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Date Permissions Signed
Date of Award
Master of Science (MS)
Suczek, Christopher A., 1942-2014
Aronoff, Steven Martin
Engebretson, David C.
The Lower Miocene Clallam Formation, located on the northern coast of the Olympic Peninsula, Washington, is an approximately 800 m thick sequence of sandstone and conglomerate that was deposited in the predominantly marine portion of a prograding delta.
As a whole, the Clallam Formation coarsens and shallows upward, progressing from fossiliferous sandstones deposited below wave base, through storm-dominated deposits characterized by climbing ripples, graded rythmites, and hummocky cross-stratification, to coarse-grained distributary mouth deposits. Within this general progression are several small-scale coarsening- and shallowing-upward cycles that reflect the occupation and subsequent abandonment of distributary channels.
Analysis of lithic and monocrystalline grain characteristics indicates that the Clallam Formation had four major types of source rocks: andesitic to dacitic volcanic rocks, chert, metamorphic rocks, and intermediate Plutonic rocks. Point count data plotted on ternary diagrams indicate that modal compositions were affected by depositional mechanisms. Sandstones deposited during storm activity contain fewer lithic constituents. When data from samples unaffected by storm deposition are plotted on ternary diagrams, mean modal compositions are Q:F:L= 40:22:38; Qm:F:Lt= 28:22:50; Qp:Lv:Ls= 21:56:23. Petrologic studies indicate that one or more of Vancouver Island, the San Juan Islands, and the Cascade Range (which contain similar rock types) was the source for the Clallam Formation sandstones.
The structural and depositional events that affected the Olympic Peninsula are intimately tied to the motions of the Pacific, Farallon, and North American plates. In the vicinity of the Olympic Peninsula, basin subsidence, uplift, and structural and metamorphic events that occurred during the Oligocene and Miocene epochs were probably the result of the morphology of the Farallon Plate, which was then being subducted under the North American plate, rather than the result of changes in its relative velocity. The passage of two fracture zones, the Aja at about 28 Ma and the Sedna at about 16 Ma, may have influenced pre- to post-Clallam Formation structural and depositional events in the vicinity of the Olympic Peninsula.
Clallam Formation, Sedimentology, Sedimentary petrology
Western Washington University
Olympic Peninsula (Wash.)
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Anderson, Kurt Soe, "Sedimentology, Sedimentary Petrology, and Tectonic Setting of the Lower Miocene Clallam Formation, Northern Olympic Peninsula, Washington" (1985). WWU Graduate School Collection. 771.