ML20151P866

Deformed Pleistocene Sediments of Tacoma Narrows, Washington, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P866
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Eggers A, Goldstein B PUGET SOUND, UNIV. OF, TACOMA, WA
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References
NUDOCS 8808100210
Download: ML20151P866 (1)

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l Deformed Pleistocene Sediments of the Tacmoa Narrows, Washington Albert A. Eggers, Barry Goldstein: Geology Department, University of Puget Sound, Tacoma, Washington 98416 (

Detailed outcrop mapping ( l 500) of pre-Fraser a@ () 15 ka) gla:tal sediments exposed in weve-cut tenenes et Pt. Evens end Pt. Defiance of the Tacome Nerrows shows that these rocks  ;

were mildly to strongly deformed during the Pleisto ne. The rocks at Pt. Defiance, unconformably underlying Yashon Drift, crop out along 2 km of wsve-cut bench and consist of rhythmically bedded dropstone-bearing silt units which are mildly folded (500maximum dips) and multiply faulted (displacements usually of a few cm). Orientation data on stuctures at Pt.

Defiance suggests eest-west compression. The rocks at Pt. Evans, unconformably underlying Vashon Drift and cropping out along more than 1 km of wave-cut bench, consist of interbedid i diamicton beds and dropstone-bearing silt units which a*e complexly Snd intensivaly falded and l faulted. Deformation of the Pt. Evans rocks is melenge-like end is characterized by closed and open folds, vertical and overturned beds, and hign-angle faults. Continuity of the sediments along the wave-cut bench for more than a kilometer indicates the deform 6 tion was not produ d by landsliding. Orientation data on Pt. Evans structures shows complex oeformation, and no single axis of compression. Comparison of orientation data on Pleistocene struct'Jres at Pt.

Defiance, Pt. Evans, and at other localities within the Puget Lowland to Yeshon ice flow directions and to the orientation of Tertiary structures in and around tne Puget Lowlsnd Indicates the structures in Pleistocene rocks at Pt. Evans and Pt. Defiance, and perhaps at other localities in the Puget Lowlend, might have been produced tectonicelly, and not by gieniotectonic and/or syndepositional pr - The literature, unpublished data, and parsonal mmmunications reveal the existance of numerous other sites where deformed Quotarnary '

sediments are exposed in the Puget Lowland. Although no systematic regional study critically evaluating these structures has been done, structures observed in Pleistocene sediments of the Puget Lowlend ere mmmonly ettributed to glaciatectonic/syndepositional pre We suggest '

that observed deformation in the Quaternary sediments of the Puget Lowland cannot bs assumed es glaclotectontc/syndepositional fn origin, that many of these structures might be of tectonic origin, end that their study might provide valuable insight into the seismicity of the Puget Lowland. '

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