PERMINERALIZED PLANTS FROM THE MARINE CLALLAM FORMATION (LATEST OLIGOCENE/EARLIEST MIOCENE), OLYMPIC PENINSULA, WASHINGTON STATE: TAPHONOMIC AND PALEOCLIMATIC IMPLICATIONS

The latest Oligocene/earliest Miocene Clallam Formation (and possibly uppermost Pysht Formation) of the Olympic Peninsula, Washington state, is a shallow marine to terrestrial deposit with marine siltstones, sandstones, and conglomerates at the base and lignite at the top. The formation has a rich invertebrate fauna including bivalves, gastropods, crabs and an extinct nautiloid. Marine siltstones and fine-grained sandstones near the base of the Clallam Formation also contain abundant fossil wood, ranging from specimens consisting of intertwined teredolites burrows surrounded by a thin layer of lignitized wood to large pieces of permineralized wood containing almost no teredinid burrows. Some permineralized wood occurs embedded in calcium carbonate concretions; other specimens consist only of permineralized wood and teredinid burrows, which can be lined or unlined. In addition to large pieces of wood, the calcium carbonate matrix of wood-bearing concretions contains finely comminuted land-plant debris, including recognizable fragments of rootlets, pine and fir (Abies) needles, fern pinnules and fern sporangia. On average, about 20% (range 5 – 50%) of the concretion matrix by volume consists of fragmentary land-plant debris.

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.