Postglacial Forest Succession and climate in the Oregon Cascades

Hansen, Henry

The postglacial chronology of pollen profiles from sedimentary columns in the southern Oregon Cascades is revealed by the record of postglacial volcanic activity. The Postglacial eruptions are defined by relationship of volcanic ejecta to moraines, glacial striae, and other glacial evidence, and by the occurrence of volcanic glass horizons interbedded in organic sediments or underlying sediments. The most widespread evidence for postglacial volcanic activity is that of the eruption of Mount Mazama, which formed the caldera holding Crater Lake. Its stratigraphic position in relation to the recorded postglacial vegetational succession, the evidence of the fluctuation of lakes in the Great Basin, and its relation to the evidence of early man in south central Oregon suggest that the eruption occurred between 10,000 and 8,000 years ago. Other postglacial volcanic activity is recorded by volcanic glass strata in both organic and inorganic sediments. The recorded vegetational succession supports the evidence offered in 60 other pollen-bearing sedimentary columns throughout the Pacific Northwest for the occurrence of a warm, dry period between 8,000 and 4,000 years ago. The correlation of this dry period with other chronological data in the Pacific Northwest, the Great Basin, eastern North America, and northern Europe, further supports the evidence for a warm, dry, postglacial climatic maximum over much of the north temperate zone.

Postglacial Forest Succession and climate in the Oregon Cascades

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