Abstract The idea that the Decade of North American Geology (DNAG) project should include preparation of a new geologic map of the continent was conceived early in the DNAG planning process. The minutes of a meeting of the Steering Committee chaired by L.T. Silver on January 29–30, 1980, record that: It was generally agreed that the geographic scope [of the DNAG project] would extend from the Arctic Ocean on the north to the southern limits of the Caribbean plate; from the Mid-Atlantic ridge on the east to the Pacifi c plate in the approximate vicinity of Hawaii. The emphasis would be placed on the geology of the continent; the adjacent sea fl oor would be carried as it is related to the continental story.… At the same meeting “the need for a new geologic map was discussed extensively with some disagreement.” However, at a meeting in May of the same year, a subcommittee appointed to examine the need for a new geologic map unanimously supported the proposal. It estimated that publication costs might be as much as $200,000, compilation costs might be $500,000, and the time required for compilation would be about 5 years. The Steering Committee agreed that a new geologic map covering the area of the DNAG project was needed, and placed compilation of the map on the list of official DNAG efforts. By May 1981, the compilers and principal cartographers had been selected, the base map chosen, the essential features of the explanation agreed upon, and

Abstract “The new Geologic Map of North America covers ~15% of Earth’s surface and differs from previous maps in several important respects: It is the first such map to depict the geology of the seafloor, the first compiled since the general acceptance of plate-tectonic theory, and the first since radiometric dates for plutonic and volcanic rocks became widely available. It also reflects enormous advances in conventional geologic mapping, advances that have led to a significant increase in the complexity of the map. The new map, printed in 11 colors, distinguishes more than 900 rock units, 110 of which are offshore. It depicts more than seven times the number of on-land units as are shown on its immediate predecessor, as well as many more faults and additional features such as volcanoes, calderas, impact structures, small bodies of unusual igneous rocks, and diapirs. When displayed at earth science institutions and libraries, this map is sure to impress viewers with the grand design of the continent and may inspire some to pursue the science of geology. The new Geologic Map of North America is also a “thinking map,” a source for new interpretations of the geology of North America, insights into the evolution of the continent, new exploration strategies for the discovery of mineral and energy resources, and the development of better ways to assess and mitigate environmental risks and geologic hazards.3 sheets (North, South, and Legend), approximately 74 x 40 inches.”

Abstract Along a transect across the Front Range from Denver to the Blue River valley near Dillon, the trip explores the geologic framework and Laramide (Late Cretaceous to early Eocene) uplift history of this basement-cored mountain range. Specific items for discussion at various stops are (1) the sedimentary and structural record along the upturned eastern margin of the range, which contains several discontinuous, east-directed reverse faults; (2) the western structural margin of the range, which contains a minimum of 9 km of thrust overhang and is significantly different in structural style from the eastern margin; (3) mid- to late-Tertiary modifications to the western margin of the range from extensional faulting along the northern Rio Grande rift trend; (4) the thermal and uplift history of the range as revealed by apatite fission track analysis; (5) the Proterozoic basement of the range, including the significance of northeast-trending shear zones; and (6) the geologic setting of the Colorado mineral belt, formed during Laramide and mid-Tertiary igneous activity.

Abstract Precambrian rocks are at or near the surface in only about 10 percent of the conterminous United States, but it can reasonably be inferred that they comprise the bulk of the continental crust beneath about 90 percent of the area. They are missing or unrecognized in the accreted terranes along the Pacific margin, but form significant parts of the crust in terranes accreted to the eastern continental margin during the Paleozoic. The total area of Precambrian rocks considered in this volume is comparable to that of the exposed Precambrian of the Canadian Shield. In comparison with the lateral extent of the Precambrian craton, the thickness of the continental crust is almost insignificant. The width of the craton in the northern conterminous United States is more than half the radius of the plant; the thickness of the continental crust is less than one hundredth of the planetary radius (Fig. 1). The volume of the continental crust is less than 2 percent of the volume of the mantle beneath it. Nevertheless, Precambrian rocks contain the only available record of the assembly and evolution of the fragile continental raft that we know as North America during more than five-sixths of geologic time. Of the areas of exposed Precambrian rocks in the conterminous United States, about half have been covered by modern reconnaissance geologic mapping (scale 1:250,000 or larger); less than a quarter hve been covered by detailed modern mapping (1:62,500) or larger. Perhaps 60 percent of the concealed Precambrian has been at least sketchily explored by drilling at spacings ranging from a few kilometers to a few tens of kilometers.

