Fossil plant resins are common in Late Cretaceous rocks of North America and in Oligocene and younger sediments of equatorial and Southern Hemisphere locations. The scarcity of amber in early Tertiary rocks, however, poses puzzling geological questions. Detailed examination of mid-Eocene amber-bearing sediments from Coalmont, British Columbia, and Seattle, Washington, indicates that resins were produced by taxodiaceous conifers, with Metasequoia occidentalis being the most likely source. Laboratory heating experiments and paleotemperature analysis of these sediments based on coal rank and vitrinite reflectance suggest that burial temperatures may significantly affect infrared spectra of amber, a phenomenon not previously recognized by researchers who have long used spectral characteristics to speculate on the botanical origin of fossil resins.
The occurrence of amber in mid-Eocene rocks of the Pacific Northwest, Arkansas, and southern California suggests that resin production ended as; the climate of North America cooled during the mid-Tertiary and that late Tertiary amber was produced by flowering tropical plants not present in the Eocene forests. This evolutionary record appears somewhat different from that of Western Europe, where progressive southward migration of coniferous amber forests occurred during mid-Tertiary time.
- Geological Society of America