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Paleogeography

Paleogeography, the study of the Earth's past geography, is a major subfield of historical geology. Relying on a broad spectrum of information made available from many other fields in geology and the natural sciences, it analyzes, interprets, and synthesizes the former configuration and distribution of oceans and lands, mountains and plains, climatic belts, paleobiological provinces, paleoecological conditions, and many related features.

Paleogeography as a clearly conceived subject is more than 100 years old. Progress in the field has been slow and often sporadic, however, largely because of paleogeography's great dependence on significant progress and advances in those other fields of study which supply much of the basic data and physical constraints upon which paleogeography is interpreted. Many contemporary ideas, methods of analysis, and data for paleogeography originate from the study of structural geology, crustal tectonics, sedimentary rocks and depositional environments, stratigraphy, fossil provinciality, taxonomy and evolution, and paleomagnetism. Also, study of recent floras and faunas; of their distributions, dispersals, provinces and realms, evolutionary histories, and ecological adaptations; of climates and climatic changes; and of community structures and interrelations have greatly influenced the development of paleogeographic concepts. Paleogeography in turn influences the individual fields that contribute to it in a number of different ways, perhaps most importantly in providing a synthesis of data and placing concepts in worldwide perspective.

Early Concepts

Development of paleogeography has proceeded along several courses. Charles Lyell, in his Principles of Geology (1830-33), stated that every part of the land had once been beneath the sea and that every part of the ocean had once been land, in an endless cycle from land to sea and back again. This dramatic statement, designed to loosen the rigid concepts of geography then prevalent, was defensible only for the land surface; knowledge of the oceans would remain too sparse for more than 125 years after that time. Only recently have studies revealed that the ocean basins have had a different history than that proposed by Lyell .

James Dwight Dana proposed (1873) that continents had always been continents but from time to time had been flooded by shallow, epicontinental seas. Dana showed that the margins of continents (at least of eastern North America) were covered by much thicker deposits than were the centers of the continents. He applied the term geosyncline to these long, linear belts of thicker sediments. Considerable study has been devoted to geosynclines; Marshall Kay (1904-75) summarized these studies between the 1940s and the 1970s. Geosynclines were recognized as regions of the Earth's surface having many features in common and as being significant to many fields of geology. The sedimentary phase of these areas was generally followed by deformation, lateral compression, uplift, and mountain building. Furthermore, the bulk of the sediments that accumulated to form the geosynclines generally did not come from the direction of the interiors of the continents but, rather, originated from the opposite direction. Also, when traced away from continental margins and into the geosynclines, the sediments changed from well-bedded quartzose sandstone, limestone, and shale into coarse, red clastic sediments or--commonly--into graywackes, basaltic volcanics, and turbidites. These sediments showed evidence of having been deformed at about the same time as they were deposited.

Dana believed that the Earth was cooling--a common hypothesis of the 1800s and early 1900s--and therefore that the Earth's surface was shrinking. He believed that this shrinking, like a shriveling orange skin, was the cause of most of the structural deformation of the Earth's crust. Dana considered (1856) the amount of deformation at the margins of the continents to be proportional to the size of the adjacent ocean--for example, the Atlantic margin of North America is less mountainous than the Pacific margin. He reasoned that the continents were fixed in position and that the ocean areas pushed against them with a force proportional to the area of the ocean. Unfortunately, this concept of fixity of continents and ocean basins was not based on an accurate geological understanding of continental-ocean margins.

Dana's idea of the fixity of continents strongly influenced paleogeographic interpretation in North America for nearly 100 years. Charles Schuchert (1858-1942), a dominant figure in paleontology in North America during the first quarter of this century, developed the concept of paleogeographic maps in order to show how relationships changed over successive intervals of geologic time. His maps included the distribution of epicontinental seas, general sediment types, and source areas (usually mountain ranges). In order to account for the sedimentary features in geosynclines, Schuchert imagined mountainous borderlands in regions of the oceans adjacent to the continents that had formerly acted as sediment sources but later foundered.

During the last quarter of the 19th and the early part of the 20th century, a number of European geologists became interested in paleogeography in order to understand the distributions of certain fossils. Henry Francis Blanford (1834-93) and Edward Suess found close similarities between fossils from parts of South America, Africa, Australia, and India. Suess, in his The Face of the Earth (1885-1909; Eng. trans., 1904-09), proposed that these areas had once been connected by land areas that had subsequently foundered in the South Atlantic and Indian oceans. Later geologists included Antarctica among these areas. Suess called this large theoretical supercontinent Gondwana ; he also originated the name Tethys for the east-west sea that extended across southern Europe and Asia during the Mesozoic Era (and in modified form during the Late Paleozoic Era and Early Tertiary Period). After the turn of the century paleogeographic studies in Europe progressed along several paths, but particularly in fossil analysis, structural geology, and tectonic analysis. Ages and extents of orogenic belts (deformed geosynclines) were outlined in detail, fossil provinciality was recognized for many groups of organisms, climatic indicators were identified and interpreted, and the relationships between sediment types and geologic environments were developed in a systematic manner.

Continental Drift

An extremely controversial hypothesis was first proposed by Alfred Lothar Wegener in 1912. Because he rejected the ideas of the permanence of ocean basins and the fixity of continents, he did not need to consider the foundering parts of continents in ocean basins. He believed that continents drifted like rafts across the surface of the Earth, and that occasionally they had joined together to form land masses having different shapes, coordinates, and geometries than those at present. Evidence of Continental Drift was assembled as a synthesis of data from many fields and was reasonably convincing, but Wegener lacked a geologic mechanism for such movements. After Wegener's death, in 1930, Alexander du Toit continued to elaborate on the hypothesis of continental drift and its paleogeographic implications and added considerably more data to the concept.

