DETRITAL ZIRCON PROVENANCE OF THE EARLY PALEOZOIC ARENITES OF THE LAURENTIAN MIDCONTINENT

Early Paleozoic quartz arenites deposited during the Sauk Transgression in Laurentia are among the most mature in the world. The provenance of these thin sheets of quartz arenite, such as the Cambrian Mt. Simon and Ordovician St. Peter sandstones, has puzzled geologists for more than a century, largely because the textural maturity and compositional purity of such deposits leave few clues of their origin. Robert H. Dott Jr. referred to these arenites as “nature’s finest distillate—almost as remarkable as a pure single malt Scotch whiskey”. Bob spent much of his life sitting atop this great sheet of quartz sand, pondering single- versus multicycling to explain the supermaturity of these rocks. His interest in these rocks was contagious. The provenance, and thus origin, of these early Paleozoic quartz arenites became constrained with the development of thermal ionization mass spectrometry (TIMS), and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MCICPMS) techniques for U-Pb age determinations of single detrital zircons. During the past few decades, thousands of zircon grains have been analyzed from Cambro-Ordovician arenites across Laurentia. Based on our data, and data from other workers, Early Paleozoic arenites in the midcontinent, several observations are evident. First, older Cambrian arenites (Mt. Simon though Galesville sandstones) contain mostly Archean age grains, and have sparse zircons from the 1400-1900 Ma interval, which is the age of much of the local crystalline basement. Second, younger Cambrian and Ordovician arenites (Jordan through St. Peter sandstones) have much higher concentrations of Grenville-age grains. Based on our analysis, the Cambrian samples reflect mixtures between the Midcontinent Rift basin (Neoproterozoic Jacobsville Sandstone and its equivalents) and the Archean basement, and the Ordovician samples appear to be mixtures the Midcontinent rift and Grenville and Archean basement. These data indicate that the younger arenites are not recycled from the older. Moreover, both the older and younger arenites contain distinct populations of zircons derived from distal sources areas as well as zircons that were recycled from more proximal Neoproterozoic strata.