PERMINERALIZED PLANTS FROM THE MARINE CLALLAM FORMATION (LATEST OLIGOCENE/EARLIEST MIOCENE), OLYMPIC PENINSULA, WASHINGTON STATE: TAPHONOMIC AND PALEOCLIMATIC IMPLICATIONS
The Clallam Formation accumulated 24.2-23.8 Ma, close to Mi-1, a transient glaciation that occurred at the Oligocene/Miocene boundary. All wood from the Clallam Formation has growth rings, indicating a temperate climate. About 75% of wood in the formation is conifer; the rest is dicot. The presence of permineralized pine and fir needles, associated with fern pinnules and sporangia in concretions suggests the presence of conifers and ferns near the shoreline, consistent with a cool temperate climate. Most invertebrates from the Clallam Formation also suggest a cool temperate climate; however the Clallam Formation contains tropical/subtropical gastropod and bivalve genera (Trochita, Cancellaria (Euclia), Ficus, Dosinia and Anadara) that occur today off the west coast of southern Baja California, Mexico.
The preservation of wood in the Clallam Formation argues for rapid rates of sedimentation and concretion formation. Teredinids and wood-eating isopods destroyed exposed wood samples placed in marine siliciclastic environments in the Gulf of Mexico within one to two years.