Abstract Precambrian rocks in the Lake Superior region underlie all or parts of Minnesota, Wisconsin, and Michigan, an area along the southern margin of the Superior province of the Canadian Shield (Fig. 1). Except on the north, adjacent to Canada, the Precambrian rocks are overlapped by sedimentary strata of Paleozoic and Mesozoic age, which constitute a thin platform cover of relatively undisturbed rocks that thicken to the west, south, and east. Inliers of Precambrian rocks are exposed locally in southern Minnesota and Wisconsin, mainly in the flat valleys of major rivers, where erosion has cut below the Phanerozoic strata. The present landscape is subdued, and is inherited largely from Pleistocene continental glaciations, which produced a variety of erosional and depositional landforms. The glacier ice scoured the bedrock in the northern parts of the region, in much the same way as throughout most of Canada, and deposited materials of diverse lithology and provenance, as much as 200 m thick, over much of the remainder of the region. The Precambrian rocks in the region record an extended interval of crustal development and evolution that spans nearly 3 b.y. of earth history. This interval of geologic time is not continuously recorded in layered and intrusive units, but instead is punctuated by specific rock-forming and tectonic events that can be deduced from geologic relations and placed in a chronometric framework by isotopic dating. (Fig. 2, also see correlation chart for Precambrian rocks of the Lake Superior region, Morey and Van Schmus, 1986; and Bergstrom and Morey, 1985.)

Abstract The Wyoming province is the region in Wyoming and adjacent states underlain by rocks of Archean age (Plate 2). It is an Archean craton bordered on the east and south by younger Precambrian provinces (Plate 1). Precambrian rocks are only exposed in the cores of the Laramide (Late Cretaceous to Early Tertiary) uplifts, and outcrops constitute less than 10 percent of the area underlain by Archean basement. Between uplifts, basement rocks are covered by thick Phanerozoic strata, so that extrapolations of geology from uplift to uplift are generally tenuous. Reconstructing the Archean history of the Wyoming province is further complicated by deformation associated with the late Mesozoic fold and thrust belt along the western margin. Although the Archean record is fragmentary, the basement uplifts generally afford excellent exposure. The northern and northwestern margins of the Wyoming province are poorly constrained. Archean rocks are known as far north as the Little Rocky Mountains of Montana (Peterman, 1981). According to King (1976), Archean and Early Proterozoic dates are intermingled at the northwest margin of the province in a wide zone that exhibits a gradational change from Archean dates in central Montana to Early Proterozoic dates to the northwest, reflecting increasing influence of post-Archean thermal events. The southwestern and southern margins of the Wyoming province are also poorly constrained. Archean rocks have been reported from several ranges in the Cordilleran orogenic belt, such as the Albion and Raft River Ranges (Armstrong and Hills, 1967) and possibly the Ruby Mountains of Nevada (A. W. Snoke, personal communication, 1986).

Abstract Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Abstract This chapter describes the Grenville orogen as it is preserved in areas of outcrop as well as in the subsurface in the United States, Late Proterozoic continental rifting that fragmented that orogen, and Precambrian rocks within terranes accreted to the rifted eastern and southern margins of Laurentia (earliest Paleozoic North America). The accretion of terranes to the eastern and southern margins of Laurentia formed the Paleozoic Appalachian (Caledonide)-Ouachita orogen. Outliers of the Grenville orogen, variously deformed by Paleozoic orogenies, crop out within the western part of the Appalachian orogen. Although protoliths as old as Archean have been identified along the northwestern margin of the Grenville orogen in Canada, as far as we know, no rocks in the areas covered by this chapter are older than Middle Proterozoic. Some rocks, however, indicate ties to older source areas. Quartzites from the Adirondack Lowlands contain detrital zircons with minimum ages of 1.65 Ga, 1.8 Ga, and 1.95 Ga (McLelland and others, 1988a). Recent work by Dunning and Cousineau (1990) and Olszewski and others (1990) on rocks of the Chain Lakes block, Maine-Quebec, and correlatives to the north in the Canadian Appalachians has shown that some zircons, possibly detrital, in diamic-tite are as old as 2.8 Ga. Sinha and Bartholomew (1984) report a discordia intercept of 1.87 ± 0.2 Ga for zircons, probably detrital, from layered gneiss of the Blue Ridge Grenvillian outlier in Virginia. Zartman and Hermes (1987) report a Late Archean inheritance in zircon from Permian granites in the southeastern New England Avalonian terrane; they attribute this to the under-plating of the Avalonian microplate by Archean crustal components, possibly of Africa, in the late Paleozoic during the collision of Gondwanaland with Avalonia.

