Abstract
Petrologic and geochemical data confirm that mudstones and sandstones of the Johnnie Formation were the initial siliciclastic deposits laid along the Cordilleran Laurentian margin following the Neoproterozoic break-up of Rodinia. Sedimentary rocks of the Johnnie Formation have corrected CIA values between 63 and 83 (or higher), which suggest moderate to intense weathering of crystalline source rocks or recycling. Based on modeling the fresh source rocks likely consisted of 90% granodiorite and 10% high-K granite. This conclusion is based on petrographic observations, major element geochemistry, and investigation of the REE: (La CN/Sm CN = 4.19 ± 1.26, Gd CN/Yb CN = 1.34 ± 0.38, Eu/Eu* = 0.63 ± 0.09 and La CN/Yb CN = 9.55 ± 2.27). Feldspars are unevenly distributed in the finer grained sedimentary rocks. Observed fluctuations in feldspar content throughout the Johnnie Formation are interpreted as a result of abrasion and hydrodynamic sorting, which concentrated feldspars in the finer grained sediment. None of the mudstone samples, including those collected just below and above the flat-pebble conglomerate in the upper Johnnie Formation, show evidence of true cold weather depositional conditions. Consequently, Johnnie Formation mudstone geochemistry does not record evidence of an extreme paleoclimatic environmental shift in the succession. Textural characteristics of Johnnie Formation sandstones are consistent with quiescent tectonic conditions characterized by low relief, and deposition of Johnnie Formation strata in a passive-margin setting.
Published Version
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