Earth’s first glaciation at 2.9 Ga revealed by triple oxygen isotopes
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Keywords: Archaean, glaciation, triple oxygen isotopes
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Abstract
Figures
 Figure 1 (a) Simplified geological map showing the distribution of the Nsuze and Mozaan groups of the Pongola Supergroup and its location within the Kaapvaal Craton. (b) Stratigraphy of the Mozaan Group of the Pongola Supergroup (Luskin et al., 2019). Distribution of δ18O in shales and diamictites with depth in PNG cores and surface samples from Klipwal Mine area. KM, Klipwal Member. |  Figure 2 Oxygen isotope evolution of shales (Bindeman et al., 2016) and diamictites (Gaschnig et al., 2016) through time, and data from this study. Stippled lines point to Palaeo- and Neoproterozoic as well as Permo-Carboniferous glaciations. |  Figure 3 Triple oxygen isotope diagram showing shale-water fractionation lines connected to various parental meteoric waters (MWL, present day meteoric water line; Surma et al., 2021) that participated in weathering. Note that Delfkom Formation shales and diamictites plot around −18 ± 2.3 ‰ meteoric water and have δ18O values lower than any other shales in the geologic record (data of Bindeman et al., 2018). Curved arrow indicates the effect of subtracting a detrital component in the samples (with crustal δ18O of +6.5) to compute a “pure weathering product” (Table S-3). |
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Introduction
The Pongola Supergroup is the tectonically least disturbed Mesoarchean volcano-sedimentary sequence on Earth (Luskin et al., 2019
Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
). Its deposition started at 3.0 Ga in an epicontinental sea that flooded the Kaapvaal Craton, which had become a stable continent ∼100 Myr earlier, but experienced extension and subsidence (Paprika et al., 2021Paprika, D., Hofmann, A., Agangi, A., Elburg, M., Xie, H., Hartmann, S. (2021) Age of the Dominion-Nsuze Igneous Province, the first intracratonic Igneous Province of the Kaapvaal Craton. Precambrian Research 363, 106335. https://doi.org/10.1016/j.precamres.2021.106335
). The Pongola Supergroup contains the oldest purported glacial deposits (von Brunn and Gold, 1993von Brunn, V., Gold, D.J.C. (1993) Diamictite in the Archean Pongola Sequence of southern Africa. Journal of African Earth Sciences 16, 367–374. https://doi.org/10.1016/0899-5362(93)90056-V
; Young et al., 1998Young, G.M., von Brunn, V., Gold, D.J.C., Minter, W.E.L. (1998) Earth’s Oldest Reported Glaciation: Physical and Chemical Evidence from the Archean Mozaan Group (∼2.9 Ga) of South Africa. The Journal of Geology 106, 523–538. https://doi.org/10.1086/516039
). Mesoarchaean diamictites of possible glacial origin were first described from the time-equivalent West Rand Group of the Witwatersrand Supergroup (Wiebols, 1955Wiebols, J.H. (1955) A suggested glacial origin for the Witwatersrand conglomerates. Transactions of the Geological Society of South Africa 58, 367–382. https://journals.co.za/doi/10.10520/AJA10120750_2097
), although formation modes other than glacial processes have been proposed, such as by cohesive debris flows (Martin et al., 1989Martin, D.M., Stanistreet, I.G., Camden-Smith, P.M. (1989) The interaction between tectonics and mudflow deposits within the main conglomerate formation in the 2.8–2.7 Ga Witwatersrand Basin. Precambrian Research 44, 19–38. https://doi.org/10.1016/0301-9268(89)90074-0
