Friday, April 26, 2013

Possible Fault on the Toandos

Back in November the Washington DNR Geology Division released their Image of the Month

Trevor Contreras at Dabob Bay outcrop
 DNR November Image of the Month

The image is of an off set of some sediments estimated to be approximately 100,000 years old. In geologic terms that is recent and more importantly suggests the potential of localized faulting and earthquakes in the area of the image. There has been a lot of looking for faulting in this area as geophysical surveys have suggested some sort of fault structure in the vicinity running up along Hood Canal and into the Dabob Bay area. This may be the first definitive exposure of faulting along this feature. Based on LiDAR imagery this fault, if it is present, has not ruptured the surface. That doesn't mean it is not active, just means that the fault movement does not break through to the surface. 

Finding fault lines in western Washington has proven to be challenging. The repeated and recent glaciations have obliterated surface features that develop along fault segments. The geophysical surveys and the general lay of the land with a steep mountain range, the Olympics, and the low lands of Puget Sound suggest a major structure.

Finding off sets that may be related to a suggested fault is exciting. The find was made while geologist Trevor Contreras with the DNR and a team were mapping the Seabeck Quadrangle. The location of the offsets and tilted sedimentary beds is on the Toandos Peninsula are indicated in a DNR post november-geology-image-of-the-month-dabob-bay-outcrop.
Map of location  

I have walked miles of the Toandos Peninsula. Typically my efforts have been related to slope stability and erosion. But I have kept a sharp eye out for fault off sets like the one shown. But darn if the location shown above is at a section of the Toandos I have not yet traversed. I was a bit relieved after seeing the map, as I initially was thinking "I must have missed it." That said, I did observe a potential off set that was poorly exposed and not very definitive to the southwest of the site. And there are some oddities in the topography of the area that could under a creative arm waving sort of way be explained by localized tectonic movement or fracturing.

I am looking forward to the DNR publishing the Seabeck Quadrangle Map. 


Meredith Payne said...

Our November Image of the Month was actually this one: The image you highlight actually shows DNR geologist, Trevor Contreras, who discovered and mapped these structures during the 2012 field season.

Dan McShane said...

Thanks Meredith. Crossed up the actual image. Had the pleasure of hearing Trevor's talk in Edmonds and am looking forward to his future work on Hood Canal and Jefferson County.