. . I,

90S 60 30 0 30 60 90N 90S 60 30 0 30 60 90N Latitude Latitude

90S 60 30 0 30 60 90N 90S 60 30 0 30 60 90N Latitude Latitude

Fig. 6.7. Equal-angle latitude histogram for evaporites. (a) Present latitudes of modern evaporites. (b) Paleolatitudes of fossil evaporites where paleomagnetic data are available. After Briden and Irving (1964), with permission from John Wiley & Sons.

precipitation in warm environments or modest rainfall in cold climates. Coal forms where the accumulation of vegetation exceeds its removal or decay. This occurs either in hot rain forests where, although decay is rapid, growth rates are high, or in cold environments where, although growth may be less rapid, decay is inhibited by cold winters. The paleolatitude spectrum of coalfields shows this effect in Fig. 6.8, where the hemispheres are plotted together using a histogram in which the class intervals vary as sin Xp (where is the paleolatitude) and therefore contain equal areas of the Earth's surface. The low paleolatitude group is mainly the Carboniferous coals of western Europe and North America, whereas the high paleolatitude group is mainly the Permian and younger coals from Canada, Siberia, and the southern continents. The two groups also contain distinct fossil floras.

An example of the paleolatitude distribution of a faunal group has been given by Irving and Brown (1964) for labyrinthodont reptiles. For large groups of widely distributed organisms, taxonomic diversity should be a maximum at or near the equator with a decrease into higher latitudes (Irving and Brown, 1964). The study of paleowind directions determined from cross stratification in eolian sandstones is another paleoclimatic indicator that can be compared with paleolatitudes derived from paleomagnetism. Opdyke and Runcorn (1960) and Opdyke (1961) have shown that in the upper Paleozoic eolian sandstones of Europe and North America, the directions observed correspond to a trade wind belt relative to the paleomagnetic equator.

The trade wind belts of the present day lie within 30° of the equator. The present day latitudes of Phanerozoic eolian deposits show a much wider scatter than this. However their paleolatitudes place most of them within 30° of the paleoequator (Drewry et al., 1974). Other paleoclimatic indicators that have been studied include oil fields (Irving and Gaskell, 1962), organic siliceous ooze or its diagenetic equivalent radiolarian chert (Drewry et al., 1974), and Phanerozoic glacial deposits (Blackett, 1961; Drewry et al., 1974). These glacial deposits

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