Terrestrial Economic Deposits
Coal, kaolinite, bauxite, and lateritic iron ores are deposited in terrestrial environments and evaporites in terrestrial or marginal terrestrial environments. All these economic deposits are dependent to some degree on rainfall. Evaporites generally are indicative of dry environments, in which evaporation greatly exceeds precipitation, whereas coal, kaolinite, latente, and bauxite are formed under wet conditions. Terrestrial deposits generally have been placed in a zonal paleoclimatic context by workers concerned with their paleoclimatic implications, for example, Briden and Irving (1964), Drewry et al. (1974), and Hallam (1984). For the most part, the zonal model is adequate for describing the distribution of terrestrial paleoclimatic indicators. However, attempts to define the zones in which the deposits should occur have been confounded by “noise” in the data. Some of the noise is attributable to inaccurate dating or to the use of data from too long an interval of time. However, other problems are inherent in the limitations of the zonal model. For this reason, it is appropriate to introduce the sections on terrestrial deposits with a discussion of the effects of the supercontinent, Pangaea, on circulation, because the zonal model is least effective for Pangaean time.
Pangaea was a single land mass whose size was unequaled during the Phanerozoic. After its assembly, the continent gradually drifted northward and, in the Triassic, was symmetrically positioned about the equator (Fig. 35; J.M. Parrish et al., in press), stretching nearly from pole to pole. Robinson (1973) recognized that this geography was likely to have had a striking
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Paleoclimates and Economic Geology
Burgeoning interest in paleoclimatology has been spurred by growing awareness of the control of paleoclimates on the formation of economic deposits. In past studies, paleoclimatic patterns were derived empirically from biogeographic patterns, and to a lesser extent, from the distributions of sedimentary paleoclimatic indicators, such as coals. The problems with this approach are numerous. In early studies, the paleoclimatic patterns appeared to make very little sense because they were reconstructed on modern continental positions. Even after the acceptance of continental drift, problems arose when the paleoclimatic indicators were poorly dated or when geologists chose paleoclimatic indicators from too long a time period, during which major paleoclimatic changes could have occurred. More recently, qualitative and quantitative models of paleoclimate have proved useful for understanding the distributions of climatically significant geologic data. These models are founded on basic principles of atmospheric and oceanic circulation as applied to global paleogeography, including reconstructed plate positions. With climate models, geologists can formulate hypotheses about the paleoclimatic patterns that might be expected during the various intervals in Earth history and test those hypotheses with the paleoclimatic indicators.