of allochthonous mostly pre-Alleghanian metamorphic rocks (Fig. 1; Plate 1). A line of rootless external Precambrian basement massifs is near and approximately parallel with the cratonward limit of early to middle Paleozoic metamorphic rocks, and the metamorphic belt contains several internal massifs of Precambrian basement rocks, mostly in windows. A more internal zone of Alleghanian amphibolite-facies metamorphism appears in both the southern and New England Appalachians.

  6. Page 319
    Abstract

    Post-Paleozoic tectonic activity in the Appalachian orogen (including the concealed basement of the passive margin) is primarily a consequence of the breakup of Pangaea and the opening of the Atlantic Ocean. It embraces a major tectonic cycle that is marked by Late Triassic-Early Jurassic rifting of the Alleghanian-Variscan orogen and by Middle Jurassic to Recent drifting of the newly forming passive margin.

  7. Page 375
    Abstract

    Paleontology is essential for the determination of world paleogeography, including that of ancient mobile belts such as the Appalachians. In this chapter we review the contributions of paleontology to interpretations of the Paleozoic paleogeographic evolution of the Appalachians–particularly that part of the Appalachians in the United States outboard of the early Paleozoic miogeocline.

  8. Page 385
    Abstract

    This chapter reviews the geophysical data in the U.S. Appalachians–including gravitational and magnetic fields, refraction and reflection seismology, terrestrial heat flow, and electrical properties. An even treatment of the various kinds of geophysical data is neither attempted nor justified. Instead, emphasis and bias are placed primarily on geophysical data that have been useful for the interpretation of the tectonic history of the orogen; of these, seismic reflection data have had the greatest impact on the development and testing of tectonic models in the U.S. Appalachians and elsewhere because of their greater resolution.

  9. Page 417
    Abstract

    Previous chapters have documented the polyphase character of Appalachian orogenesis, and have outlined many difficulties in resolving overprinting relationships. A problem along the length of the orogen is to determine the extent to which the tectonothermal record within eastern crystalline terranes developed concomitantly with the late Paleozoic westward vergent folds and thrusts that characterize deformed portions of the foreland. Except for a few locations in southeastern New England, overstepping late Paleozoic cover sequences are absent, and establishing the chronology of superimposed tectonothermal events must be largely based on collaborative field and geochronological investigations. Resolution of the extent and nature of superposed metamorphic events may best be gained by comparing a set of systematically determined argon mineral ages with radiometric dates provided by other more refractory isotopic systems such as Rb-Sr whole-rock and/or U-Pb zircon. This chapter will discuss how argon mineral ages may be used to more clearly resolve the timing and regional extent of late Paleozoic tectonothermal events within U.S. portions of the Appalachian orogen. A brief review of argon dating methods and the problems inherent in these methods is presented initially. This is followed by a review and interpretation of available data from several representative areas. For simplicity of presentation, the discussion is arbitrarily divided into sections dealing with northern and then southern portions of the orogen. All radiometric ages presented have been calculated on the basis of the decay constants and isotopic abundance ratios listed by Steiger and Jäger (1977).

  10. Page 445
    Abstract

    Paleozoic rocks exposed in the Appalachian Mountains are bordered on the east and south by the Atlantic and Gulf Coastal Plains, and rocks of the Paleozoic orogenic belt may be traced beneath the cover of post-orogenic Mesozoic-Cenozoic Coastal Plain strata (Plate 6). Large-scale lithotectonic components of the exposed Appalachian orogen include the foreland fold-thrust belt and internal metamorphic belts (collectively called the Appalachian Piedmont), which locally contain internal basement massifs. External basement massifs and discontinuous belts of low-grade metamorphic rocks are distributed along the boundary between the fold-thrust belt and the Piedmont. Accreted terranes are identified in the internal metamorphic belts.

  11. Page 459
    Abstract

    The Appalachian Highlands, with the inclusion of the Appalachian Plateaus, form a major geomorphic region that comprises approximately one tenth of the area of United States, and also includes part of Canada. The U.S. boundaries of the Appalachian Highlands were delineated in 1930 by a committee of geomorphologists headed by N. M. Fenneman. The work was published as the Map of Physical Divisions of the United States, distributed by the U.S. Geological Survey in 1946. The area described in this chapter is divided into six provinces as shown in Figure 1.

