Paleogeography and paleocurrents

Paleogeography is the geography of a past geologic time and the science of its determination. Paleocurrents are currents existing within a paleogeographic environment or region.

Ancient Geography

Paleogeography is the geography or the study of the geography of the past. Paleogeographic interpretations or maps may refer to any geographic phenomenon or group of phenomena, so there are many special types of paleogeographic maps. Paleobiogeographic maps show the distribution of fossil species, ecological assemblages, floras, or faunas. Paleoclimatological maps plot ancient climatic realms, and paleo-physiography maps depict ancient landforms. Paleotectonic maps outline regions of deformation, including crustal compression, crustal extension, persistent uplift or depression, and stable areas. Paleobathymetric maps describe the depths of ancient bodies of water. Maps defining past patterns of circulation in the sea, a lake, or stream are called paleocurrent maps. Any geographic property, in fact, may be the subject of a specialized paleogeographic map. Paleogeographic maps are interpretive maps—they are not maps of existing conditions or objects but describe conditions inferred from the rock record. They cannot be verified by direct observation in nature. Paleogeographic maps and interpretations may illustrate the geography of the entire globe, whole continents, or specific regions or features.

Global maps generally show continents and ocean basins as they are inferred to have existed during a major time interval, such as a geologic period. The continents shown on these maps may be parts of present-day continents or may be composed of the merged parts of several modern continents. Furthermore, the ancient continents are not likely to be located in their present position in reference to latitude and longitude. On many of these maps, ancient continental landmasses and archipelagos are plotted over the whole Earth, with the outline of present continents and parts of continents superimposed. A series of such maps thus records the manner in which segments of continental crust have continually divided, split, merged, and moved about throughout geologic time. The maps for the Precambrian, which are the least reliable, record the appearance of many “protocontinents,” coalescing blocks of continental crust, lighter rocks of the composition of granite that form an incomplete outermost layer of the rocky Earth.

These protocontinents eventually gathered to form perhaps five ancestral continents. During the Early Paleozoic, these gathered in two large continents: Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere. During the Late Paleozoic, Laurasia and Gondwanaland coalesced in a single continental mass, Pangaea. A large embayment, the Tethys Sea, indented the eastern side of Gondwanaland and is the remote ancestor of the Mediterranean Sea. During the Mesozoic and Cenozoic, this single large continent progressively separated into the modern suite of continents: North America, South America, Eurasia, Africa, Australia, and Antarctica. These continents gradually moved to their present positions relative to the poles and the equator. Global paleogeographic maps of this sort first appeared in the middle twentieth century, as the theory of plate tectonics and paleomagnetic concepts were discovered and accepted.

Past Positions of Continents and Ocean Basins

Knowledge of the global position, size, and interconnections of the continents and ocean basins is required to place past geologic events in a geographic context. It is also needed for a full understanding of present global geologic relationships. The past position of crustal blocks, continents or parts of continents, and ocean basins may be inferred in part from the remanent magnetism of rocks of known age. Remanent magnetism is a magnetic field imprinted in a rock that conforms to the world's magnetic field existing at the time the rock was formed. This remanent magnetic field remains fixed, even if the rock is subsequently rotated or moved. It is, therefore, a permanent record of the original latitude and orientation of the rock. Thus, the azimuth direction of the remanent magnetism in a rock indicates the position of the pole with respect to the locality of a rock when it was deposited or intruded. The polarity of the remanent field reveals the polarity, normal or reversed, at the time the rock formed. The inclination of the vertical component of the remanent field gives the latitude at which the rock formed. Also, regions in which the remanent magnetic fields in rocks of common age are oriented uniformly may be considered parts of the same continental blocks or fragments. The remanent field is susceptible to destruction only by heating the rock to near-melting temperatures. The longitudinal position of a rock cannot be determined by paleomagnetic analysis, so other phenomena must be called upon to place ancient continental crustal blocks in their proper positions on the globe.

