Accretionary Wedge #41 Contribution: Slip-Sliding Away

I’ve been thinking about my own contribution for December’s Accretionary Wedge for over a month now (hopefully giving everyone else all the cover they’d need for late entries), but the time has come to write it up and move on.

I’ve been fairly fortunate in my time as a geologist to witness a volcano erupting firsthand in Hawaii. If I tried to tell you that I’ve directly experienced a more memorable geological event (at least to this point in my life), I’d be lying. I’ve also had the chance to feel a couple of very small earthquakes firsthand in New Jersey (Ramapo Fault), Hawaii (Kilauea Volcano), and Kansas (Oklahoma quake that triggered this Accretionary Wedge topic). I slept through an even bigger quake (M7.3 Landers, ~150 miles away) during my first field season, and saw the damage firsthand before the day was out. This past spring I saw the Mississippi River at a record flood crest from the safety of the levee near the Old River Flood Control structure and drove across the Morganza Spillway with 19 gates open, sparing New Orleans from another watery apocalypse. These last two may have been the most significant geological events I’ve witnessed firsthand. Nonetheless, the event I’m choosing to write about is a different event altogether.

The geological event that is the focus of this post is the Nardi Road/Goulais River landslide of October 13, 2003. Although this event was of a more local scope than some of the ones I enumerated above, I was able to witness firsthand the profound consequences for those who experienced it most directly. I was fortunate to be able to see and document the consequences with my Geoenvironmental Systems class less that 24 hours after the slide occurred. Mr. and Mrs. Norlin, whose safely rode out the slide in their red-roofed house (which features prominently in a number of images below, were kind enough to grant us permission to examine the slide in firsthand detail, and it must be noted that their spirits were remarkably good, despite the ordeal they had so recently survived.

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Norlin House and Suburban

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For the most part, I’ll let the pictures do the talking – the full set of images can be seen on Flickr. The immediate triggering event for the landslide (or more accurately, a rapid, progressive series of rotational slumps) is unclear and may never be known. In a larger sense, this area was located on a cutbank of a meandering section of the Goulais River. The slump was rooted in a thick sequence of varved clays deposited at the end of the Wisconsinan Ice Age, as well as alluvial sands. There have been a number of other slides of this type in the region historically – in fact, our class had already examined the deposits of one such slide on the south shore of the Saint Mary’s River just a few miles west of Sault Ste. Marie, MI earlier in the semester. Although this event took place long before I had my first GigaPan, I did already own my first digital camera, and I shot a number of panoramic shots along with the many individual images seen below.

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Dennis Guimond’s House, next door.

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The Norlin’s House and a shed

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The Goulais River, near the toe of the slide.

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Location of the Nardi Road/Goulais River Slide (Google Earth)

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Evolution of the Landslide Toe, April 2005 – May 2007 – March 2010

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Varved glacial clays are visible in a crevasse beneath a thin mantle of sand. Detachment surface is locally dipping about 45 degrees. Faint “slickenline”-like traces were still visible in the loose sand on the detachment surface in the days immediately after the slumping event.

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Tension gashes formed in the “lower plate” of this detachment surface within 24 hours of the main slip event.

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Kelsey Anderson points out a normal fault where the “lower plate” of this detachment surface (now horizontal) has been offset within 24 hours after the main slump event.

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General Chaos and Mayhem, LSSU students for scale.

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High angle detachment surface near headwall. Note slip direction on the detachment surface is delineated by sand and soil still coating the muddy surface. Varved clay sediments make up the footwall of this block.

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Headwall of the slide complex. Sandy alluvial sediments and soil make up the highest layers; varved clays are lower down. The Suburban had been parked outside the carport and buried itself vertically during the event. Geology students for scale.

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Slicklenlines and boot prints in the lower portion of the slide.

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Detachment surface and varved clays near the headwall of the slump.

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Evidence for late-stage lateral shearing. These horizontal slickenlines were formed as a cluster of maple trees was dragged laterally toward the end of the slide sequence.

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Trace “fossil” where a tree trunk indented the varved clay sediments of this block.

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Tree trunks with clay from the impact above.

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The “Birch Tree Block”

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Another great day in the field.

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You can also consider this the final call for submissions for Accretionary Wedge #41. I’ll be writing up the summary post next (tomorrow).

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