Abstract The distribution of Middle and Late Proterozoic sedimentary and metasedimentary cover that lies unconformably on Early Proterozoic and Archean crystalline basement has been known for decades, but recent work, employing techniques of paleomagnetic correlation, sedimentology, sequence stratigraphy, and analysis of tectonic subsidence has led to modifications of some long-accepted correlations and tectonic models. Within the context of both older classical studies and this new work, the stratigraphy, correlation, tectonic setting, fossil content, and mineral potential of Middle and Late Proterozoic rocks of parts of the Rocky Mountain, Colorado Plateau, and Basin and Range provinces of the United States are discussed. A problem common to interpretation of all Proterozoic strata is a widespread lack of fossil control on age and paleoecology, which makes correlations inherently uncertain and interpretation of depositional environments more difficult. We present current hypotheses about these topics and stress the uncertainty of some of our conclusions. The apparent polar wander path for the North American craton, as derived from the Middle and Late Proterozoic sedimentary cover, is central to our modifications of stratigraphie correlation, especially of Middle Proterozoic rocks. The reader is asked to view the work and summaries presented here in the light of ongoing scientific debate about strata that are chronically stubborn in yielding information. The authors of sections of this chapter include both those who have performed classical studies, which are the foundation of our present understanding, and younger geologists who have been busy refining and modifying early interpretations, using different methods of study. The treatment in this chapter is therefore variable depending on which generation of investigators is speaking.

Abstract The preceding chapters, which deal with myriad facets of the Precambrian geology of the conterminous United States, necessarily emphasize local descriptions and details. In this chapter we consider some broader regional and global relations. Hamilton discusses some of the difficulties with conventional views of the early evolution of the crust, and speculates that the geology of Venus, as elucidated by the new Magellan imagery, may provide a model for terrestrial Archean tectonics. Farmer and Ball describe the Nd-Sm isotopic system and discuss the applications of Nd-Sm studies to the determination of model crustal ages and delineation of major crustal provinces. Finally, we summarize the development of the southern part of Laurentia, the Late Proterozoic to early Paleozoic supercontinent that was fragmented during the Paleozoic and Mesozoic to produce the North American craton. Geologic processes have changed greatly as the Earth has progressively lost heat, and plate-tectonic models that work well for the Phanerozoic (where my own expertise mostly lies) become progressively less applicable to successively older Precambrian assemblages. These models nevertheless can be fitted with reasonable success to Proterozoic geology, as Hoffman has been the leader in demonstrating; his 1989a paper provided excellent syntheses of the geology and tectonic significance of the Proterozoic tracts of Canada and the United States in such terms. Accretionary wedges, ophiolites, fore-arc and back-arc basins, oceanic and continental magmatic arcs, and rifted-margin sedimentary wedges are all recognizable in Proterozoic assemblages although different in important ways from modern ones. Archean rock and tectonic assemblages are markedly less yet like modern ones. Although an increasing consensus (as, Hoffman, 1989a; until recently, I concurred) holds that Archean geologic processes also were dominated by plate tectonics, major features now appear to me inexplicable in such terms.

