Vitrinite reflectance values from two mid-Late Cretaceous sedimentary sections in the Yukon-Koyukuk basin are within the oil window when plotted on a time-temperature-reflectance nomogram. However, preliminary paleomagnetic studies in other areas indicate a higher temperature overprint. To establish a more consistent record of paleothermometry in the basin, over 500 paleomagnetic samples from 35 localities and 25 K-Ar dating samples were analyzed. The samples include cobbles from conglomerates to test for remagnetization and argon loss.

Our analysis indicates that (1) some sample localities fail the conglomerate test (cobbles show consistent magnetic directions) and have experienced temperatures of 300°-500°C, (2) other localities, along the lower Yukon River, display primary paleomagnetic signals that pass both conglomerate and regional fold tests and have reversals, and (3) K-Ar ages of detrital components (volcanic pebbles and muscovite) from the lower Yukon River localities are older than their age of deposition, suggesting that temperatures have not exceeded the K-Ar blocking temperatures of these components (~300°C for muscovite and lower for whole rocks).

The complex thermal history of the Yukon-Koyukuk basin is a result of at least three magmatic episodes. The overprinted areas are closest to igneous occurrences; other areas were thermally unaffected by these widespread magmatic pulses.

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