lower latitudes the alternation of arid and humid climates results in alternating cycles of alluviation and incision in river valleys as well as an alternation of soil-forming processes.

The late Cenozoic-Pleistocene glaciations resulted in a series of sea-level falls of increasing amplitude. Erosion of exposed unconsolidated clastic shelf sediments and consequent isostatic compensation has resulted in large masses of sediment being off-loaded from the continental shelves onto deep-sea fans and abyssal plains by turbidity currents. Modern continental shelves with clastic sediments are adjusted to the Pleistocene low stands of sea level. The present widespread development of estuaries is a result of this condition. Interglacials are too short and the sediment supply is too small to allow shelves to build back to equilibrium grade with high sea-level stands. Although during the interglacials a few large rivers can build deltas to the shelf break and spill sediment into the deep sea, most estuaries do not fill with sediment before the next sea-level fall. In contrast, carbonate sediments become cemented by fresh water infiltration when sea level falls and, as a result, carbonate-dominated shelves and banks reflect equilibrium with high sea-level stands. Carbonate extraction from the ocean is at a maximum as shallow shelf seas flood during the late stages of deglaciation.

The largest supplies of clastic sediment come from the largest regional uplifts, the Tibetan Plateau-Himalayas and western North America. Evidence suggests that both of these regions were uplifted in the late Pliocene and Pleistocene. Their uplift may be the ultimate cause of the onset of continental glaciation and the global climatic alternations that have characterized the Earth's recent past.

Pleistocene-Holocene fluxes do not reflect the long-term state of the planet but are the result of a set of unusual conditions. It is, however, against this rapidly changing background that future global change will take place.

RECOGNITION, DEFINITION, AND LENGTH OF THE QUATERNARY AND HOLOCENE

Information on the distribution and thickness of Pleistocene and Holocene deposits is often highly inconsistent. Ordinarily, geologic maps do not show glacial deposits but do show other Quaternary sediments and volcanics, and often large areas are simply designated Quaternary alluvium. Maps of the thickness of glacial sediments have been published for a few regions, but for most areas there are not even cross sections from which thicknesses can be estimated. The areas shown as Quaternary on geologic maps, even excluding the areas covered by glacial drift, are much larger than the areas of older deposits. Gilluly (1969) reported the measured area of Quaternary sediment shown on the Geologic Map of North America (Geological Society of America, 1965) to be 2.185 x 106 km2 and the sum of older Cenozoic sediments to be 2.075 x 106 km2. He noted that the Geologic Map of South America (Geological Society of America, 1950) shows almost twice as large an area covered by Quaternary sediments, 4.276 x 106 km2, and only 2.889 x 106 km2 of Tertiary sediment.

The base of the Quaternary is taken to be the base of the Pleistocene, originally defined by Lyell (1839) as sediments characterized by fossil content containing more than 70 percent living species of mollusc. The Pleistocene later became associated with Northern Hemisphere glaciation, but because it was recognized that the beginnings of glaciation were only vaguely known, the base of the Pleistocene came to be defined by the first significant cooling of the Mediterranean, marked by the Calabrian Stage in Italy. Dispute over indications of the first cooling of the Mediterranean has led to disagreement over the criteria used to define the base of the Pleistocene in the boundary stratotype region in southern Italy. There are also difficulties in extending correlation of the base of the Pleistocene to other parts of the world. Even more serious discrepancies in the use of the term Quaternary come from those who would consider the base of the Pleistocene to be marked by the climatic change to glacial conditions in the Northern Hemisphere even though this is now thought to be between 3 and 4 million years ago (Ma). Hays and Berggren (1971), Jenkins et al. (1985), and Berggren et al. (1985) have presented useful reviews of the problems associated with the Pliocene-Pleistocene boundary.

Many of the estimates of Pleistocene sediment masses involve calculation or extrapolation of accumulation rates, so that knowledge of its duration is critically important. The age of the base of the Pleistocene was thought for many years to be about 2 Ma; this was revised to 1.8 Ma by Hays and Berggren (1971), and has more recently determined to be 1.6 Ma (Haq et al., 1977). The boundary is currently thought to be just above the top of the Olduvai Normal Magnetic Polarity Event.



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