Steptoe Butte is a quartzite "island" in the sea of loess known as the Palouse Hills, in eastern Washington and western Idaho in the United States. According to Wikipedia:

The rock that forms the butte is over 400 million years old

with no source to this claim. Looking around, I found this page from the Washington Geological Survey, which claims that Steptoe Butte is part of the Belt Supergroup, with an age of 1.4$-$1.5 Ga. Similar information and age can also be found on this University of Idaho webpage. Now comes this Eastern Washington University page, which claims (emphasis mine):

The butte of quartzite is well over 400 million years old, metamorphosed from a protolith of sandstone that went under searing temperatures and pressure into a hardy metamorphic rock known as quartzite which is almost entirely made of interlocking grains of quartz and other minerals (see photos of quartzite). On the northern lower part of the miraculous summit, it has vitreous quartzite with feldspathic matrix that was initially interpreted as Precambrian Belt Supergroup (Waggoner, 1998 and the attached geologic map). But, using the age distribution of detrital zircons resulted in a dominant age peak of 1.7 to 1.9 billion years old (Ellis et al., 2004), which correlates with the age peak of detrital zircons from the Cambrian period (e.g. Ross and Villeneuve, 2003; Linde et al, 2017). This information means that the quartzite is not as old as it was once thought to be, but is still quite old.

I can't make sense of this last passage. The cited abstract (Ellis et al., 2004) goes:

The youngest grains observed constitute a dominant age peak at approximately 1.7-1.9 Ga. Older minor peaks occur at 2.45, 2.6-2.7, 2.9 and 3.3 Ga. Despite the apparent difference in degree of deformation, quartzites on Steptoe and Kamiak Buttes are likely correlative. The detrital zircon signatures are consistent with those determined for easterly derived units of the Belt-Purcell Supergroup which contain a strong Laurentian signature (Ross and Villeneuve, 2003), but lack the younger 1.45-1.5 Ga syn-depositional detrital zircon signature that characterizes some sections of the Belt-Purcell sequence such as the nearby Wallace Formation.

But I don't see how this tells us that Steptoe Butte is not part of the Belt Supergroup and is thus of Cambrian age, as the previous text implies. So, I'd like to ask: how old is Steptoe Butte?


1 Answer 1


Without directly knowing anything about WA geology myself, you're dealing with two ages here: the age the rock deposited as a sandstone (protolith age, 1.4-1.5 Ga), and the age of metamorphism (400 Ma).

Detrital zircon populations constrain the depositional age because the U-Pb system in zircon stays robust below 900 °C. These are transported from their original host rock via fluvial systems and other sedimentological processes. Eventually you get a rock (in this case a sandstone) with a pastiche of necessarily older detrital zircons that can give you a minimum depositional age. I am not sure why it is considered 200-300 million years younger than its oldest zircon population; it is possible there is simply known sedimentation at 1.4-1.5 Ga.

Thereafter, it was metamorphosed from a sandstone to a quartzite at 400 Ma, likely during the assembly of Pangaea going by the age.

  • 2
    $\begingroup$ It makes sense, thanks. Then which is considered the age of the rock in such case? The age of the protolith, or the age of the metamorphic episode? I think in the case of the Acasta gneiss, the generally reported age of ~4 Ga is the igneous age (formation of the TTG protolith), while the rock records several, more recent metamorphic events: doi.org/10.1007/s004100050465 I guess it's more a philosophical question... $\endgroup$ Oct 13, 2022 at 15:25
  • 1
    $\begingroup$ Yes, I did indeed. Edited. I generally see metasedimentary rocks referred to by their depositional age, with timing of metamorphism labelled as such. Not sure why your first link quotes the metamorphic age, unless by 'well over 400 million years old', they are accounting for over a billion years with that 'well over' and an arbitrary time stamp... $\endgroup$
    – desander
    Oct 14, 2022 at 12:50
  • $\begingroup$ The text of this link makes no sense. It implies that the Steptoe Butte quartzite is not part of the (Precambrian) Belt Supergroup, while the quoted paper says the opposite... Floodexplorer.org about page says that "most of the stories were created by EWU students", so I guess the one who wrote this one misunderstood the literature he quoted. So it all comes down to a case of poor editing! $\endgroup$ Oct 14, 2022 at 15:52
  • $\begingroup$ I think being mostly buried in olumbian flood basalts had an effect. $\endgroup$
    – Spencer
    Oct 16, 2022 at 13:22
  • $\begingroup$ The Columbian basalts are Quaternary, and so are unlikely to be related to the Cordilleran metamorphism that the Steptoe Butte quartzites record. $\endgroup$
    – desander
    Oct 16, 2022 at 16:36

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