Constraining Ages of Pre-Quaternary Eolian Dust Deposits as a Means for Accessing High-Resolution Climate Archives
Loess (terrestrial eolian silt) is abundant today, and well recognized as a high-fidelity archive of terrestrial climate change. Long neglected in the pre-Quaternary, loess(ite) is becoming increasingly well recognized in the late Paleozoic (latest Devonian through Permian). Both true ‘loessite’ and eolian silt deposited in a variety of continental and marine environments occur.
High-resolution dating remains a hurdle to unlocking the climate archives housed in ancient loess deposits owing to their unfossiliferous character, but detrital zircon geochronology is providing unexpected inroads. For example, the Maroon loessite of Colorado has a published age of Middle Pennsylvanian on the basis of enclosing strata; acicular detrital zircons from the base and top of a 700 m section, however, reveal a maximum age of early Permian, and apparent high rates of loess deposition. Young detrital zircons have also been recovered from the Cutler loessite of Utah. The acicular, sharply terminated morphology of these magmatic grains together with their presence in an eolian unit suggests minimal temporal storage prior to deposition. Dust within Pennsylvanian marine carbonates from the Midland basin also contain volcanic-derived grains, apparently reflecting the proximity of late Paleozoic arc volcanism.
Constraining the depositional ages of eolian dust with geochronology enables the potential for high-resolution paleoclimate studies from these deep-time datasets. For example, cm-scale analyses conducted on the Maroon and Cutler loessite units, as well as dust-infused carbonate successions document variations in multiple climate-sensitive parameters such as grain size, magnetic susceptibility, and whole-rock geochemistry that reflect changes in wind patterns and intensity as well as precipitation-evaporation. Improved age dating of these deposits enable transformation of these variations into the temporal domain, and are beginning to suggest sub-Milankovitch variations.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009