March/April Geological Society of America Bulletin media highlights

The Appalachian orogenic belt records a series of collisions between ancient North America (Laurentia), arc terranes, microcontinents, and larger continental masses, which culminate in the assembly of the late Paleozoic supercontinent Pangea. While the tectonic evolution of the northern Appalachians includes the apparent subduction of Laurentia, or peri-Laurentian blocks, beneath an exotic island arc(s) during the Ordovician Taconic orogeny, Tull et al. show that the southernmost Appalachian Talladega belt, which formed along the Alabama continental promontory, was tectonically isolated from this collisional event. Instead, a Middle Ordovician suprasubduction metavolcanic sequence, the Hillabee Greenstone, records evidence of a Laurentian continental margin arc. Both geochemical analyses and field mapping suggest the bimodal Hillabee Greenstone is an accreted fragment of a previously more extensive Ordovician volcanic complex that was thrust onto the middle Paleozoic shelf along a cryptic fault during the Late Devonian or Early Carboniferous. Emplacement of these marginal arc rocks may have resulted from a collision between the Laurentian Alabama promontory and the Ouachita arc.

U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada
Robert Rainbird (corresponding author) and W.J. Davis, Geological Survey of Canada, Continental Geoscience, Ottawa, Ontario K1A 0E9, Canada. Pages 314-328.

Keywords: U-Pb, geochronology, provenance, Paleoproterozoic, Dubawnt Supergroup, western Churchill Province.

This paper reports on the ages of zircon (ZrSiO2) sand grains extracted from sandstones in sedimentary basins of early Proterozoic (approximately 1.8–1.6 billion years old) age from northern-central Canada (western Churchill Province). The ages of the zircon grains tell us that certain continental blocks, (e.g., Superior and Slave provinces), were colliding with the western Churchill block at the same time that these sedimentary basins were forming. Similar collisions were occurring elsewhere on Earth at this time (e.g., Australia), as part of a global-scale amalgamation known as a supercontinent. Uplift and erosion of mountain belts that formed as a result of this amalgamation shed large volumes of detritus (sand and gravel) via large river systems, into the interior of the supercontinent, where it was captured in sedimentary basins like those presently exposed in the western Churchill Province of northern-central Canada. Such studies will help geoscientists to piece together this ancient supercontinent puzzle.