CONTINENT-SCALE FLUVIAL SYSTEMS REVEALED BY DETRITAL ZIRCON GEOCHRONOLOGY OF PROTEROZOIC CRATONIC SHEET SANDSTONES

Prior to 400 Ma, the lack of substantial vegetative cover allowed for high rates of physical weathering and sediment production. This probably was complemented by greater rates of chemical weathering as a function of higher atmospheric CO2 and attendant acidic precipitation. Anomalously thick (by present-day comparison) quartzose, cratonic sheet sandstones, which characterize the Proterozoic-early Paleozoic stratigraphic record are testament to the effects of this rigorous weathering regime.

Dispersal of detritus from vast ancient mountain ranges by extensive braided river networks is indicated by sedimentology and U-Pb detrital zircon provenance analysis of Proterozoic cratonic sheet sandstones. Examples are preserved in early Neoproterozoic fluvial quartzarenites that stretch from the southwest U.S. to Arctic Canada and late Paleoproterozoic intercontinental basin sandstones of the U.S. and Canadian mid-continent region.

Early conventional detrital zircon studies of the Shaler and Mackenzie Mountains supergroups in northwestern Canada revealed sources in the Grenvillian orogeny, which formed on the opposite side of Laurentia, up to 3000 km away from the sites of original deposition of the sandstones. The “big river” hypothesis was supported by regionally consistent paleocurrent patterns and later SHRIMP investigations of proximal parts of the river system, some located on other continents (e.g. Siberia, Europe).

Recent detrital zircon provenance studies of late Paleoproterozoic fluvial sheet sandstones, preserved in the Athabasca and Thelon basins, indicate overwhelming input from the adjacent 1.9-1.8Ga Trans-Hudson orogen, consistent with west-directed paleocurrents that dominate the basin fill. The detrital zircon age profile for fine-grained sandstones from the correlative Muskwa basin, located on the western margin of Laurentia ~800km away, is remarkably similar suggesting that it hosts probable distal equivalents of this broad cratonic sheet. Strata from the upper Athabasca Group have southerly provenance, which is accompanied by the appearance of relatively young (ca. 1770-1650 Ma) detrital zircons that have no local source. We postulate that they were derived from the Yavapai and Mazatzal orogens, implying transport by a >1500 km long river system draining northward.