). Despite their importance for the evolution of Earth’s climate through time, a glacial setting for the Pongola diamictites remains a matter of debate. As the oxygen isotopic composition of surface materials is strongly dependent on latitude and climate, we have applied triple oxygen isotope analysis to scrutinise their origin. We report δ18O values of shales and diamictites lower than any siliciclastic deposits analysed so far, supporting glacial conditions during deposition of the Pongola Supergroup.top
Geological Setting
The Archaean Pongola Supergroup is a volcano-sedimentary succession that was deposited between 2.99 and 2.87 Ga on continental crust of the south-eastern part of the Kaapvaal Craton (Fig. 1a; Gumsley et al., 2015
Gumsley, A., Olsson, J., Söderlund, U., de Kock, M., Hofmann, A., Klausen, M. (2015) Precise U-Pb baddeleyite age dating of the Usushwana Complex, southern Africa – Implications for the Mesoarchaean magmatic and sedimentological evolution of the Pongola Supergroup, Kaapvaal Craton. Precambrian Research 267, 174–185. https://doi.org/10.1016/j.precamres.2015.06.010
; Luskin et al., 2019Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
). It is lithologically and stratigraphically similar to the Witwatersrand Supergroup and underlying Dominion Group, and together they form the oldest preserved cratonic cover succession (Beukes and Cairncross, 1991Beukes, N.J., Cairncross, B. (1991) A lithostratigraphic-sedimentological reference profile for the Late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. South African Journal of Geology 94, 44–69. https://journals.co.za/doi/10.10520/AJA10120750_593
; Paprika et al., 2021Paprika, D., Hofmann, A., Agangi, A., Elburg, M., Xie, H., Hartmann, S. (2021) Age of the Dominion-Nsuze Igneous Province, the first intracratonic Igneous Province of the Kaapvaal Craton. Precambrian Research 363, 106335. https://doi.org/10.1016/j.precamres.2021.106335
). Two groups comprise the Pongola Supergroup: the lower Nsuze Group, dominated by volcanic rocks ranging from flood basalt to rhyolite, and the upper Mozaan Group, dominated by shallow marine siliciclastic sedimentary rocks (Luskin et al., 2019Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
). Chemical sedimentary rocks, including banded iron formation (BIF) and stromatolitic carbonates, Mn-rich shales, and palaeosols provide an exceptional record of surface processes in the Mesoarchean and indicate shallow-marine oxygen oases under an anoxic atmosphere (Siahi et al., 2016Siahi, M., Hofmann, A., Hegner, E., Master, S. (2016) Sedimentology and facies analysis of Mesoarchaean carbonate rocks of the Pongola Supergroup, South Africa. Precambrian Research 278, 244–264. https://doi.org/10.1016/j.precamres.2016.03.004
; Eickmann et al., 2018Eickmann, B., Hofmann, A., Wille, M., Bui, T.H., Wing, B.A., Schoenberg, R. (2018) Isotopic evidence for oxygenated Mesoarchaean shallow oceans. Nature Geoscience 11, 133–138. https://doi.org/10.1038/s41561-017-0036-x
; Ossa et al., 2018Ossa, F.O., Hofmann, A., Wille, M., Spangenberg, J.E., Bekker, A., Poulton, S.W., Eickmann, B., Schoenberg, R. (2018) Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea. Earth and Planetary Science Letters 500, 28–40. https://doi.org/10.1016/j.epsl.2018.07.044
; Heard et al., 2021Heard, A.W., Aarons, S.M., Hofmann, A., He, X., Ireland, T., Bekker, A., Qin, L. Dauphas, N. (2021) Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa). Geochimica et Cosmochimica Acta 295, 1–23. https://doi.org/10.1016/j.gca.2020.12.005
).Figure 1 (a) Simplified geological map showing the distribution of the Nsuze and Mozaan groups of the Pongola Supergroup and its location within the Kaapvaal Craton. (b) Stratigraphy of the Mozaan Group of the Pongola Supergroup (Luskin et al., 2019
Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
). Distribution of δ18O in shales and diamictites with depth in PNG cores and surface samples from Klipwal Mine area. KM, Klipwal Member.The Mozaan Group overlies the Nsuze Group disconformably, the latter as young as 2954 ± 8 Ma, the youngest age of the correlative Dominion Group (Paprika et al., 2021
Paprika, D., Hofmann, A., Agangi, A., Elburg, M., Xie, H., Hartmann, S. (2021) Age of the Dominion-Nsuze Igneous Province, the first intracratonic Igneous Province of the Kaapvaal Craton. Precambrian Research 363, 106335. https://doi.org/10.1016/j.precamres.2021.106335
). The Mozaan Group is ∼5 km thick and consists largely of shallow marine sandstone and shale deposited in an intracratonic sea subjected to sea-level oscillations (Beukes and Cairncross, 1991Beukes, N.J., Cairncross, B. (1991) A lithostratigraphic-sedimentological reference profile for the Late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. South African Journal of Geology 94, 44–69. https://journals.co.za/doi/10.10520/AJA10120750_593
). Despite regional metamorphism to lower greenschist facies grade, rocks are extremely well preserved, not requiring the usage of terminology for metamorphic rocks. Following Luskin et al. (2019)Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
and references therein, it is subdivided into ten formations (Fig. 1b). At the base is the Sinqeni Formation, consisting of two fluvial to shallow-marine sandstone units separated by a unit of shale and BIF. The Ntombe Formation is a transgressive sequence of ferruginous shale, siltstone, and sandstone and is overlain by compositionally similar marine highstand deposits of the Thalu Formation. Both formations contain shales enriched in Mn-carbonate, derived diagenetically from Mn(IV)-oxyhydroxides that precipitated in oxygenated surface waters (Ossa et al., 2018Ossa, F.O., Hofmann, A., Wille, M., Spangenberg, J.E., Bekker, A., Poulton, S.W., Eickmann, B., Schoenberg, R. (2018) Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea. Earth and Planetary Science Letters 500, 28–40. https://doi.org/10.1016/j.epsl.2018.07.044
). The Hlashana Formation is dominated by cross-bedded sandstone of a current-dominated shelf sea environment and is overlain by the Delfkom Formation, composed of sandstone, shale, and diamictite, the focus of this study. The youngest concordant detrital zircon date of 2903 ± 14 Ma from a sandstone of this formation provides a maximum depositional age (Zeh and Wilson, 2022Zeh, A., Wilson, A.H. (2022) U-Pb-Hf isotopes and shape parameters of zircon from the Mozaan Group (South Africa) with implications for depositional ages, provenance and Witwatersrand–Pongola Supergroup correlations. Precambrian Research 368, 106500. https://doi.org/10.1016/j.precamres.2021.106500
). For the description of the units higher up in the Mozaan Group stratigraphy, the reader is referred to Luskin et al. (2019)Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
. A dolerite sill dated at 2869 ± 5 Ma provides a minimum age for Mozaan Group deposition (Gumsley et al., 2015Gumsley, A., Olsson, J., Söderlund, U., de Kock, M., Hofmann, A., Klausen, M. (2015) Precise U-Pb baddeleyite age dating of the Usushwana Complex, southern Africa – Implications for the Mesoarchaean magmatic and sedimentological evolution of the Pongola Supergroup, Kaapvaal Craton. Precambrian Research 267, 174–185. https://doi.org/10.1016/j.precamres.2015.06.010
). The Delfkom Formation and its diamictites were thus deposited between 2.90 and 2.87 Ga.top
Sampling and Petrography
