The unnamed Middle Eocene sandstones of Scow Bay are well exposed on the beaches of Indian and Marrowstone Island, northwest Washington. The rocks strike approximately east-west and dip to the north except on the east coast of Marrowstone Island where the structure is more complex. Each of the four coastal sections was described in detail, and the most complete was also measured.
The sandstone beds are typically thin- to very-thick-bedded, structureless lithic arenite. Locally abundant sedimentary structures include dish structures, vague horizontal lamination, load casts, flame structures, soft sediment deformation, and shale clasts. Interbedded with the sandstones are minor siltstones and shale. The siltstones are typically thin bedded although at least two very thick siltstone beds are present. At least eleven thinning and fining upward sequences were indentified on the east coast of Indian Island. The sandstones of Scow Bay were most likely deposited as channel fill sequences on the midfan region of a subsea fan. Deposition was probably from high-density turbidity currents, liquefied flows, and fine-grained debris flows. Low- density \u27classic\u27 turbidites probably deposited most of the fine grained sediment.
The sandstones of Scow Bay are lithic arenite with QFL values of 29, 15, 56. When polycrystalline quartz and chert are included in the lithic category, the resultant QmFLt values are 19, 15, 66. The lithic grains are dominated by sedimentary and metasedimentary lithic grains with lesser amounts of polycrystalline quartz and chert and volcanic and metavolcanic lithic grains (Qp14 Lv20 Ls66 ). Volcanic and metavolcanic lithic grains are composed of about equal amounts of micrilitic and lathwork volcanic lithic grains with lesser amounts of low-potassium felsic volcanic lithic grains. Quartz-plagioclase plutonic rock fragments are common.
The sandstones of Scow Bay have undergone a complex multi-stage diagenesis. Cements include calcite (two different stages), laumontite, phyllosilicates (two stages), pyrite, quartz overgrowths, and albite overgrowths. Of these, only the first three are present in large amounts.
The source area for the sandstones of Scow Bay must have been rich in immature clastic sedimentary rocks with lesser amounts of volcanic rocks and chert. The source area also probably included low-potassium felsic intrusive and extrusive igneous rocks, possibly part of an ophiolite sequence. The sediment in the sandstones of Scow Bay was most likely not transported very far from the source area.
Possible source areas for the sandstones of Scow Bay include the North Cascades and the Chuckanut Formation, the area of deposition of the Puget Group, the Olympic Peninsula terrane, Vancouver Island, the Leech River unit, and the San Juan Islands terranes. Of these, only the San Juan Islands terranes consist of dominantly immature clastic sedimentary rocks with lesser volcanic rocks and chert. Also, the plagiogranite and keratophyre of the Fidalgo Ophiolite could provide the quartz-plagioclase plutonic rock fragments and felsic volcanic lithic grains. Therefore, the sandstones of Scow Bay were most likely derived from a local uplift in the San Juan Islands terranes although some sediment could also have been derived from Vancouver Island.
The sandstones of Scow Bay support the model of Fairchild (1979) and Fairchild & Armentrout (1984), which places a tectonic suture between rocks with North American affinities and the Olympic Peninsula terrane along the Leech River Fault and the Discovery Bay fault zone. A possible tectonic model for the Middle Eocene deposition of the sandstones of Scow Bay involves uplift and erosion of the San Juan Islands terranes during the Middle Eocene followed by accretion of the Leech River unit and the Crescent/Metchosin seamounts along the San Juan, Leech River, and Discovery Bay faults during the Late Eocene
