February 28, 2012
Hundreds Of Millions Of Years Of Change In Cordilleran Terranes Of Western North America
The March GSA TODAY, the Geological Society of America´s open-access science and news magazine, is now online at http://www.geosociety.org/gsatoday/archive/22/3/. This month´s science article, by Todd LaMaskin of the University of North Carolina at Wilmington, presents uranium-lead dating analyses of detrital zircon grains in Paleozoic-Mesozoic basin sediments in the Cordillera of western North America.
LaMaskin's analysis shows a systematic variation in age distribution within the depositional age and setting of the host sediments. Four such age distributions, which he terms "detrital zircon facies," are identified: (1) a Paleoproterozoic-Archean facies mainly found in Paleozoic and early Mesozoic accretionary complexes; (2) a Proterozoic-Phanerozoic facies found in Jurassic basins; (3) a Triassic-Jurassic facies found in Late Jurassic-Early Cretaceous basins; and (4) a Jurassic-Early Cretaceous facies found in Cretaceous marginal basins.These "facies" reveal the influence of a dynamic interplay of orogenic (mountain-building), erosional, and sediment-transport systems (such as wind and water) on sediment provenance in the southern Cordillera. They are interpreted to record stages of basin formation during phases of the supercontinent cycle and to reflect long-term secular variation (changes in Earth´s magnetic field measured in years, decades, and centuries) in this active plate tectonic setting.
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