Weathering intensity in the Mesoproterozoic and modern large-river systems: A comparative study in the Belt-Purcell Supergroup, Canada and USA

摘要

The Mesoproterozoic Belt-Purcell Supergroup (BPS) preserves a thickness of 17. km of dominantly siliciclastic rocks deposited between 1470 and 1400. Ma. The total range of Chemical Index of Alteration (CIA) values, corrected for diagenetic K-addition, are 62-88 for argillites and 55-80 for sandstones, with averages of 72 ± 6 and 68 ± 7, respectively. More intense CIA values, in conjunction with low absolute contents of Sr, Ca and Na, and high Rb/Sr ratios (average 4), reflect an intensely weathered provenance in a hot-wet climate with hot-arid intervals resulting in evaporitic sediments. Covariations of CIA-Eu/Eu* and Sr-Eu/Eu* are consistent with a large catchment area including extensive provenance terranes of weathered recycled sedimentary rocks for the most extreme CIA and Eu/Eu* values, with smaller less intensely weathered juvenile terranes represented by lower CIA values. Accordingly, variations of CIA within the BPS stratigraphic sequence may record some combination of shifting catchment terranes and weathering intensity. Stratigraphic trends in CIA within the Appekunny and Grinnell Formations of 80 ± 6 to 66 ± 4 to 79 ± 5 record the variation in this combination with time. Siliciclastic rocks record a first order trend of CIA values from CIA ～80-100 in the Mesoarchean, through ～80-90 in the Neoarchean, and ～70-85 in the Proterozoic, to ～72 for global Phanerozoic shales. These values reflect progressive drawdown of greenhouse gases that promote silicate weathering by their sequestration into carbonates and black shales, as preserved in the geologic record. Second order secular peaks in CIA values correlate in time with mantle plumes that emit greenhouse gases, which enhance silicate weathering. Some of the more intense CIA values in the BPS may also stem from release of volcanic gases during magmatism that accompanied rifting of Laurentia during breakup of the Supercontinent Columbia at ～1.4. Ga. Overall, CIA values are within the range of modern humid-temperate and humid-tropical climatic catchment areas drained by large river systems such as the Orinoco, Nile and Amazon rivers. Proterozoic rivers have been viewed as mostly braided systems due to the lack of influence of rooted vegetation, which resulted in fast channel lateral migration, high run-off rates, and low bank stability. Many large-scale Proterozoic siliciclastic basins have been preserved, formed by river systems up to pan-continental scale. However, their significance as archives of continental weathering intensity remains under-explored. This study suggests that BPS CIA values reflect more aggressive chemical weathering, since Proterozoic rivers had less sediment residence times due to a lack of vegetation cover, and therefore faster transport time than their modern counterparts. To achieve high CIA values in shorter periods of time without vegetation cover, more intense chemical weathering conditions must have been present.