『Abstract
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±5 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 Formation 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-temperature
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
nave 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.
Keywords: Proterozoic; Belt-Purcell Supergroup; Chemical Index
of Alteration; River systems; Weathering; Paleoclimate』
1. Introduction
2. Geologic setting
3. Sample datasets and methodology
3.1. Sampling
3.2. Petrography, clay mineralogy analysis
3.3. Post-depositional chemical changes versus weathering
3.4. Chemical Index of Alteration
3.5. K-addition
4. Results
4.1. Mineralogy and sedimentary structures
4.2. Argillites
4.3. Sandstones
5. Discussion
5.1. Continental crust, sedimentary rocks, CIA and flora
5.2. Climate factor dynamics and secular variations
5.3. The weathering index in modern river systems
5.4. The weathering index in Proterozoic river systems
5.5. CIA, Eu/Eu*, Sr, and K/Cs systematics
5.6. Archean-Proterozoic climate and geodynamics
5.7. Paleoclimate during deposition of the BPS
5.8. Sediment residence time and weathering intensity
5.9. Possible caveats
5.10. Mesoproterozoic weathering intensity
6. Conclusions
Acknowledgments
Appendix A. Supplementary data
References