DETRITAL ZIRCONS OF MIDDLE-LATE ORDOVICIAN QUARTZ ARENITES SURROUNDING THE TRANSCONTINENTAL ARCH RECORD PROVENANCE AND REGIONAL TECTONISM OF LAURENTIA

Widespread Middle-Upper Ordovician eolian to marine quartz arenites surrounding the Transcontinental Arch (TA) of Laurentia commonly were deposited within thick sections of marine carbonate or shale. This juxtaposition suggests these units were sourced from the TA as first cycle sediments (since there weren’t thick siliciclastic units to recycle), likely during humid conditions along the Middle-Late Ordovician paleoequator. These quartz arenites were deposited diachronously, the units east of the TA (e.g. St. Peter Sandstone, Simpson Group, Cable Canyon Sandstone) being the oldest; whereas units west of the TA (e.g. Eureka Quartzite, Swan Peak Quartzite, Mt. Wilson Quartzite) are younger.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) of detrital zircons from Middle-Late Ordovician quartz arenites of the Midcontinent have abundant 0.9 – 1.1, ~ 1.4, 1.6 – 1.7, and 2.5 – 2.9 Ga grains. Conversely detrital zircons of Late Ordovician quartz arenites of western Laurentia (Northwest Territories, Alberta, Idaho, Nevada, and Mexico) are predominantly 1.8 – 1.9, or 2.5 – 2.9 Ga grains with minor abundances of 2.05 – 2.1 and 0.9 – 1.1 Ga grains. The lack of 1.8 – 1.9 Ga grains in the Midcontinent suggests there was a drainage divide along the TA that separated western and eastern Laurentia.

During the Middle Ordovician, sediment (and their zircons) that compose the quartz arenite were derived locally from the TA and areas to the north that included Grenville Basement and/or the Midcontinent Rift (0.9 – 1.1 Ga), Yavapai-Mazatzal terrane (1.6 – 1.7 Ga), so-called “Anorogenic” Granites (~1.4 Ga) and the Superior Craton (2.5 – 2.9 Ga). However, the uniformly consistent detrital zircon signature of western Laurentia units indicate they were sourced almost solely from the Trans-Hudson Orogen (1.8 – 1.9 Ga) and the Wyoming Craton (2.5 – 2.8 Ga). The change in sediment sources during the Middle-Late Ordovician is likely due to changes in the tectonic uplift of the TA related to Taconic Orogenesis. When subduction was active the TA was uplifted, and as the locus of loading moved northward the sediment source also moved north and west, and when most of the subduction ceased, the TA collapsed, siliciclastic sedimentation ceased and the TA was covered by widespread Upper Ordovician carbonate.