Abstract

The history of the North American continent starting roughly a billion years ago is preserved remarkably in the Appalachian, Ouachita, Cordilleran, Franklinian, and East Greenland (Caledonian) geosynclines. All except the Ouachita geosyncline are known to contain great thicknesses of Proterozoic terrigenous sediments, in many places overlain conformably by fossiliferous Cambrian strata. The similarity in the history of the Ouachita and Appalachian geosynclines and their apparent direct connection in Alabama imply that the Ouachita trough also originated in Proterozoic time. As Dana noted, the size and height of the mountains resulting from the deformation of a geosynclinal mass correlate with the size of the adjacent ocean basin. Also the climax of the orogeny may correlate with the geographic position of the geosyncline in respect to the adjacent oceon basin. For example, in the North Atlontic Ocean basin, the East Greenland Caledonian orogeny climaxed in Late Silurian—Early Devonian time; farther south, the Acadian orogeny climaxed in Devonian time; and still farther south in the Southern Appalachians, in Carboniferous time. The climax of the deformation of the Franklinian geosyncline in pre-Middle Pennsylvanion time—much later than that of the converging East Greenland geosyncline—may result from its less active reaction with the relatively small Arctic Ocean basin.

The first phase of geosynclinal sedimentation was characterized by quartzose and feldspathic sands and muds which accumulated slowly in a simple downwarp. Bordering the geosyncline on the outer or ocean basin side was a geanticlinal upwarp of Archeozoic crystalline rocks which supplied much sediment. The basal unit of the Proterozoic Ocoee Group of the Southern Appalachians came from the southeast (Appalachia) and much of the Belt and Purcell of the Cordilleran geosyncline appears to have come from the west (Cascadia) including northwestern Washington where recent zircon-age determinations prove the existence of Archeozoic crystalline rocks. The ultramafic, anorthositic, and associated crystalline rocks of the northern tip of the continent (Ellesmere Island) probably represent Pearya; certainly the Devonian clastic wedges of the Franklinian geosyncline were derived from a northern source. The Proterozoic clastics of the East Greenland Caledonian geosyncline, originally thought to have been derived entirely from the Archeozoic shield on the west, now are thought to have come, at least in part, from the Atlantic and/or the north.

The second phase of geosynclinal sedimentation, as exemplified by the Early Cambrian Chilhowee Group of the Southern Appalachians and the Windermere Group of the Canadian Rockies, bears more feldspathic sands and grits accompanied locally by the appearance of polymictic gravels. The more rigorous tectonic regimen they suggest also is supported by a shifting cratonward of the geosynclinal axis. In the Cordilleran, East Greenland Caledonian, and Appalachian geosynclines, the first two phases may have represented up to half a billion years or more. By contrast, the third phase may have lasted about 100 m.y., and it was characterized by the accumulation on a slowly subsiding shelf of shallow-water clean-quartz sand, dolomite, limestone, and some mud. It is exemplified in the Southern Appalachians by the Lower Cambrian Erwin Quartzite and the overlying carbonates and shales, whose ages range up to the Middle Ordovician. The fourth phase was longer and was characterized by active syntectonic sedimentation. For example, in the Southern Appalachians, where the fourth phase includes Middle and Upper Ordovician limestone and shale, Silurian sandstone, and Devonian sandstone and shale, more than 10,000 ft of structural relief had developed between the Blue Ridge anticlinorium and the complementary synclinorium on the northwest by Middle Ordovician time. The molossic fifth phase accumulated as the depositional axes migrated farther cratonward. The Carboniferous coal, sandstone, and conglomerate of the Southern Appalachians ore a prime example. The first appearance of coal marks the beginning of the end of the geosynclinal cycle. Overthrusting accompanied this phase and represents the climax but not the end of the orogenic cycle. In the thrustfaulted Southern Appalachians, as much as 10,000 ft of structural relief was developed locally after thrusting, as compression continued to be transmitted through the overridden rocks. The Cordilleran and probably the Appalachian eugeosyncline developed much later than the initial geosyncline and on its borderland or oceanbasin side and evolved much more rapidly.

The protracted evolution of North American geosynclines reflects a fundamental compressional reaction between the margin of the continent and the adjacent ocean basins. The gently deformed Ordovician to Devonian strata of the Suwannee basin of Florida and adjacent states are interpreted to have been deposited on the outside of the main zone of reaction, the geosyncline, and are in part the basis for the prediction that fossiliferous Paleozoic strata will be cored from the Atlantic Ocean basin in the foreseeable future.

The protracted nature of North American geosynclinal development, the difference in the time of the orogenic climax along the Atlantic side of the continent, and the climax of other North American geosynclines indicate that the fundamental tectonic mechanism is not a catastrophic operation that began belatedly in post-Permian time. The geosynclines of North America resulted from stress systems that began in Proterozoic time and continue to operate to this day. The Triassic basins of eastern North America parallel older Appalachian trends, and the axis of seaward tilting of the Cretaceous and Tertiary sediments of the Atlantic Coastal Plain parallels both Triassic and Appalachian trends. The Gulf Coast geosyncline parallels Ouachita trends on the north, and the similar history of the Gulf and Atlantic Coastal Plains suggests that the reaction of the continent with the Gulf of Mexico is similar to that with the Atlantic Ocean basin. The trend of both the Pacific Coast Ranges and the axis of uplift and Cenozoic volcanism of the Sierra-Cascade province indicates vigorous continuation of the same stress system that formed the Proterozoic Belt trough and the Rocky Mountain system on the east. In the Arctic Archipelago, the structural downwarping of the Sverdrup basin with its late Paleozoic to early Tertiary sediments represents continuation of the same north-south-directed stress system that developed the Franklinian geosyncline in Proterozoic time and deformed it prior to Middle Pennsylvanian time. The protracted, three-dimensional, sphincterlike compression to which North America has been subjected for roughly a billion years represents a reaction or motion antagonistic to the proposed separation of the continents associated with the postulated opening of the Atlantic Ocean basin in post-Permian time. Particularly important is the fact that the north-south-directed systems responsible for the Ouachita and Gulf Coast geosynclines and for the Franklinian and Sverdrup troughs have a tectonic history similar to that of the rest of the continental margin.

The time has come to destroy once and for all the myth that the Cambrian-Precambrian boundary is, from a tectonic standpoint, of special significance. Moreover, no matter how additional evidence from the ocean basins affects the theories of continent drift and sea-floor spreading, proponents and opponents alike should be able to agree that all must continue to support fully and enthusiastically the exploration of this challenging vast frontier. In view of the tremendous geologic record preserved on the continent—a record based on fossils and radiometric-age determinations—and in view of the history of drilling, which shows that additional exploration, especially to greater depth, usually produces important results, the writer asks the supporters of the continental-drift theory to consider the meaning and implications of this tectonic record and to withhold acceptance of the theory or theories they support at least until they can agree on a fundamental cause.

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