ML20151H080

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Episodic Tectonic Subsidence of Late-Holocene Salt Marshes in Oregon:Clear Evidence of Abrupt Strain Release & Gradual Strain Accumulation in Southern Cascadia Margin During Last 3,500 Yrs, Presented at 880412-15 Meeting in Olympia,Wa
ML20151H080
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Site: Trojan File:Portland General Electric icon.png
Issue date: 04/12/1988
From: Darienzo M, Peterson C
Oregon State University, CORVALLIS, OR
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NUDOCS 8808010134
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i EPISODIC TECTONIC SUBSIDENCE OF LATE. HOLOCENE SALT MARSHES IN OREGON: CLEAR EVIDENCE OF ABRUPT STRAIN RELEASE AND GRADUAL STRAIN ACCUMULATION IN THE SOUTHERN CASCADIA h  !

MARGLN DURING THE LAST 3,500 YEARS.

by Curt D. Peterson and Mark E. Darienzo College of Oceanography Oregon State University Oceanography ADMIN BLDO 104 Corvallis OR 97331-5503 l

Multiple Buried Marsh Horizons in Oregon Bays and Estuaries Coastal marshes from northern, central and southem Oregon have been cored to 6 m depth to i establish late Holocene records of relative sea level and associated coastal neotectonics. Multiple buried marsh horizons (4-6 in number) have been identified in Netarts Bay (45.5 0 latitude),

1 Nestucca Bay (45.2 0 latitude), Alsea Bay (44.4 0 latitude) and South Slough, Coos Bay (43.3 0 l latitude). The marsh horizons,10 cm to 1 m thick, have been traced laterally (over I km in distance) within individual estuarine systems by stratigraphic correlation of marsh and sediment burial sequences. Burial sequences are generally observed to include 1) venically rooted or l rhizome rich muds grading upward to peaty sediments (marsh layer) which are overlain by 2) I barren sands or muds which commonly grade upwards to finely laminated or bioturbated muds (sediment burial layer). Fresh water diatom assemblages (high marsh) in some buried marsh 1

deposits are consistently overlain by brackish water diatom assembbges, confirming marsh l

subsidence and subsequent burial by tidal flat muds. Contacts between marsh layers and overlying buriallayers are typically sharp, indicating abrupt subsidence. However, some widely traced l

contacts are clearly gradational (see later section). Sediment capping layers on top of some buried marsh horizons range from 20 cm to less than 1 cm in thickness and often include intemal l laminations of sand or mud. The sediment capping layers are wide spread in the lagoonal marsh  !

system of Netarts Bay but are less well developed in fluvially influenced marsh systems of Nestucca and Alsea Bays. Ages of buried marsh surfaces have been estimated by radio-carbon dating of peats in Netans Bay and indicate approximate ages ollocal subsidence events:

Surface Depth (MSL) Calibrated Age (>Ts BP) 1st Buried marsh top 0.7 350+-60 2nd Buried marsh top 1.5 1220+-60 3rd Buried marsh top 1.7 1640+-80 t 4th Buried marsh top 2.2 1760+-60 5th Buried marsh top 4.4 3170+-90 6th Buried marsn top 5.3 3290+ 100 8808010134 SSO61b PDR ADOCK 05000344 P PDC A

Comparitive Salt Marsh Stratigraphies From Subduction and Transform Margins In an effort to constrain the tectonic mechanisms of coastal marsh subsidence observed in the l southem Cascadia Margin we have performed comparison studies of marsh stratigraphy from the l 1

San Andreas transform raargin near Point Reyes, Califomia (38.2 0 latitude). Marsh cores (9) l taken in Tomales Bay (formed within the San Andreas fault zone) corc.ned a maximum of 6 l

buried marsh horizons extending to a depth of 5 m below the mode.n marsh surface. However, l unlike the buried marsh layers of the Cascadia Subduction zone, the Tomales Bay buried marshes

1) are not widely correlated within the basin,2) do not have sharp upper contacts with overlying sediments and 3) do not have distinctive sediment capping layers on top of the buried marshes even though sand is abundant within the upper basin. Burial of the Tomales Bay marshes appears to have occured by incremental subsidence. The preserved marsh layers and intervening sediments in Tomales Bay show no sign ofliquefaction or severe disturbance even though the San Francisco

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earthquake of 1906 was centered near Tomales Bay.

As a control to the study of marsh sequences in the seismically active San Andreas fault zone, an investigation of marsh development was also undertaken in the Schooner Bay arm of Drakes Estero, located about 5 km due west of the fault zone. Uninterrupted peat accumulation was observed in cores to 8 m depth from this tectonically stable setting on the Salinian Block. Episodic tectonic subsidence in the northern Califomia transform margin is limited to the transform fault zone itself. Significantly, the continuous marsh development in the tectonically stable Drakes Estero also demonstrates that marsh burial by potential fluctuations in custatic sea-level did not occur in late Holocene time. Since marshes of the Schooner Bay arm have developed in a sediment starved, micro tidal environment they should have been particularly sensitive to any fluctuations in custatic sea level that might have influenced marsh development on the U.S. west coast.

