Recalibrating Rodinian rifting in the northwestern United States

A lack of precise age constraints for Neoproterozoic strata in the northwestern United States (Washington State), including the Buffalo Hump Formation (BHF), has resulted in conflicting interpretations of Rodinia amalgamation and breakup processes. Previous detrital zircon (DZ) studies identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted to reflect mostly first-cycle sourcing of unidentified but proximal magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga. Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent a distal source with deposition occurring during the early phases of Rodinia rifting, more than 250 m.y. after zircon crystallization. We combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to evaluate these opposing models. Our study of similar to 2000 DZ grains from the BHF identified for the first time a minor (similar to 1%) yet significant ca. 760 Ma population, which constrains the maximum depositional age. This new geochronology implies that the BHF records early rift deposition during the breakup of Rodinia and correlates with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.

Automated summary

The lack of age constraints for Neoproterozoic strata in northwestern United States has led to conflicting interpretations of Rodinia amalgamation and breakup processes, including the Buffalo Hump Formation. A new study identified a minor yet significant population of ca. 760 Ma in the BHF, suggesting early rift deposition during the breakup of Rodinia and correlation with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.