PALEOGEOGRAPHY

also spelled Palaeogeography, the geography of selected portions of the Earth's surface at specific times in the geologic past. This geography consists of interpreted reconstructions, produced in map form, which represent visual summaries of a wide variety of complex geologic information. The maps provide a series of relatively instantaneous views of the Earth through geologic time. Paleogeographic maps may be as simple as those that merely give the former distribution of ancient lands and seas. They may be extremely comprehensive, however, showing the occurrence and distribution of fossil, plant, and animal communities, environments of sedimentation (e.g., deltas, reefs, deserts, or deep-sea basins), areas undergoing uplift and erosion or subsidence and deposition, and major climatic zones. Interpretations of the distant geologic past are based largely on observations of the rocks of the Earth's crust in light of knowledge of present geologic processes. Virtually all the scientific specialties within the geologic sciences contribute to paleogeography. The disciplines of stratigraphy and sedimentology, paleontology and paleoecology, structural geology and geophysics, and petrology and geochemistry are particularly important. Usually, a paleogeographic map covers a certain interval of time and thus represents the average geologic situation during that interval. Time intervals for any particular paleogeographic reconstruction should, however, be short, relative to the rates of geologic change in the area. There are three main techniques for determining geologic time intervals. One is by measuring the radioactivity of rocks. The technique of dating rocks by measuring ratios of radioactive parent-to-daughter isotopes always involves certain intrinsic sources of error, and the time of formation of a rock is given as a range of years rather than as a unique point in time. The second technique for dating rocks is by correlating the similarity of fossil remains. Differing evolutionary stages of organisms provide a useful chronology for establishing the time equivalence of rocks in separate parts of the world. Rates of evolutionary change, however, usually are not sufficiently great to discriminate events of less than several million years' duration. A relatively new technique for establishing instants in time is that provided by study of paleomagnetic reversals. At specific times in the past the polarity of the Earth's magnetic field reversed, and each reversal was essentially instantaneous. A sequence of reversals has been established for the last 7 million years from the polarity and orientation of fine-grained iron oxides found in deep-sea sediments and terrestrial lava flows. Paleomagnetic reversals may ultimately provide the instant-time signatures so necessary for accurate reconstruction of the Earth's geologic history. The simplest kind of paleogeography is that showing the location of ancient lands and seas. The data that bear on such a reconstruction include the distribution of marine and nonmarine sedimentary rocks as defined by their fossil flora or fauna. Fossil organisms, however, can provide much more detailed information than whether their enclosing sedimentary rocks were deposited on land or in the sea. Terrestrial and marine environments include a wide range of habitats (e.g., upland plains or plateaus, deserts, forests, river valleys, swamps and lakes, tidal flats, beaches, lagoons, reefs, deltas, or open ocean) in each of which there lives an assemblage of animals and plants specially adapted to the ecology of the particular habitat. Knowledge of the ecological requirements of fossil organisms thus allows more refined interpretation of the paleogeography than simply designating land or sea. Fossil land plants have long been used as climatic indicators in paleogeographic studies. When internal cellular structure is preserved, it is possible to determine if the plant lived in an arid or wet climate and if there were marked seasonal changes in temperature or rainfall. In geologically younger rocks containing fossil plants that have living descendants, one can define the past climate from the environmental preferences of the living form. Plants provide other fossil traces besides bits and pieces of leaves, stems, or fruits. In fact, most paleoclimatological interpretation is based upon the study of fossil spores and pollen, the microscopic reproductive bodies of plants that have extremely resistant outer coverings. Because of the great durability of spores and pollen, and their very great initial abundance, fossil spores and pollen can be obtained from many terrestrial or even marginal marine sedimentary deposits that may otherwise lack larger plant remains. Marine plants, especially the calcareous algae, are also found and recognized in the fossil state. Not only will they indicate the marine origin of the rocks in which they occur but, because of the need for light for photosynthesis, the marine environment will be identified as being relatively shallow (solar radiation does not penetrate much beyond several hundred feet of seawater). Fossil vertebrates, whether fish, amphibians, reptiles, birds, or mammals, are also useful in paleogeography. Like plants, individual vertebrate species have characteristic adaptations to their environment, so that their distribution in ancient rocks is a guide to the occurrence and extent of those environments. Shelly marine invertebrates, including microscopic calcareous and siliceous protists, sponges, corals, ectoproct bryozoans, brachiopods, mollusks, echinoderms, and arthropods, are also used regularly by paleontologists to define environments within ancient seas. Not all evidence of past life found in rocks is so direct as the presence of the hard skeletal remains of shelly invertebrates, calcareous algae, or plants. Indeed, much of the evidence is quite indirect. The occurrence of oil and natural gas, whose organic constituents are derived from fossil marine life, usually defines the origin of the containing strata as marine, although, after formation in the source rock, oil and gas can migrate to the reservoir rocks whose origin might actually be nonmarine. Coprolites, which are the fossilized fecal remains of animals, can occasionally provide some paleogeographic data. Other interesting indirect evidence of life is that of tracks, trails, and burrows made by ancient animals. Paleogeographic data are not confined to fossil remains. Rocks of all sorts, fossiliferous or not, are used for paleogeographic reconstruction. Igneous, sedimentary, and metamorphic rocks are the products of a variety of processes occurring at the Earth's surface or within its crust. The environments in which rocks originate are distributed along geographic trends, and the composition, texture, and internal structures characteristic of these environments are often recorded within the individual rock types. Analysis of the rock record at a specific point in the geologic past allows definition and interpretation of these major rock-forming environments and indicates their geographic distribution.