The results of the nonhem Califomia studies are significant in that they demonstrate a southem boundary to the abrupt subsidence style of marsh burial seen in Oregon and Washinton. In addition, the lack of severe sediment disturbance in the seismically active San Andreas fault zone demonstrates that marsh and sediment disruption are not neccessarily produced by catastrophic e:~.hquakes. Little or no disruption of marsh or sediment burial sequences are observed in abruptly subsided deposits of coastal marshes in the southern Cascadia Margin. The lack of sediment disruption in the marsh records of the Cascadia Margin does not argue against recent seismic activity in this subduction zone.

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Discrimination of Flood and Tectonic Events in Coastal Marsh Records Regional climatic mechanisms of potential marsh burial have been investigated on a preliminary I basis in endmember marsh systems of the southem Cascadia margin. Flood overbank deposition provides a means by which marsh burial could possibly occur independently of tectonic subsidence in fluvially dominated estuaries of the southem Cascadia Margin. In an effort to identify potential marsh burial by flood deposition, several cores (4-5 m depth) we e taken in a flood plain estuarine marsh of the Little Nestucca Riverin nonhem Oregon. Several prominant sand and/or gravel layers 10-30 cm in thickness were observed in an upstream flood plain core site, indicating abundant sand supply to the downstream marsh system. A series of 3-5 buried marsh layers were observed in two core sites 0.5-0.75 km downstream of the upper flood plain site. Although this riverine-marsh environment should have been influenced by major flood events and associated sand supply there are no sand layers associated with the buried marshes or with overlying burial sequences. He buried marshes (10-30 cm thick) have sharp upper contacts and are buned by laminated muds20-100 cm in thickness. '

To test the flood hypothesis more rigorously, a total of 10 marsh cores were taken to depths of 4-7 m in the fluvially dominated upper reaches of Alsea Bay in central Oregon. Several buried marsh layers (10-30 cm thick) are preserved in the upper 3 m of this marsh system and two of the buried marshes are capped by sand layers. .Two orientated core transects, both normal and parallel to the major estuarine riverine channel, showed no evidence of increasing sand layer thickness (1-5 cm thick) with increasing proximity to the channel margin or with increasing distance upstream. A l thourough search for evidence of the 1964 flood, estimated to have exceeded the 100 yr flood level for this drainage system, showed no evidence of marsh burial or sand accumulation within the modem marsh. Preliminary indications of the marsh studies in Nestucca and Alsea Bays suggest l that mash burial by riverine floods have not occured during late Holocene time in these fluvially dominated basins.

Temporal Transitions Between Strain Release and Strain Accumulation While most of the buried marsh sequences we have observed in the southern Cascadia margin have sharp upper contacts (peat-sediment transitions < 1 cm thickness) some buried marshes show gradational contacts (peat sediment transitions > 5 cm thickness). One such contact at about 4.4 m depth (MSL) is laterally persistent in Netarts Bay, northern Oregon, as the 5th buried marsh layer (see previous section). This gradual subsidence event (3.170+-90 yrs BP) occurs very shortly after an abrupt subsidence event (3290+-100 yrs BP) but long before the next subsidence event (an abrupt subsidence at 2,040+- 70 yrs BP). Two independent measures of the transition from marsh to tidal flat deposits have been performed on this gradational contact in different core sites. Both

dry weight loss on ignition (a tneasure of the abundance of peaty material) and a diatom indices of

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salinity (freshwater assemblage =high marsh, brackish assemblage = low marsh or tidal flat) are l shown for one core site below.

4 Depth m (MSL) Loss on Ignition (% Org) Diatom Salinity 3.60 5 Brackish 3.96 7 Brackish 4.20 8 Brackish = Fresh 4.33 12 Fresh > Brackish 4.36 24 Fresh 4.40 28 Fresh The more common events of marsh burial by abrupt subsidence in Oregon are clearly related to tectonic strain release while the events of gradual marsh burial appear to be related to tectonic strain accumulation. Interestingly, the transition between the two modes of subsidence (rapid strain release and gradual strain accumulation) can occur over very briefintervals (less than 300 years).

Additonal evidence of recent subsidence by strain accumulation might be provided by some recent marshes (Netarts Bay and South Slough) which have well defined erosional scarps 0.5-1m in height. The most reent marshes in Alsea Bay and Nestucca Bay have relatively sharp bases (dense rhizome mats over barren sediment) and have prograded over high-energy tidal flats (sand) that have not previously maintained significant marsh development. The unusually broad coverage of these most recent marshes suggest an initial period of coastal emergence by strain accumulation or by strain release. Finally, close spaccd couplets of thin marsh layers of similar age might indicate a futher complexity of tectonic movement. Such sequences are observed in Netarts Bay and possibly indicate abrupt subsidence (marsh burial) followed by emergence (rapid marsh progradation). Additional stratigraphic studies of submergent and emergent marsh sequences in the southern Cascadia Margin are needed to establish the complex tectonic cycles of strain accumulation and strain release that have ocurred along this active-margin during late Holocene time.

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