  12. Page 471
    Abstract

    Students of mineral deposits have generally bypassed the Appalachians in preference to studying deposits of the western United States. A “western outlook” in economic geology is not surprising. For the mining and exploration geologist, the western United States has traditionally represented a wide-open land with excellent rock exposure, no glacial cover or saprolite, and vast acreages of public land available for prospecting. Furthermore, many economic geologists have felt that virtually all important deposits of the Appalachians were discovered and thoroughly studied long ago. Hence, a generation or two of economic geologists, with the exception of a few hardy academic, government, and exploration geologists, have given short shrift to the study of Appalachian mineral deposits.

  13. Page 495
    Abstract

    The Appalachian basin is an elongate asymmetric synclinorium that extends from Lake Ontario southwestwardfor 1600 km through New York, Pennsylvania, Ohio, WestVirginia, Virginia, eastern Kentucky, Tennessee, and Georgia to Alabama (Fig. 1). The basin consists of Paleozoic strata ranging from 600 to 900 m thick on its west flank, along the Cincinnati arch, to more than 13,700 m thick on its east side in central Pennsylvania adjacent to an allochthonous metamorphic terrain. As defined herein, the basin-filling rocks underlie most of the central and southern parts of the Appalachian mountain chain. To the northeast in New England, coal-bearing strata of Pennsylvanian age underlie approximately370 sq km in the Narragansett Basin.

  14. Page 511
    Abstract

    The preceeding chapters in this book constitute syntheses of our knowledge of the tectonic history of the U.S. Appalachians following their inception as a rifted and passive margin after the Grenville orogeny to the present state of decay. The purpose of this chapter is to summarize the evidence for the major events of the Paleozoic and later history of the Appalachians, to bring to light similarities and differences between along-strike segments, and to explore aspects of Appalachian history from the perspective of the tectonic map (Plate 1) that may not have been brought out in the previous chapters. This, as all syntheses, represents only a progress report whose total complexion may change with the appearance of new data. Details of stratigraphy and structure were outlined in previous chapters.

  15. Page 537
    Abstract

    Late Paleozoic orogenic structures exposed in the Appalachian Mountains of Alabama and in the Ouachita Mountains of Arkansas extend from opposite directions beneath a cover of post-orogenic Mesozoic-Cenozoic strata in the Mississippi Embayment of the Gulf Coastal Plain (Fig. 1; Plates 6, 9). Although the physiographic expressions of the Appalachian and Ouachita Mountains end at the edge of the Coastal Plain, the orogenic belt neither ends nor changes abruptly along strike at the present onlap limit of Coastal Plain strata. Instead, as shown by data from deep wells and geophysical surveys, a continuous belt of Paleozoic orogenic structures extends beneath the post-orogenic Coastal Plain cover (Fig. 1; Plates 6, 9). Nevertheless, both the Paleozoic stratigraphic sequence and details of structural style exposed in the Ouachita Mountains contrast strongly with those in the nearest Appalachian outcrops, and projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the Coastal Plain cover.

  16. Page 555
    Abstract

    The name “Ouachita orogen” or “Ouachita orogenic belt” applies to the belt of deformed Paleozoic rocks flanking the southern margin of the North American craton. It is about 2,100 km in length, extending from the subsurface of Mississippi to the Marathon region of Texas; about 80 percent of this distance lies in the subsurface, buried beneath the Mesozoic and Tertiary sediments composing the Gulf Coastal Plain. The two major areas of outcrop of the orogenic belt are in the Ouachita Mountains of Arkansas and Oklahoma and in the Marathon region of west Texas (Fig. 1). The map trace of the Ouachita Orogen, from Mississippi to north Texas, defines a broad arcuate salient (Thomas, 1977a) extending into the North American continent. From north Texas the trace continues southward, passing to the east and southeast of the Llano Uplift of central Texas, where it forms a recess before bending abruptly northwestward to the Marathon region. Southward, from the Marathon region, small, widely scattered outcrops show that the Ouachita Orogen extends at least 450 km into Mexico (Handschy and others, 1987); from there, its southward or westward projection is conjectural

  17. Page 563
    Abstract

    The Paleozoic rocks of the Ouachita Mountains (westcentral Arkansas and southeastern Oklahoma) consist primarily of sparsely fossiliferous deep-water deposits. Until recently, graptolites have provided most of the biostratigraphic evidence for the correlation of these rocks with sections in other parts of North America. Conodonts have been known from the Arkansas Novaculite for over 60 years, but their presence in limestones within the dominantly shaly Ordovician part of the sequence was demonstrated only recently (Repetski and Ethington, 1977). Shelly fossils are almost unknown from the pre-orogenic rocks of the Ouachita Mountains, although fragments of brachiopods, bryozoans, and trilobites have been found in acid residues of limestones thatwere processed for conodonts.

