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Documentation of an ancient ice age depends on both direct and indirect evidence, all fitted into an acceptable paleogeographic reconstruction. The direct evidence consists of striated and polished basement surfaces, commonly displaying friction cracks, roches moutonnées, and other geomorphic forms. Sedimentary strataoverlying may be identifiable as lodgment and other tillite and glacial facies. Where icebergs have carried glacial debris to distant basins and have dropped the material into contrasting facies, the existence of glaciers someplace may be shown but their locations are uncertain. Regional reconstructions of facies within a limited time slice, utilizing all methods of correlation available, provide the most trustworthy method of ice age documentation. In the Proterozoic, however, correlations are imprecise so that our knowledge of ice ages is quite inexact.

Indirect results of continental glaciation include strong and rapid changes in sea level. These eustatic changes contrast with those resulting from other causes, including tectonics, which are likely to be very much slower or of much less magnitude. Transgressions and regressions in the Proterozoic record need comparison and correlation with known glacial sequences. They should be traced from stratigraphic section to section employing every correlation technique available, including magneto-stratigraphic and seismostratigraphic methods.

Some paleomagnetic observations indicate glaciation at equatorial and low latitudes, whereas the Phanerozoic record shows that glaciation takes place in near-polar regions. Controversy has therefore arisen between those investigators who accept low-latitude glaciation and those who are skeptical and look for a way to explain the paleomagnetic measurements that is compatible with high-latitude glaciation. It may be that rapid crustal movements in combinations with postdepositional imprint of the magnetic vector by 10 or 15 m.y. have carried glaciated localities from a high- to a low-latitude region during the interval. More research is therefore called for on rock and mineral magnetics with respect to diagenetic changes. Searches are also needed for other geological consequences of such a marked climatological or orbital reorientation over the 120 or so m.y. interval between the latest Proterozoic glaciations, and the polar Ordovician-Silurian glaciations.

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