Several other departures in paleogeographic thinking appeared briefly in North America between the 1880s and the 1920s, but because of their strong conflict with orthodoxy none gained acceptance. In general, geologists who were located farthest from Gondwanan continents were the least inclined to accept the hypothesis of continental drift. Both Charles Bartlett Warring (1825-1907) and Howard B. Baker anticipated the general portion of Wegener's hypothesis of continental drift, but neither had convincing hypotheses. Both were catastrophists; thus their ideas were given little credence in geological circles. Frank B. Taylor (1860-1938), using studies in structural geology, volcanology, and geophysics (as those fields existed in 1910), approached the problem of orogenic belts in a geologically more convincing manner. He was particularly interested in the Pacific margin and the Tethyan orogenic belts. Taylor advocated horizontal crustal movements caused by tidal forces. Most North American geologists rejected Taylor's general ideas because of a perceived similarity between them and Wegener's theory of continental drift. S. Warren Carey (1911- ) in the 1950s and 60s expanded the structural geologists' approach to crustal tectonics and showed that many crustal features, such as orogenic belts, could be viewed as having poles of rotation, compressive arcs, and discrete, small, semirigid blocks. Further, he considered the Earth's mantle to be expanding and that fragments of the continental crust are being spread apart as new crust is added in the ocean basin.

The ideas of the permanence of ocean basins and the fixity of continents and the reluctance to accept continental drift in its original form also influenced the interpretations of the origins and dispersals of present-day faunas and floras. Alfred Russel Wallace published (1875) a two-volume study of present-day animal distributions. His general map, based mainly on the distribution of families of organisms, has been only slightly modified by later studies, although the environmental and phylogenetic bases for his maps have been significantly clarified. In order to explain many present-day distributions, various possibilities for dispersals were studied through the stimulation developed by George Gaylor Simpson between the 1930s and the 1960s. Land bridges, isthmian links, island hopping, and long-distance (sweepstakes) dispersals received increasingly close attention and commonly drew on statistical methods and probability theory to lend support to various models of species and generic dispersals. During the course of such studies a great deal has been learned about the processes that control dispersals and about the present geographic distribution of plants and animals.

Modern Interpretation

The actual amount of departure from the present configuration of continents and ocean basins during the geologic past was unclear for many years. During the 1960s and '70s intensified studies by geophysicists, sedimentologists, and paleontologists of the ocean basins and their Paleomagnetism, Earthquake patterns, and heat flow demonstrated that the earlier understanding of ocean-continent relationships had not been correct. The result was the theories of seafloor spreading and plate tectonics. These studies showed that the oldest parts of the Atlantic Ocean basin originated during the Late Jurassic or Early Cretaceous periods, and that the oldest parts of the Pacific and Indian Ocean basins were formed during the Late Triassic or Early Jurassic periods. The ocean basins are geologically very young in comparison to the continents. Very different arrangements for continents prior to the Jurassic were thus possible. Seafloor spreading and plate tectonics can be used as the mechanisms to construct the continental configuration of the supercontinent Pangea for the Carboniferous, Permian, and part of the Triassic periods. The steps that led to the gradual accretion of continental fragments to form such a supercontinent during Early and Middle Paleozoic time are currently being explored; the Precambrian relationships are not completely known.

Other Paleogeographic Tools

The reconstruction of Late-Paleozoic Pangea is based mainly on fitting together the geometric shapes of the broken edges of continental fragments, tracing the geosynclines and orogenic belts and comparing their histories, and determining apparent pole positions through paleomagnetic studies (polar wandering). These criteria are less readily interpreted for pre-Carboniferous times.

Climatic belts similar to those that exist today were present in the geologic past; thus climatic indicators are helpful in interpreting approximate latitudinal positions (paleoclimatology; paleotemperature). Glacial deposits such as tillites, glaciomarine strata, and glacially grooved surfaces generally indicate cold climates. On the other hand, thick, light gray limestone deposits, particularly large reef, bank, and shoal deposits that are rich in calcareous algae and large calcareous foraminifers, indicate warm, tropical, or subtropical water. Red beds may indicate warm, moist lateritic climates or warm, oxygen-rich depositional environments, or both. Dune sands may indicate nearness to a beach, desert, or some other type of eolian environment. Coal deposits commonly represent reducing environments where oxidation of carbon compounds is slow. Coals may occur in many latitudes, but those having fossil plants with annual growth rings are considered to have formed outside tropical latitudes. Evaporites suggest warm to hot, dry climates. Thick clastic wedges of coarse sediment are the erosional product of nearby mountain ranges or orogenic uplifts.

Faunal provinciality is another tool of paleogeographers. Phylogenetic similarities, particularly at the species and generic level, between geographically different fossil assemblages of the same age and same ecological communities suggest interbreeding or only recently separated fossil populations. When entire fossil faunas are compared on this basis, an evaluation of common elements and general similarities can be made. In a broad way, close similarities indicate geographic continuity of favorable ecological environments. Also, the number of species are usually more numerous among tropical communities than among polar communities. The resulting species-diversity gradient forms an additional basis for estimating latitudinal position.

Charles A. Ross

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