Global temperature decline associated with the Eocene–Oligocene transition resulted in extinctions of plants and animals in both marine and nonmarine environments. The extensive stratigraphic exposures, well-preserved palynological assemblages, and interbedded coal seams of the nonmarine Amphitheatre Formation, Burwash Basin, Yukon Territory, provide a comprehensive record of this transition. The formation spans a paleoclimatically significant interval otherwise poorly represented in high-latitude deposits of the northwestern Cordillera.

Palynological data constrained by the chronologic and stratigraphic framework established for the Amphitheatre Formation indicate that the global temperature decline resulted in a shift from warm temperate, angiosperm-dominated to cooler temperate, gymnosperm-dominated (mainly coniferous) forest types. This turnover is seen in the increase in the percentage of gymnosperm compared to angiosperm pollen upsection. The effect of climate change is also recorded in the systematic composition of the angiosperm pollen spectra. The highly diverse palynoflora of the lower Amphitheatre Formation is dominated by angiosperm pollen characteristic of plants favoring warm temperate climates, and consistently includes pollen of broad-leaved evergreen taxa. In contrast, the angiosperms most conspicuous in the low-diversity palynoflora of the upper Amphitheatre Formation are characteristic of plants favoring a cooler temperate climatic setting. Petrographic compositional changes in the coals document the same plant community changes. The floristic data also indicate that, at high latitudes, there may have been a change to a wetter and less seasonal climate during the overall global cooling trend.

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