SEDIMENTARY PROVENANCE OF THE UPPER CAMBRIAN WORM CREEK QUARTZITE, IDAHO, USING U-PB AND LU-HF ISOTOPIC ANALYSIS OF ZIRCON GRAINS

The Worm Creek Member of the Upper Cambrian St. Charles Formation is an anomalous feldspathic sandstone within the Cordilleran carbonate platform in southeast Idaho. It contains two or three decameter-scale siliciclastic to carbonate cycles, ranging from 5 to over 150 meters thick, that each contain a basal feldspathic sandstone and an upper limestone with varying amounts of quartz sand. Detrital zircons within the Worm Creek display a strong unimodal 500 Ma age-peak. This age population is also recognized in the Upper Cambrian DuNoir Limestone at Wind River Canyon, WY and the Pilgrim Formation at Melrose, MT.

U-Pb zircon results from Beaverhead-Big Creek belt plutons yield ages of 505-490 Ma and are identical to the unimodal age peaks of detrital zircons in the Worm Creek Member which suggests exhumation and erosion of crust overlying the hypabyssal plutons immediately after intrusion. Four samples from the Beaverhead and Deep Creek plutons yielded εHf values between -6.3 and 2.7, suggesting an isotopically evolved source with a depleted mantle model age of 1370 to 1470 Ma. εHf values for the 500 Ma-age peaks of six sandstones from the Worm Creek Member from the northern Bannock Range and the Bear River Range, southeast Idaho, have similar evolved εHf signatures with a range of -8.0 to 5.4. The simplest mantle derivation model for the plutons suggests segregation from the mantle at 1370 to 1470 Ma, which overlaps the age of Mesoproterozoic granitic intrusive rocks west of Salmon, Idaho and suggests an extensive lower crustal magma chamber of that age in east-central Idaho.

Evidence of active Cambrian magmatism and exhumation of the central Idaho Lemhi Arch requires syndepositional plutonism during formation of the Cordilleran “passive” margin. The areal extent of siliciclastic influx from the unroofed Cambrian plutons of the Lemhi Arch allows an estimate of at least 10 meters of relative sea-level drawdown with a volume of over 700,000 km³. The overlap of εHf values, along with the same depleted mantle age for the detrital grains and the igneous crystals from plutons, suggests that Beaverhead-belt plutons are the source of the 505-490 Ma detrital zircon population in the Worm Creek Member, and that this uplift event is related to the Sauk II to III sea-level drawdown in the northern Rocky Mountains.