Detrital Zircon Geochronology of Supermature Quartz Arenites in Western North America Indicate Provenance from Transcontinental Arch Tied to Long-Term Sea Level Fluctuations

Detrital zircon geochronology can be an incredibly useful tool in determining provenance for siliciclastic rocks. However, too often results from individual, far-separated samples from widespread units are used, negating the abundance of data available through sampling the same unit multiple times within its sequence stratigraphic framework. Research on the provenance of Middle-Late Ordovician quartz arenites of the western Laurentian passive margin showed these units are unique among Phanerozoic sandstones on this margin in having a mostly bimodal distribution of Paleoproterozoic (1.8-2.0 Ga) and Archean (2.5-2.8 Ga) grains (Gehrels, 2000, personal comm. 2006). These sediments are likely 1st-cycle quartz arenites produced by intensive equatorial weathering of the Transcontinental Arch. Integration of detailed stratigraphic descriptions, sequence stratigraphic interpretations and increased detrital zircon sample density is providing a more detailed picture of the depositional history of these units; we are beginning to study similar units in the U.S. Midcontinent (Simpson Group sandstones, St. Peter Sandstone) in the same manner.

The Middle-Late Ordovician quartz arenites of western Laurentia preserve sedimentary structures indicating deposition occurred on a shallow-water high-energy siliciclastic shelf that passed basinward into interbedded Deepwater shale and sandstone beds. Commonly two or three, 3rd-order sequences (1-10 My duration) are preserved within each unit that represents a composite 3rd-order, or 2nd-order sequence (10-20 My duration). Detrital zircons were sampled from the LST/TST and HST of each 3rd-order sequence at multiple sections within a small geographic area. Samples from Idaho, north and south of the Snake River Plain indicate that during the LST/TST of these units Archean grains predominate over Paleoproterozoic grains suggesting a proximal sediment source from the Wyoming Craton. As sea level rose during the HST the proximal source was at least partially covered and Paleoproterozoic grains from the Trans-Hudson Orogeny predominate over Archean grains. Longshore transport of sediment was likely a very important process that produced widespread quartz arenites with such homogenized detrital zircon signatures.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California