Stratigraphically resolved sampling of diamictite and shale from the Delfkom Formation was conducted in the Klipwal Mine area (Fig. 1a; von Brunn and Gold, 1993
von Brunn, V., Gold, D.J.C. (1993) Diamictite in the Archean Pongola Sequence of southern Africa. Journal of African Earth Sciences 16, 367–374. https://doi.org/10.1016/0899-5362(93)90056-V
). At this locality, four diamictite units have been described that are interbedded with ferruginous shale, quartz arenite and conglomerate (Fig. S-1). The most prominent of the diamictite units is the Klipwal Member that has been sampled for this study together with several samples of underlying shale. The shales consist largely of silt-sized detrital quartz, detrital and diagenetic feldspar, euhedral chlorite, disseminated magnetite and some euhedral pyrite and Ti-oxide (Fig. S-2). The composition of diamictite has been described by von Brunn and Gold (1993)von Brunn, V., Gold, D.J.C. (1993) Diamictite in the Archean Pongola Sequence of southern Africa. Journal of African Earth Sciences 16, 367–374. https://doi.org/10.1016/0899-5362(93)90056-V
and Young et al. (1998)Young, G.M., von Brunn, V., Gold, D.J.C., Minter, W.E.L. (1998) Earth’s Oldest Reported Glaciation: Physical and Chemical Evidence from the Archean Mozaan Group (∼2.9 Ga) of South Africa. The Journal of Geology 106, 523–538. https://doi.org/10.1086/516039
, and its matrix is compositionally similar to the shale (Fig. S-3).Shale samples were also obtained from two deep borehole cores (PNG2, PNG3) drilled in 1988 by the AngloGold Exploration Division (Fig. 1a). The boreholes intersected lower stratigraphic units of the Mozaan Group, including the Sinqeni, Ntombe and lower Thalu formations (Figs. 1b, S-4). A limited geochemical dataset of samples from PNG2 have been reported by Ossa et al. (2018)
Ossa, F.O., Hofmann, A., Wille, M., Spangenberg, J.E., Bekker, A., Poulton, S.W., Eickmann, B., Schoenberg, R. (2018) Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea. Earth and Planetary Science Letters 500, 28–40. https://doi.org/10.1016/j.epsl.2018.07.044
. The shales are ferruginous, consisting largely of silt-sized detrital quartz in a predominantly chlorite matrix with total organic carbon contents of <1 wt.%.top
Isotopic Composition
All samples were subjected to oxygen (including Δ’17O) and hydrogen isotope analysis at the University of Oregon (Table S-1) and all data are reported relative to VSMOW. Samples from the Klipwal Mine area were also analysed for major element contents at the University of Johannesburg (Table S-2). Analytical procedures are outlined in the Supplementary Information.
Shale samples from drill core PNG2 (n = 8) cover the Ntombe and lower Thalu formations. The δ18O values are relatively constant throughout the core, averaging 8.1 ± 0.9 ‰. The δD values average −74.8 ± 4.6 ‰, excluding the two stratigraphically uppermost samples that range between −119.9 and −100 ‰.
Drill core PNG3 intersected the Thalu Formation. The δ18O values of shale samples (n = 9) from this core are lighter compared to those of PNG2, averaging 6.6 ± 0.6 ‰. The δD values are relatively constant throughout the core, averaging −56.3 ± 7.2 ‰.
Shale (n = 5) and diamictite (n = 5) samples obtained from fresh outcrop around Klipwal Mine show even lighter δ18O values, with shale averaging 2.9 ± 0.3 ‰ and diamictite averaging 4.7 ± 1.4 ‰. The δD values show some scatter, with shale of the Delfkom Formation averaging −73.2 ± 16.4 ‰ and diamictite averaging −83.1 ± 14.5 ‰. There are moderate positive correlations between δ18O values vs. SiO2 (R2 = 0.42) and K2O (R2 = 0.41). δD values neither correlate with major element contents nor with δ18O values (Fig. S-5).
