THE BORING BILLION AS TOLD THROUGH THE MESOPROTEROZOIC BELT AND PURCELL SUPERGROUPS

The Mesoproterozoic Belt and Purcell supergroups record nearly 18 km of conformable fluvial and lacustrine deposition within the Nuna supercontinent during the “boring billion”. Sheetfloods entered the basin from all sides along nearly flat surfaces that were episodically drowned by expanding lakes. Sediment packages contain detrital zircon dates of 1.49-1.61 Ga, 1.65-1.80 Ga, and 1.75-1.90 Ga from the west, south, and east respectively. Detrital zircon dates > 2.0 Ga occur proximal to the Medicine Hat and Wyoming Blocks. No erosional unconformities or incised fluvial channels are documented, indicating base level stability for tens of millions of years. Thickness changes suggest steep block faults within the basin, but the angular unconformities and footwall block erosion required for horizontal extension of the upper crust are absent. Assigning these unique attributes to a tectonic regime has proved difficult. By quantifying dissimilarity between 72 cataloged detrital zircon samples we reveal first-order trends in space and time which provide fundamental constraints for viable tectonic models. Multi-dimensional scaling reveals that stratigraphic position alone does not predict detrital zircon signature because detrital zircons are not evenly distributed throughout the basin. Detrital zircon signatures have low internal consistencies and carry less than ~200 km. North American Magmatic Gap dates (1.49-1.61 Ga) are only dominant within 200 km of the Priest River block and remain present throughout deposition. Earliest Paleoproterozoic and Archean dates (> 2.0 Ga) are localized near the Perry and Vulcan Lines. These constraints disagree with prevailing rift models. Deposition occurred at grade in a low-relief landscape without fault block rotation, sudden removal of source terrains, or significant exhumation of the surrounding Archean blocks. In the Belt basin, kilometers of vertical accommodation space were filled in the absence of significant horizontal extension of the upper crust, consistent with the hypothesis that vertical motions dominated the “boring billion”.