QUARTZITE BUTTES IN THE PALOUSE REGION, SE WASHINGTON: THEIR RELATIONSHIP TO THE BELT BASIN ON THE BASIS OF U-PB LA-ICPMS DETRITAL ZIRCON DATA

A number of buttes (e.g., Kamiak and Steptoe Buttes) and small hills (e.g., Smoot Hill) underlain by quartzite protrude through the Miocene flood basalts of eastern Washington and northwestern Idaho. Quartzites of Steptoe Butte and isolated outcrops to the north contain a well developed foliation defined by mica growth. The grain size of these quartzites is uniformly fine to medium sand and sedimentary structures are rarely preserved. However, on Kamiak Butte the grain size of the quartzite varies from fine to coarse with local pebbly lenses and sedimentary structures, especially cross-bedding, are more common. The pebbly lenses are discontinuous and contain ubiquitous intrusive igneous clasts up to 2 cm diameter.

The depositional age of these isolated quartzite exposures is currently poorly known. Using available constraints, they likely may (1) represent Mesoproterozoic pre-Belt basin sediments, (2) be correlative with sediments along the western margin of the Mesoproterozoic Belt basin (e.g., Ross and Villeneuve, 2003), or (3) occur as remnants of Paleozoic sediments deposited along the western margin of Laurentia. Detrital zircons were separated from samples collected from both Kamiak and Steptoe Buttes for analysis by the laser-ablation ICPMS method. Plotting ²⁰⁷Pb/²⁰⁶Pb ages on cumulative probability plots, both samples yield similar age spectra. 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.

(Ross, G. M., and Villeneuve, M., 2003, Geological Society of America Bulletin, v. 115, p. 1191-1217.)