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Discussion
Diamictites of the Mozaan Group were first described by von Brunn and Gold (1993)
von Brunn, V., Gold, D.J.C. (1993) Diamictite in the Archean Pongola Sequence of southern Africa. Journal of African Earth Sciences 16, 367–374. https://doi.org/10.1016/0899-5362(93)90056-V
. They reported the presence of a highly varied suite of extra-basinal clasts, some of which being striated and faceted, and argued for their emplacement by gravity flows derived from highland glaciers. Young et al. (1998)Young, G.M., von Brunn, V., Gold, D.J.C., Minter, W.E.L. (1998) Earth’s Oldest Reported Glaciation: Physical and Chemical Evidence from the Archean Mozaan Group (∼2.9 Ga) of South Africa. The Journal of Geology 106, 523–538. https://doi.org/10.1086/516039
noted moderate CIA values (Chemical Index of Alteration, 66.8 on average) and high Fe-oxide contents in diamictites and interbedded shales. This was regarded in support of a glacial origin of the diamictites, by analogy with Neoproterozoic glaciogenic diamictites. They further reported the rare presence of dropstones, indicative of clast-charged floating glacier ice at the time of deposition.The oxygen isotope composition of shales (and other mud-dominated rocks such as diamictites) largely reflects weathering conditions, specifically the isotopic composition of the meteoric water during formation of clays at weathering temperature, generally leading to higher than crustal δ18O values, and well above the mantle value of 5.7 ‰ (Bindeman et al., 2016
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
and references therein). Values of δ18O broadly increase through Earth history from ∼10 ‰ in the Archaean to ∼15 ‰ in the Phanerozoic (Fig. 2), linked to the combined effects of crustal differentiation and incorporation of isotopically heavy weathering products into the crust over time. Strong downward shifts of δ18O values by several per mille in the Palaeoproterozoic and Neoproterozoic (Fig. 2) have generally been attributed to glaciations, as meteoric waters in glacial settings have very low δ18O values, and glacial rock flour contains less weathered clastic materials (Nesbitt and Young, 1982Nesbitt, H.W., Young, G.M. (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. https://doi.org/10.1038/299715a0
) lower in δ18O.Figure 2 Oxygen isotope evolution of shales (Bindeman et al., 2016
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
) and diamictites (Gaschnig et al., 2016Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
) through time, and data from this study. Stippled lines point to Palaeo- and Neoproterozoic as well as Permo-Carboniferous glaciations.Average δ18O values for Mesoarchaean (3.2–3.0 Ga) shales have been reported as 9.7 ± 2.4 ‰ (Bindeman et al., 2016
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
). Our values for shale samples from PNG2 from the lower part of the Mozaan Group fall within this average. Stratigraphically upwards, shales from PNG3 show lower values, and this decrease culminates in δ18O values for shales and diamictites of the Delfkom Formation lower than mantle. These data indicate weathering of Thalu and Delfkom formations source materials by meteoric waters with progressively decreasing δ18O values, potentially culminating in weathering by ultra-low δ18O waters. Thus, there is strong evidence for climatic cooling during deposition of the lower Mozaan Group, eventually giving rise to a glacial environment during Delfkom Formation deposition. We note that diamictites of the Mozaan Group and Afrikander Formation of the West Rand studied by Gaschnig et al. (2016)Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
also contain low (<6 ‰) δ18O values overlapping with our data (Fig. 2).Positive correlations of δ18O values with SiO2 and K2O in Delfkom Formation shales and diamictites indicate higher δ18O values for detrital quartz and K-feldspar compared to the matrix. The δD values of our samples are similar to those of average modern shales and crustal fluids (Sheppard and Gilg, 1996
Sheppard, S.M.F., Gilg, H.A. (1996) Stable isotope geochemistry of clay minerals: “The story of sloppy, sticky, lumpy and tough” Cairns-Smith (1971). Clay Minerals 31, 1–24. https://doi.org/10.1180/claymin.1996.031.1.01
