--> Paleogeographic Interpretation of Several Eocene Floral Assemblages from Coast Range of Northwestern Oregon, by John E. Ries; #91024 (1989)

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Paleogeographic Interpretation of Several Eocene Floral Assemblages from Coast Range of Northwestern Oregon

John E. Ries

Several freshwater and brackish water deposits are exposed throughout the Coast Range of northwestern Oregon. Occurring as interbeds in the Tillamook Volcanic Sequence and the Cowlitz Formation, these strata are stratigraphically equivalent to hydrocarbon bearing strata in the Mist gas field to the north.

From analysis of macrofossils, species appear to be present from two different paleoaltitude zones. A coastal swamp stream deposit has produced specimens of Sabalites, Platanophyllum, Alnus, Lastera, and rooted Equisetum. Both floristic and physiognomic analysis of the flora indicates a moist subtropical climate.

Deposits exposed in the Tillamook uplands are apparently lacustrine shales. The flora preserved is not as diverse, represented by specimens of Cornus, Chamaecyparis, Ailanthus, Pinus, and Picea. The climate here appears significantly more temperate than the assemblage preserved to the northeast, although deposited at approximately the same time. This difference can be explained by the existence of volcanic uplands backing the coastal swamp.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.