The distribution of climatic regions on the Earth is controlled by a complex interaction of many phenomena, including solar flux (i.e. latitude), wind directions, ocean currents, elevation, and topographic barriers (Sections 13.1.2, 13.1.3). The majority of these phenomena are only poorly known in the geologic record. On a broad scale, however, latitude is the major controlling factor of climate and, ignoring small micro-climatic regions dependent on rare combinations of other phenomena, it appears likely that the study of climatic indicators in ancient rocks can be used to infer, in a general sense, their ancient latitude. Consequently,
paleoclimatology, the study of past climates (Frakes, 1979), may be used to demonstrate that continents have drifted at least in a north-south sense. It must be realized, however, that the Earth is presently in an interglacial period, and so parallels between modern and ancient climates may not be completely justified. The important paleolatitude indicators are listed below.
1 Carbonates and reef deposits. These deposits are restricted to warm water and occur within 30° of the equator at the present day where temperatures fall in the narrow range 25-30°C.
2 Evaporites. Evaporites are formed under hot arid conditions in regions where evaporation exceeds seawater influx and/or precipitation, and are usually found in basins bordering a sea with limited or intermittent connection to the ocean proper (Section 13.2.4). At the present day they do not form near the equator, but rather in the arid subtropical high pressure zones between about 10° and 50° where the required conditions prevail, and it is believed that fossil evaporites formed in a similar latitudinal range (Windley, 1984).
3 Red-beds. These include arkoses, sandstones, shales, and conglomerates that contain hematite. They form under oxidizing conditions where there is an adequate supply of iron. A hot climate is required for the dehydration of limonite into hematite, and at present they are restricted to latitudes of less than 30°.
4 Coal. Coal is formed by the accumulation and degradation of vegetation where the rate of accumulation exceeds that of removal and decay. This occurs either in tropical rain forests, where growth rates are very high, or in temperate forests where growth is slower but decay is inhibited by cold winters. Thus, coals may form in high or low latitudes, each type having a distinctive flora. In Wegener's compilation of paleoclimatic data for the Carboniferous and Permian (Fig. 1.3), the Carboniferous coals are predominantly of the low latitude type, whereas the Permian coals of Gondwana are of the high latitude type. Younger coals were typically formed at high latitudes.
5 Phosphorites. At the present day phosphorites form within 45° of the equator along the western margins of continents where upwell-ings of cold, nutrient-rich, deep water occur, or in arid zones at low latitudes along east-west seaways.
6 Bauxite and laterite. These aluminum and iron oxides only form in a strongly oxidizing environment. It is believed that they only originate under the conditions of tropical or subtropical weathering.
7 Desert deposits. Care must be employed in using any of these deposits because desert conditions can prevail in both warm and cold environments. However, the dune bedding of desert sandstones can be used to infer the ancient direction of the prevailing winds. Comparison of these with the direction of the modern wind systems found at their present latitudes can indicate if the continent has undergone any rotation.
8 Glacial deposits. Glaciers and icecaps, excluding those of limited size formed in mountain ranges, are limited to regions within about 30° of the poles at the present day.
The results of applying these paleoclimatic techniques strongly indicate that continents have changed their latitudinal position throughout geologic time. For example, during the Permian and Carboniferous the Gondwana continents were experiencing an extensive glaciation (Martin, 1981) and must have been situated near the south pole (Fig. 3.9). At the same time in Europe and the eastern USA, coal and extensive reef deposits were forming, which subsequently gave way to hot deserts with evaporite deposits. The northern continents were thus experiencing a tropical climate in equatorial latitudes (see also Fig. 1.3).
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