ML20151P805

Discrimination of Flood,Storm & Tectonic Events in Coastal Marsh Records of Southern Cascadia Margin, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P805
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Darienzo M, Peterson C Oregon State University, CORVALLIS, OR
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References
NUDOCS 8808100192
Download: ML20151P805 (1)

Text

Ii DISCRIMINATION OF FLOOD, STORM AND TECTONIC EVENTS IN COASTAL MARSH RECORDS OF THE SOUTHERN CASCADIA MARGIN by Curt D. Peterson and Mark E. Darienzo, College of Oceanography Oregon State University, Oceanography ADMIN BLDG 104 Corvallis, OR 97331-5503 phorv (503)-754-2759 Regional climatic mechanisms of potential marsh burial have been investigated on a prelmunary basis in endmember marsh systems of the southern Cascadia margin. Depo 3ition by flood or storm surge provides means by which marsh burial could possibly occur independently of tectonic subsidence in some estuades of the southem Cascadia Margin. In an effon to identify potential marsh burial by flood deposition, selected marsh sites were cored to 3-5 m depth in flood plain and estuarine marshes of the Little Nestucca, Alsea and Siltetz Rivers in northem Oregon. Sand deposits are associated with riverine-tidal channels of each bay, providing abundant sand supply to the downstream estuarine marsh systems. However, vertical sequences of 3-5 buried marsh horizons either 1) lack sand capping layers or 2) are overlain by sand capping layers that show no indication of increasing thickness with increasing proximity to river channel axes or with I increasing distance upstream. A thourough search for effects of the regional 1964 flood (100 year flood event)in modern marsh deposits of Alsea Bay (probably 300 years in age) showed no l

evidence of sand accumulation or marsh burial associated with this historic flood. Preliminary l study results indicate that sand and coarse silt fall out of suspension upstream of estuarine marshes during maximum riverine floods when ebb flow is backed up by constricted tidal inlets. Seaward I transport of river sand resumes in inter-tidal or sub-tidal channels with decreasing discharge levels I i

and flood sand deposition rarely if ever reaches supra tidal marsh settings.

l Sediment transpon and deposition by oscillatory and tidal current flow during storm sarge conditions represent other potential mechanisms by which supra tidal marshes might be buried by excessive sedimentation. Fresh water diatom assemblages in buried marsh horizons of Netarts Bay establish supra tidal settings for the upper marsh surfaces. Detailed examination of sand capping  !

layers above 3-5 buried marsh sequences in exposed settings of Netarts Bay and Alsea Bay in nonhern Oregon, demonstrate a lack of any intemal cross bedding associated with traction current transport under sub-critical flow conditions. Marsh burial sequences in exposed reaches of both bays include sand and silt laminations fining up to silt and clay laminations between successive marsh layers. The burial sequences, up to 100 cm thick, are too thick to be formed by single storm events and yet they lack alternating sand and mud layers in the upper pans of the sequences which would be expected from deposition by repetitive storm events overprinted on a condition of rising sea level. Neither flood nor storm deposition can account for cbserved marsh burial sequences in southern Cascadia coastal marshes without prior tectonic subsidence of marsh horizons to low inter tidal elevations. The observed fining-up burial sequences represent tectonic subsidence followed by vertical sediment accretion and/or tectonic emergence to supra tidal elevations.

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