Abstract This wide-ranging discussion of Precambrian rocks includes contributions from a diverse array of authors actively engaged in investigations of various aspects of U.S. Precambrian geology. Summary discussions by the editors of the five major chapters place these contributions in a logical regional framework. A concluding chapter explores Archean crustal processes from the point of view of lunar and planetary analogies, discusses the significance of Sm crustal provinces, and provides an overview of the development of the southern parts of Laurentia. Accompanying plates include a newly compiled map of the Precambrian rocks of the conterminous U.S., maps showing relationships of the Precambrian geology to magnetic anomalies and to isostatic residual gravity, and a new correlation chart for U.S. Precambrian rocks.

Abstract Precambrian rocks are at or near the surface in only about 10 percent of the conterminous United States, but it can reasonably be inferred that they comprise the continetal crust beneath about 90 percent. They are missing or unrecognized in the exotic terranes along the Pacific margin of North America, but they probably form significant parts of the crust in exotic terranes or continental fragments accreted to the eastern part of the continent during Paleozoic time. Thus, the total area of Precambrian rocks to be considered in this volume is comparable to that of the exposed Precambrian of the Canadian Shield. It is important to remember that in spite of the enormous lateral extent of the craton the volume of the continental crust is almost insignificant. The width of the North American craton is more than half the radius of the planet, but the thickness of the continental crust is less than one hundredth of the planetary radius (fig. 1). The volume of the crust is less than 2 percent of the volume of the mantle beneath the U.S. part of North America. Precambrian rocks contain the only available record of the assembly and evolution of the fragile continental raft that we know as North America during more than four fifths of geologic time. Of the areas of exposed Precambrian rocks in the conterminous United States, about half have been covered by modern reconnaissance geologic mapping (scale 1:250,000 or larger); less than a quarter have been covered by detailed modern mapping (scale 1:62,500 or larger).

Abstract Washington, D.C., is the first and largest planned city in the United States. The city lies along the Fall Line at the boundary between the Atlantic Coastal Plain and the Piedmont Plateau and at the head of navigation on the estuary of the Potomac River. This site combines the engineering complexities of two vastly different geologic terranes with the other complications introduced by the terraces and channels of a major river-estuary system. The western part of the city and most of the suburbs to the west and north are on the Piedmont Plateau, an upland underlain by complexly deformed metasedimen-tary and metaigneous rocks of late Precambrian or early Paleozoic age. These crystalline rocks are mantled by soil, saprolite, and weathered rock to depths of as much as 50 m, which adds both to their geologic inscrutability and to the problems of excavation and design of structures. The Atlantic Coastal Plain is underlain by unmetamorphosed and little deformed fluvial and marine strata of Cretaceous through Miocene age. These deposits form a prism that thickens southeastward from a wedge edge at the Fall Line to as much as 450 m in the southeastern part of the metropolitan area. Unconformities, facies changes, and variations in physical properties with age and depth of burial add spice to the life of the engineering geologist dealing with these strata. Terrace deposits ranging in age from Miocene(?) to Holocene bevel across the contact between the Coastal Plain deposits and the crystalline rocks of the Piedmont. The oldest deposits underlie a broad, deeply dissected upland that stands at an elevation of 80 to 90 m southeast of the Fall Line; isolated outliers cap hills and interfluves at elevations of as much as 150 m northwest of the Fall Line. Lower and younger terraces flank the major drainages and occur at various levels down to the modern flood plains. Much of the central city is built on low terraces of Sangamon or Wisconsin age. These younger terraces locally fill and conceal deep bedrock channels cut by the ancestral Potomac during low stands of sea level during the Pleistocene. The terrace deposits show conspicuous differences in degree of weathering and soil development, depending on their age and physiographic position. Estuarine and marsh deposits flank the tidal reaches of the Potomac and Anacostia Rivers, and considerable parts of the central city are built on artificial fill over these deposits. Considerable experience in underground excavation has been gained in the last decade during construction of METRO, a regional rapid transit rail system. Tunneling techniques have been developed for both crystalline rocks and Coastal Plain deposits, but cut and cover methods are generally used in the young materials, which are generally weakest. Foundation and slope stability problems are widespread in some geologic units in the metropolitan area and are locally serious. They affect structures ranging from single family dwellings to the Washington Monument.