Multiple Deformation Darrington Formation

I had a venture into the Northwest Cascades that included a chance to check a couple of old quarries. The quarries were readily apparent in lidar bare earth imagery, but a bit obscured on the ground as the forest had grown up around them. The first quarry mine floor was being pioneered by a mix of cottonwood and red alder trees. 

The geology of this area east of the South Fork Nooksack River is mapped as Darrington Phyllite and glacial drift and the quarries were in both formations with the one pictured above being in the Darrington Phyllite. The quarry exposed the highly deformed phyllite that includes a range of tight folds, kink bands and mini slip zones.

The Darrington Phyllite covers an extensive area of the Northwest Cascades with outcrops from as far east as near Marblemount to the west end of Samish Island. The same multiple deformation fabrics I observed at the quarry are also present miles away at excellent exposures on Samish Island. Dunhan (2010) working from excellent exposures on Samish Island did a detailed analysis of the deformation including how the kink bands so prominent in the formation developed.  

The Darrington is part of a larger terrain, the Easton Metamorphic Suite, a tectonic terrain of ocean crust that accreted to North America about 140 million years ago. The Darrington is mostly former mudstone that has been metamorphosed into phyllite. The multiple deformations in the Darrington represent different tectonic events in the history of the Easton Suite. Metamorphic minerals within the former ocean floor basalt record one time very deep burial of these rocks indicating the Easton Suite went deep down the subduction trench. Further deformation would have taken place when these rocks were exhumed or uplifted out the deep burial in the trench and then deformed yet again when subsequent tectonic events emplaced them in their current position with additional deformation during the most recent mountain building events. A hard story that has taken time to piece together by lots of geologists with still much to resolve and interpret.    

Just to the east of the quarry the geology changes abruptly into an entirely different tectonic terrain, or more appropriately multiple tectonic terrains termed the Bell Pass Mélange. In the midst of the mélange of terrains is a large block of dunite derived from the mantle of the planet. This block of mantle forms the jagged peaks of the Twin Sisters Range. The Twin Sisters were in and out of the clouds during my venture.

Twin Sisters Range