Palynology and paleoecology of the tertiary Weaverville formation, Northwestern California, U.S.A.

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Abstract
The pollen flora of the mid‐Tertiary Weaverville Formation complements the leaf flora described in 1937 by MacGinitie in recording numerous additional taxa from the regional flora that allow a fuller reconstruction of the vegetation and paleoenvironment and a refined interpretation of the age. Sediments of the Weaverville Formation comprise tuffs, lacustrine shales, and fluvial auriferous conglomerates that are preserved in small fault‐bounded basins. Stratigraphic sections from two basins 25 km apart yielded a total of 151 types of palynomorphs, which include 85 genera or tribes in 51 families. The pollen and leaf floras have few elements in common, due to variable pollen production and differential preservation of pollen and leaves. The most notable floral elements derived from the pollen flora are Betulaceae, Onagraceae, Ericaceae, Sterculiaceae, Carya, and several groups of gymnosperms. By comparison with modern taxa, the Weaverville flora represents a mixed broad‐leaved evergreen and deciduous forest and a mixed coniferous forest, with a Tax‐odium‐Nyssa swamp forest near the site of deposition. A subtropical to warm temperate climate is inferred, although the flora also includes taxa that today are tropical to subtropical (Bombacaceae, Sterculiaceae) and others that are temperate to boreal (Larix, Picea). This seemingly anomalous composition may have resulted from depositional mixing of pollen from several vegetational units or from less distinct ecological zonation of vegetation in the Tertiary. An interesting association of typically Eocene taxa (Pistillipollenities, Bombacaceae, Triumfetta) with Oligo‐Miocene taxa (Compositae, Malvaceae, Polygonaceae, Chenopodiaceae, Gramineae) occurs in both measured sections, and these types are not restricted stratigraphically. Overall, the strati‐graphic ranges of Weaverville taxa and the diversity of herbs suggest that the age of the Weaverville Formation is Early to early Middle Miocene.