). The lack of correlation between δ18O and δD values (Fig. S-5) suggests isotopic re-equilibration with diagenetic and/or metamorphic fluids during recrystallisation of the chlorite precursor material and chlorite itself. No weathering history of the sediment source is thus preserved in the δD values of the Mozaan Group.The CIA values of our samples (Table S-2) are not unlike pre- and post-2.9 Ga shales (Bindeman et al., 2016
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
). They range from 60, translating to ∼20 % clay weathering product and ∼80 % unweathered silicates, to 97, representing almost pure clay. The average CIA value is 69.9 for diamictite and 75.2 for shale. This makes sense as diamictites contain unweathered clasts of quartz and feldspar, as also indicated by slightly higher δ18O values compared to shales. Via mass balance, we can derive the “weathering product” in equilibrium with weathering waters, by subtracting 0–80 % of unweathered siliciclastic detritus with an average δ18O crustal value of +6.5 ‰ (Table S-3). Such a procedure was used by Bindeman et al. (2018)Bindeman, I.N., Zakharov, D.O., Palandri, J., Greber, N.D., Dauphas, N., Retallack, G.J., Hofmann, A., Lackey, J.S., Bekker, A. (2018) Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago. Nature 557, 545–548. https://doi.org/10.1038/s41586-018-0131-1
in their global shale inversion. Then we employed a mathematical inversion of measured δ18O and δ17O values of our samples and computed a weathering product (Fig. 3, Table S-3), by using the modern meteoric water line (MWL) equation δ17O = 0.528 × δ18O + 0.033. As we have two fractionation equations for 1000lnα18Oshale–water, 1000lnα17Oshale–water, and the MWL equation, we are able to obtain solutions for δ18Owater, δ17Owater and temperature (see Table S-3 for further details of these computations). We obtain δ18O values of weathering waters of −18.9 ± 2.3 ‰ (bulk sample) and −20.9 ± 4.8 ‰ (weathering product), values found in polar regions today. Computed temperatures are 37–41 °C.Figure 3 Triple oxygen isotope diagram showing shale-water fractionation lines connected to various parental meteoric waters (MWL, present day meteoric water line; Surma et al., 2021
Surma, J., Assonov, S., Staubwasser, M. (2021) Triple Oxygen Isotope Systematics in the Hydrologic Cycle. Reviews in Mineralogy and Geochemistry 86, 401–428. https://doi.org/10.2138/rmg.2021.86.12
) that participated in weathering. Note that Delfkom Formation shales and diamictites plot around −18 ± 2.3 ‰ meteoric water and have δ18O values lower than any other shales in the geologic record (data of Bindeman et al., 2018Bindeman, I.N., Zakharov, D.O., Palandri, J., Greber, N.D., Dauphas, N., Retallack, G.J., Hofmann, A., Lackey, J.S., Bekker, A. (2018) Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago. Nature 557, 545–548. https://doi.org/10.1038/s41586-018-0131-1
). Curved arrow indicates the effect of subtracting a detrital component in the samples (with crustal δ18O of +6.5) to compute a “pure weathering product” (Table S-3).As Archaean seawater may have been lower in δ18O (e.g., Bindeman, 2021
Bindeman, I.N. (2021) Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments. Reviews in Mineralogy and Geochemistry 86, 241–290. https://doi.org/10.2138/rmg.2021.86.08
), we additionally performed computations assuming Archaean seawater with δ18O of −5 ‰, generating a MWL equation of δ17O = 0.528 × δ18O + 0.078, having higher Δ'17O values per lower δ18O in seawater (e.g., Sengupta and Pack, 2018Sengupta, S., Pack, A. (2018) Triple oxygen isotope mass balance for the Earth’s oceans with application to Archean cherts. Chemical Geology 495, 18–26. https://doi.org/10.1016/j.chemgeo.2018.07.012
). Under this assumption, with a meteoric water line higher in Δ'17O, δ18O values of weathering waters are −26 to −28 ‰ at 6–8 °C.The computed temperatures reflect the conditions during isotopic closure, during the last equilibrium between the weathering waters and sediment, possibly at the time of expulsion of pore waters during compaction, leading to a reduction in porosity and hydrologic impermeability (Bindeman, 2021
Bindeman, I.N. (2021) Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments. Reviews in Mineralogy and Geochemistry 86, 241–290. https://doi.org/10.2138/rmg.2021.86.08
