Thursday, October 9, 2014

A Break from the CRBG - Violent Ellensburg Formation

A recent day of field work which mostly entailed navigating complex topography to get to where I needed go had lots of basalt outcrops of the Grande Ronde basalt member of the Columbia River Basalt Group (CRBG). The Grande Ronde is the most massive of the lava flows that make up the CRBG. The two flow sections I was navigating through west of the Kittitas Valley are interpreted by Tabor and others (1982) on the Wenatchee 1:100,000 geology map as two upper flows of the Grande Ronde based on stratigraphic position and polarity of the iron minerals in the flow. These are the same flows that are exposed in the bottom off the Grande Ronde River Canyon in the southeast corner of the state grande-ronde-basalts-at-grand-ronde.  

Cliffs of basalt 

Top of cliff

Scree of weathered basalt

My entire day was occupied looking at rather uniform basalt cliffs, scree slopes and thin soils full of fractured basalt. But at the very end of the day I came across something new - an exposure of welded hard volcanic material full of bits of rock fragments firmly embedded within a finer matrix. My initial though before taking a closer look was it was glacial till.

The Tabor and others (1982) map indicates Ellensburg Formation volcanics about a mile to the east. This exposure fits the Ellensburg Formation from the area. The Ellensburg Formation does interfinger with CRBG meaning that when the early massive CRBG flood basalts flowed across southern Washington approximatley 16 million years ago there was also a rather violent separate volcanic field in the area with explosive eruptions. Some of the explosive eruptions can be viewed along Highway 10 northwest of Ellensburg ellensburg-formation-side-trip  


Kat said...

I'm reading Rare Earth, and was struck by the description of gabbro. Is there a good place to see some anywhere near Ellensburg?

Dan McShane said...

Kat: there are mafic magma rocks on Highway 97 on the north side of Blewit Pass. Been awhile since I worked in that area so I do not know a specific spot. Rare Earth is a must read for any geotypes.