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<br /> <br />INDICA TORS FOR CHARA CTERIZING ALLUVIAL FANS <br /> <br />3 <br /> <br />Oxygen isotope values <br />a 1SO (a units) <br />2 1 0 -1 <br /> <br />o <br /> <br /> <br />0.5 <br /> <br />o <br />'" <br />. <br />. <br />. <br />. <br />~ 1.0 <br />o <br />. <br />c <br />~ <br />::0 <br /> <br />1.5 <br /> <br />2.0 <br /> <br />Glaciations <br /> <br />6/ <br /> <br />-2 <br /> <br />Present <br />interglaclalion <br />I <br />Last <br />interglaciation <br /> <br />Interglaciations <br /> <br />FIGURE 3-3 Quaternary period Iimescale illustraling <br />oscillating climatic conditions from full glacial (cool) to <br />interglacial (warm). SOURCE: Reprinled with permis- <br />sion from Skinner and Porter (1995). <br /> <br />Pleistocene and Holocene epochs. In many regions, the post-Pleistocene change of climate <br />resulted in a reduction in the rate of sediment supply to fans, whether by streams or debris flows. <br />As a result, the discharges presently available are able to move the sediment supplied on a lower <br />slope than that formed during the Pleistocene, so fan-head incision is occurring on some fans. <br />One of the most common causes of the abandonment of large parts of an alluvial fan is a <br />change in elevation of local streams. Elevation change can result from a change in climatic <br />conditions or in rates of tectonism. Climatic change might result in a decrease in the size of large <br />streams and/or lakes at the toe of the fan, as in the case of a change from braided, post-glacial <br />meltwater streams to smaller, meandering streams incised into the braided gravel deposits. <br />