Former continuity between separated continental blocks may be inferred from their shape, as in the apparent match between the eastern coastline of the Americas and the western coasts of Europe and Africa. This pattern was noted, and a relationship was inferred as early as the first half of the nineteenth century. Such “jigsaw puzzle” techniques are a significant aid in continental placement. It should be noted, however, that the land area of a continent is not what is being matched. Much of the continent is submerged, so the location of the continental margin and slope is essential.

However, the apparent continuity of ancient geologic features on the margin of separated continental masses suggested former connections before the advent of plate tectonics, which still reinforces paleogeographic reconstructions and informs further plate tectonics theories and research. For example, the apparent continuation of the Appalachian orogenic belt and the band of carboniferous coal deposits from Texas to Newfoundland, the British Isles, and Western Europe supports the idea of former continuity between the landmasses. Common occurrences of glacial deposits, coal beds, similar volcanic extrusives, and disjunct occurrences of fossil organisms incapable of crossing open oceans in Australia, India, Antarctica, South America, and Africa all support the reconstruction of the Gondwanaland continent. Before the advent of plate tectonics, now-submerged continent-sized landmasses in the South Atlantic and Indian Oceans or long isthmian land bridges were postulated as crossing those oceans to explain the present distribution of the Gondwanaland fauna and flora.

Paleogeographic Maps

Paleogeographic maps of continental masses may show the distribution of ancient geographic features on modern-day continents, or they may refer to continents as they existed at some former time. Maps of the distribution of the land, sea, and ancient mountain ranges on present-day continents have been produced since the mid-nineteenth century. These maps typically show the epicontinental seas, seas which consist of marine waters overflowing continental crust, as opposed to the seas in deep ocean basins. Such maps also show the continental shelf or inundated continental margin, the relatively steep continental slopes at the very edge of the continental crust, and dry-land areas. In addition, major belts of deformation, broad areas of uplift, and broad areas of subsidence, or basins, are frequently shown on these maps. More detailed and generally more specific maps will show such things as deltas, ancient volcanic tracts, coastal lagoons, estuaries, reefs, dune fields, mountains, and glaciers.

Regional maps may show the paleogeography of areas defined by geologic interest, or they may be bounded arbitrarily. Examples of the first would include a map of the Late Paleozoic Ancestral Rocky Mountains. A set of paleogeographic maps for a state would exemplify the second. Regional maps of smaller areas generally are more detailed than continental or global maps and are more directly derived from local geology and paleontology. Latitude and pole position may, however, be shown and are determined by the same techniques employed in producing global or continental-scaled maps. At the regional level, paleogeography may be concerned with the distribution of land and sea, mountain ranges, volcanic regions, lakes, flora and fauna, and the climate.

Paleogeographic maps are constructed by correlating and establishing the extent of rocks of the age in question. The conditions under which contemporaneous rocks were deposited are then determined. In this way, areas of uniform or nearly uniform environment may be outlined and mapped. Areas lacking rocks of the age under consideration were regions of non-deposition or intrusion or were areas subsequently eroded to remove rocks of that age. Areas undergoing deformation and areas suffering igneous intrusion or extrusive volcanism are also defined and mapped.

Such maps are more accessible in the twenty-first century than ever before because of technology like plate reconstruction and modeling software and three-dimensional mapping. The National Aeronautics and Space Administration (NASA) supports the PALEOMAP Project—an open resource that provides paleogeographic maps, digital data sets, software, and educational materials for students. Other databases and publications offer similar and conflicting information, creating a need for standardizations across the field and forcing scientists to properly discriminate between reconstructed paleogeographic models.

Principal Terms

azimuth: degrees of arc measured clockwise from the north

correlation: the determination of identity in age, fossil content, or physical continuity of rocks observed in different areas

endemic species: species confined to a restricted area in a restricted environment

epicontinental sea: a sea overlapping continental crust, as opposed to a sea underlain by oceanic crust

isotopes: forms of an element having the same atomic number but differing atomic weights

planetary wind system: a global atmospheric circulation pattern, as in the belt of prevailing westerlies

remanent magnetism: the magnetic field imposed upon a rock at the time of its formation in accord with the global magnetic field

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