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Date of Award
Master of Science (MS)
Suczek, Christopher A., 1942-2014
Dragovich, Joe D.
DeBari, Susan M., 1962-
The late Pleistocene Whidbey Formation occupies a region on and around Whidbey Island, Washington, that received both northern-derived glacial sediments and eastern-derived nonglacial sediments. Distinguishing between glacial outwash and nonglacial sediments of the Whidbey Formation and the younger Olympia beds has been problematic where stratigraphic sections are incomplete. For the Whidbey Formation, discontinuous deposits of sand, clay, peat and silt that stem from meandering streams on floodplains have not been fully characterized, petrographically. Recent documentation (Dragovich and others, 2005) of dacitic channel and lahar-runout facies on northern Whidbey Island spurred petrographic analysis of various sands from across the whole region of known outcrops for the Whidbey Formation.
Sands from the Whidbey Formation and surrounding stratigraphic units were point counted and compared. Most nonglacial sands had higher amounts of hypersthene, biotite, hornblende and augite. Glacial sands had higher amounts of chert and polycrystalline quartz. Felsic volcanic and greenstone lithics were the most telling sand components, the latter linked to glacial sands and the former, in concentrations of over 10%, to nonglacial sands.
Comparison between the Whidbey Formation and Olympia beds proved inconclusive both for a lack of coarse sand in the Olympia beds and the uncertain assignment of some sands to the two units. The nonglacial sands from northern Whidbey and Camano Islands typically contain over 20% felsic lithics; felsic lithics are less abundant among the nonglacial sands from southern Whidbey and southern Camano Islands, the Kitsap Peninsula, and the mainland. Throughout the Whidbey Formation most sands contain over 10% felsic lithics, although a few contain sedimentary lithics instead of felsic lithics. Sands of the Olympia beds also contain felsic lithics, the concentration of which exceeds 50% at Long Point. Differing amounts of felsic lithics in nonglacial sands reflect episodic input of volcanogenic material from Glacier Peak via either the Skagit or Stillaguamish Rivers. Sands of the Olympia beds near the Snohomish delta and the Olympic Peninsula contain over 2% serpentine lithics, whereas sands of the Whidbey Formation contain less than 2% serpentine. Beyond this, the petrography of the Olympia beds is apparently indistinguishable from that of the Whidbey Formation.
Whidbey Formation, Petrography
Western Washington University
Subject – LCSH
Petrology--Washington (State)--Whidbey Island; Geology--Washington (State)--Whidbey Island; Sedimentation and deposition--Washington (State)--Whidbey Island; Geology, Stratigraphic--Pleistocene
Whidbey Island (Wash.)
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Peterson, Jonathan, "Petrographic Signature for the Whidbey Formation" (2007). WWU Graduate School Collection. 835.