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Date of Award
Master of Science (MS)
Suczek, Christopher A., 1942-2014
Engebretson, David C.
Beck, Myrl E.
The upper Eocene Aldwell Formation crops out in an east-west trending band across the northern Olympic Peninsula, Washington. Petrographic analyses indicate that the Aldwell Formation has lithologically distinctive eastern and western portions. The rocks of the eastern Aldwell are basalt-rich lithic greywackes; their mean compositions are Qm-F-Lt: 5- 22-73 and polycrystalline quartz-volcanic rock fragments-total sedimentary rock fragments: 1-78-21. The rocks of the western Aldwell are chert-rich lithic arenites; their mean compositions are Qm-F-Lt: 27-19-54 and polycrystalline quartz-volcanic rock fragments-total sedimentary rock fragments: 66-11-23.
Similarities in texture, composition, and alteration products suggest that the basalts of the Crescent seamount province as the likely source for the sediments of the eastern rocks. Since the basalts of the Crescent seamount province crop out both to the north (Metchosin Volcanics) and south (Crescent Formation) of the Aldwell Formation, no definite conclusions can be drawn regarding the provenance of the eastern Aldwell sediments. However, a northern source is inferred based on the restriction of abundant basaltic detritus to the eastern rocks and a southward transport direction for the olistostromal blocks in a conglomerate unit near Lake Aldwell.
The provenance of the sediments in the western portion of the Aldwell Formation is speculative. A terrane located to the southwest of Vancouver Island during the late Eocene is proposed as the source of these sediments.
The rocks in the eastern portion of the Aldwell Formation are predominantly thin-bedded, laminated siltstones with interbedded fine sands. The rocks in the western portion of the Aldwell Formation are thin-bedded siitstones with interbedded fine sands, commonly in thin lenses; coarse sandstone and conglomerate are also present. The eastern rocks are interpreted as having been deposited in an outer fan environment and the western rocks were deposited in a mid-fan environment, but the different lithologies indicate that deposition took place on two separate fans within the Juan de Fuca basin rather than in two areas of the same fan.
A model for the tectonic setting of the Juan de Fuca basin and surrounding area during the deposition of the Aldwell Formation is proposed in which the Crescent seamount province, moving northeast on either the Kula or Farallon plate, converged against an allochthonous terrane then located near Vancouver Island, resulting in uplift and southerly transport of Aldwell sediments.
Petrography, Aldwell Formation
Western Washington University
Olympic Peninsula (Wash.)
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Marcott, Keith, "The Sedimentary Petrography, Depositional Environment and Tectonic Setting of the Aldwell Formation, Northern Olympic Peninsula, Washington" (1984). WWU Graduate School Collection. 744.