  18. Page 575
    Abstract

    Pre-orogenic rocks of the Ouachita Mountains include nearly 3,500 m of strata ranging in age from Late Cambrian to Mississippian (Osagian; Fig. 1). They crop out in the Benton and Broken Bow uplifts and in several smaller uplifts within the zone of frontal thrust faults in Oklahoma, including the Potato Hills and Black Knob Ridge (Fig. 2). The sequence is dominated by fine-grained clastic units, including shales of the Collier, Mazarn, and Womble Formations. Interbedded quartzose sandstone and detrital limestone units are present in the lower part of the section and chert is abundant near the top.

  19. Page 591
    Abstract

    Post-Arkansas Novaculite strata comprise the major part of the Ouachita stratigraphic sequence, both in outcrop area and rock thickness. These flysch successions are almost exclusively deep marine and show a marked contrast with typical shallowwater, coal-bearing Carboniferous rocks of the southeastern United States. The Carboniferous rocks of the Ouachitas are much thicker than underlying pre-flysch rocks and include virtually no chert or novaculite.

  20. Page 603
    Abstract

    Paleozoic rocks that were deposited along the southeastern margin of North America during Paleozoic time and that make up the Ouachita orogen extend from Arkansas across Oklahoma and Texas and have been traced almost to Mexico. Rocks at the southwestern end of the Ouachita orogen are exposed only in the Marathon and Solitario uplifts in west Texas, but the western edge of the sequence in the subsurface of Texas is fairly well known from well and seismic data (e.g., Flawn and others, 1961; Nicholas and Rozendal, 1975). The Marathon uplift is a broad domal uplift of early Tertiary age and is more than 125 km in diameter (King, 1937). Erosion of Cretaceous and younger strata from the crest of the uplift produced the topographic Marathon Basin in which are exposed, in an area approximately 50 by 75 km, deformed Paleozoic rocks that have a composite stratigraphic thickness of 5000 m. Permian strata in the Glass Mountains unconformably overlie older rocks alongthe northwestern edge of the Marathon uplift (Fig. 1). The Solitario uplift, 65 km to the southwest, provides exposures of Paleozoic rock approximately 8 by 15 km at the crest of a buried intrusion.

  21. Page 621
    Abstract

    The outcrop regions of the Ouachita system cover but a small fraction of the area occupied by the entire system, yet they may serve as a general model for muchof the belt in the subsurface, where it lies unconformably beneath younger rocks of the Gulf Coastal Plain. Sparsity of penetrations by wells, similarity of late Paleozoic strata, and structural complexity make a detailed tectonic resolution in the subsurface an exceedingly difficult if not impossible task. An understanding of the structural styles and evolution in the outcrop regions of Oklahoma-Arkansas and west Texas assists in unraveling the regional problems of the entire system between the Appalachians and northern Mexico. However, one has to remember that both the Ouachita-Arkoma and the Marathon outcrop region probably represent just the frontal elements of an orogenic belt and they may not give much insight into the makeup of the orogenic interior and hinterland.

  22. Page 635
    Abstract

    The Benton and Broken Bow uplifts form the central core of the Ouachita Mountains of Arkansas and Oklahoma, respectively (Plate 8; Fig. 1). These uplifts are made up of complexly deformed, early to middle Paleozoic rocks and constitute the largest exposures of the Upper Cambrian to Mississippian, preorogenic strata of the Ouachita folded belt (Lowe, this volume). The oldest formation is the partly exposed Collier Shale, and the youngest is the Devonian to Mississippian Arkansas Novaculite (Ethington and others, this volume). Also described in this chapter are regions of Carboniferous orogenic strata that flank the older rocks of the uplifts.

  23. Page 661
    Abstract

    The exposed parts of the Ouachita system, the Ouachita Mountains and the Marathon uplift, are located in structural salients along the frontal margin of the orogenic belt (Plate 9). Between these exposures, the Ouachita system is concealed for about 1,000 km along strike beneath nearly flat-lying Mesozoic and Tertiary rocks of the Gulf Coastal Plain, but its course has been defined by subsurface data from hundreds of wells and by geophysical information. Borehole data also indicate that Ouachita strata extend southeastward from the Ouachita Mountains into the Mississippi Embayment (Thomas, this volume).

