I’ve been asked about the source for the quartzite clasts in glacial till in the Puget Lowland. I mentioned them in the field trip guide to the Fragrance Lake trail. The clasts are diagnostic of a British Columbian provenance for the Puget Lobe of the last (Fraser) advance that left the Vashon till behind, and the several great glaciations that preceded it. Because they do not match any lowland bedrock, they are glacial erratics.
What are the temperature and pressure conditions needed to create quartzite? Where in B.C. does quartzite come from? Why was quartzite not created anywhere in the North Cascades?
I am not well-versed on these geology topics, so have had recourse to my shelf-full of petrology texts, better-informed colleagues, and, yes, even the internet.
There are, unfortunately, two different rocks called quartzite. Only one is metamorphic. True quartzite, also known as metaquartzite is metamorphosed from sedimentary parent rock rich in quartz grains. The parent rocks are usually sandstone consisting essentially of quartz grains (often from eroded sand dunes, themselves eroded from other quartzose formations), or marine chert. Quartzite is usually 90 – 99% quartz. Quartz-rich rocks, when subjected to sufficient heat and pressure, become the non-foliated metamorphic rock quartzite. The original grains are fused to form a crystalline, essentially structureless rock. Fully formed metaquartzite has lost all sedimentary structures such as the original quartz clasts, bedding planes, and fossils. Quartzite that preserves some sedimentary structures should probably be called metasandstone or metachert. Metaquartzite requires fairly high temperature, pressure, and time to develop.
The other type of quartzite is called orthoquartzite and this is the type we find in Vashon till. This rock forms at low pressure and temperature (i.e. the rock is not buried all that deeply) when circulating fluids fill the spaces between sand grains with silica or possibly carbonate cement. Orthoquartzite is not a metamorphic rock because the original mineral grains are still there, and bedding planes and other sedimentary structures are still evident. Unfortunately, ‘quartzite’ remains a part of this rock’s name, because it is rich in silica. You can commonly find quartzite clasts in Vashon till that have thin layers of darker rock cutting across them. This is relict sedimentary bedding. Also, fossil burrows have been described in Vashon till quartzite clasts (Mustoe, 2001). There is a subtle gradation between orthoquartzite and metaquartzite, and I probably shouldn’t even have raised the subject!
Quartzite can be differentiated from less thoroughly cemented quartz sandstone because fractures in the sedimentary rock run around the sediment clasts (quartz grains), but run through the quartz grains in the metamorphic rock.
As an aside, the Chuckanut Formation in northwest Washington, though rich in sandstone, does not contain a high percentage of quartz clasts, and is unlikely to be metamorphosed to quartzite, if it survives erosion and is some day metamorphosed by pressure + heat. The Chuckanut is an ‘arkosic’ sandstone, rich in feldspars and lithic fragments but with less quartz sand. There are a lot of quartzite pebbles in the Chuckanut, eroded from the Rocky Mountain highlands and carried to the pre-Cascade flood plains of what is now Whatcom and Skagit Counties.
All quartzites are very hard and resistant to both physical and chemical weathering. Broken clasts of quartzite that are carried as sediment in mountain streams will eventually be milled to the smooth round cobbles and pebbles we see in till. But due to the hardness of the quartz, made even more so by metamorphism, rounding takes a lot of time and stones must be carried a long distance and bang into a log of other hard rocks to get rounded.
Where does the quartzite in our tills come from? WWU’s George Mustoe published a paper studying this question, and you can read it here in Washington Geology (see page 15). Both he and Jon Riedel, geologist at North Cascades National Park, an authority on northern Cascade till provenance, believe that the quartzite clasts mainly come from the Hamill Group ** in the Cariboo and Selkirk Mountains in east-central BC. This group of marine sedimentary formations was deposited in the late Cambrian Columbia Basin about 580 M years ago. George’s paper describes a fossil in a quartzite cobble that is likely the oldest fossil yet reported from Washington State.
The Cariboo Mountains are the northernmost subrange of the Columbia Mountains, which run down into northeast Washington, northern Idaho, and northwestern Montana. The Selkirks are east and south of the Cariboos, extending into the US. The Columbia’s also include the Monashees and Purcells. (from Wikipedia.)
The quartzite was eroded from the western flank of these ranges and carried by streams or glaciers westward into the Fraser River valley in central BC. Continental ice sheets eventually incorporated the rounded stones and carried them south, down the Fraser Valley and into the coastal lowlands. This is about 300 miles, or 480 km. From, there, it’s just a hop, skip, and jump to end up in glacial tills around the Salish Sea.
The Okotoks erratic is the largest known glacial erratic in the world, and is quartzite. It lies south of Calgary, Alberta, after being carried a couple hundred miles eastward onto the plains from the Rocky Mountains.
Why no quartzite in the Cascades? Rocks here started out the wrong way. Parent rocks were not eroded from quartz-rich sand dunes. There are thin chert beds in the Bell Pass Melange west of Mount Baker, but they are not metamorphosed quartzite.
Sources: Figure 2 is from http://www.pitt.edu/~cejones/GeoImages/6MetamorphicRocks/Quartzite.html
Mustoe, G., 2001, Washington Geology, v. 29, no. 3/4, p. 17-19