  24. Page 673
    Abstract

    In 1937, P. B. King published his classic treatise on the rocks of the Marathon Basin. Today, that work stands as the definitive reference and primary source for the west Texas part of the Ouachita fold-thrust belt. This paper summarizes the present state of knowledge about the basin (emphasizing recent insights), raises more questions than answers, and demonstrates the soundness of King's work to this day.

  25. Page 681
    Abstract

    A variety of major basins and uplifts are found on the foreland adjacent to the present limit of the Ouachita foldbelt (Fig. 1). These are both parallel and transverse to the foldbelt. Individual smaller structures within these basins and uplifts are generally parallel to the structural trend of the major features. The parallel structures are a series of basins that appear to have a generally analogous structural history and style and that can, in turn, be related to Ouachita events. The transverse structures do not share a common history and their relationship to Ouachita deformation is unclear.

  26. Page 689
    Abstract

    The Ouachita system is a major orogenic belt whose importance can be overlooked because of its limited surface exposures. Since it is mostly buried, we must rely to a considerable degree on a few deep drill holes and geophysical data to infer its nature and extent. These data provide much valuable information, but as other papers in this volume document, many important questions will remain unanswered for many years.

  27. Page 695
    Abstract

    The tectonic history of the Paleozoic Ouachita orogenic belt composes a complete Wilson cycle: the early history records the rifting of the southern margin of the North American craton and the opening of an ocean basin in which marginal and basinal pre-orogenic sediments were deposited; the later history records the closing of that ocean basin by south-directed subduction and the accompanying deposition and deformation of synorogenic clastic sediments. The off-shelf, pre-orogenic rocks and the deepwater, synorogenic rocks compose the “Ouachita fades” (Fig. 1; Plates 9, 11), deposited beside but not on the North American craton, although the youngest synorogenic sediments lapped onto the southern margin of the craton. Rocks of the Ouachita facies were thrust onto the southern margin of the North American craton during the final stages of ocean closing and constitute the Ouachita orogenic belt. Geologic structures related to the Ouachita orogeny extend beyond the boundaries of the orogenic belt into the cratonal interior.

  28. Page 729
    Abstract

    The Ouachita Mountains are host to a wide variety of mineral deposits. The metallic deposits have been ascribed to two major metallogenic events: a Late Pennsylvanian-Early Permian event and a Late Cretaceous event. The Paleozoic event is intimately related to the occurrence of hydrothermal quartz veins (Miser, 1943, 1959; Engel, 1952) from which adularia crystals have yielded radiometric ages of approximately 260 ± 10 m.y. (Bass and Ferrara, 1969; Denison and others, 1977; Shelton and others, 1986). The Mesozoic event is related to Late Cretaceous alkalic igneous activity associated with the Magnet Cove and Potash Sulfur Springs intrusive complexes (Erickson and Blade, 1963).

  29. Page 739
    Abstract

    Hydrocarbons have been identified at several localities along the 2092 km length of the Ouachita trend. By far the greatest concentration of occurrences is in southeastern Oklahoma and adjacent parts of north Texas and western Arkansas (Fig. 1). A few deep wells in central Texas have encountered showings of asphaltic material; one has encountered a small flow of natural gas. In the west Texas portion of the Ouachita belt there are two small oil fields and a three-well gas field (Diemer and others, 1980; Gaines and others, 1982). Some oil in shallow wells and bituminous layers in outcrops have been reported in the Marathon Basin of far west Texas (King, 1937). At the eastern end of the Ouachita belt, in eastern Arkansas, Mississippi, and western Alabama, there are no reports of hydrocarbons. In that large area, however, there is only a handful of deep wells and, of course, no surface exposures.

  30. Page 747
    Abstract

    Happily, our understanding of an orogenic belt is never complete. During the past two decades, much has been learned about the Ouachitas, but many problems remain. The following conjectures about the path of future research are a personal and may not speak to the questions that others would ask. Geologists collect data not at random but with a specific problem in mind, yet it is their good fortune that the data themselves are cosmopolitan and may be applied to a range of problems. With that justification for being parochial, here are some of the unsolved problems of Ouachita geology as I see them.

Purchase Chapters

Recommended Reading