). We consider it was then that most oxygen atoms from the hydrosphere were taken up by the bulk of the silicates and their precursors analysed here. The rocks carry no textural evidence of percolation of post-diagenetic fluids, however the effects of subsequent metamorphism and tectonic uplift, namely dehydration of rocks to potential re-hydration, on bulk isotope values of rocks are considered in the Supplementary Information and produce <1 ‰ shifts in δ18O values but affect δD and [H2O] significantly, potentially explaining “reset” δD values.Assuming no major change between 2.95 and 2.90 Ga in the degree of continentality and elevation of the sedimentary source terrain, the isotopic data are consistent with deposition of the Delfkom Formation under cool climatic conditions with continental weathering involving low δ18O meteoric waters. Palaeomagnetic data constrain the Kaapvaal Craton to mid to high latitudes at the time of deposition of the Pongola Supergroup (de Kock et al., 2021
de Kock, M.O., Luskin, C.R., Djeutchou, C., Wabo, H. (2021) Chapter 12 - The Precambrian drift history and paleogeography of the Kalahari craton. In: Pesonen, L.J., Salminen, J., Elming, S.-Å., Evans, D.A.D., Veikkolainen, T. (Eds.) Ancient Supercontinents and the Paleogeography of Earth. Elsevier, Netherlands, 377–422. https://doi.org/10.1016/B978-0-12-818533-9.00019-9
). Climatic cooling may be linked to drift of the Kaapvaal Craton towards a pole. Alternatively, climatic cooling was a global phenomenon pending verification on different crustal blocks. Stabilisation of the Singhbhum and Kaapvaal cratons at ∼3.1 Ga (Hofmann et al., 2022Hofmann, A., Jodder, J., Xie, H., Bolhar, R., Whitehouse, M., Elburg, M. (2022) The Archaean geological history of the Singhbhum Craton, India – a proposal for a consistent framework of craton evolution. Earth-Science Reviews 228, 103994. https://doi.org/10.1016/j.earscirev.2022.103994
) and the Pilbara Craton at ∼2.9 Ga (Hickman, 2023Hickman, A.H. (2023) Archean Evolution of the Pilbara Craton and Fortescue Basin. Modern Approaches in Solid Earth Sciences, v. 24, Springer Nature, Cham. https://doi.org/10.1007/978-3-031-18007-1
) allowed for subaerial emergence, enhanced continental weathering, and the drawdown of atmospheric CO2, providing suitable conditions for the development of continental glaciers. In addition, muted S-MIF signatures reported from Pongola strata may hint to atmospheric oxidation and destabilisation of greenhouse methane (Ono et al., 2006Ono, S., Beukes, N.J., Rumble, D., Fogel, M.L. (2006) Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. South African Journal of Geology 109, 97–108. https://doi.org/10.2113/gssajg.109.1-2.97
). However, there is no intracratonic record for the 2.90 to 2.87 Ga time interval apart from the Kaapvaal Craton, necessitating a search for glacial deposits in Archaean greenstone successions that formed around that time, but under deeper water conditions. The presence of continental ice sheets may explain well developed sedimentary cyclicity recorded in the Pongola Supergroup (Beukes and Cairncross, 1991Beukes, N.J., Cairncross, B. (1991) A lithostratigraphic-sedimentological reference profile for the Late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. South African Journal of Geology 94, 44–69. https://journals.co.za/doi/10.10520/AJA10120750_593
) due to glacio-eustatic sea-level changes.top
Acknowledgements
AH is supported by the DST-NRF Centres of Excellence CIMERA and Palaeosciences (Grant 86073) and thanks AngloGold-Ashanti for access to drill core samples. INB is supported by US-NSF grant 1822977. We thank Hartwig Frimmel for review. This article is dedicated to the memory of Nic Beukes, who kindly provided the PNG core logs.
Editor: Romain Tartèse
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References
Beukes, N.J., Cairncross, B. (1991) A lithostratigraphic-sedimentological reference profile for the Late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. South African Journal of Geology 94, 44–69. https://journals.co.za/doi/10.10520/AJA10120750_593
Show in context It is lithologically and stratigraphically similar to the Witwatersrand Supergroup and underlying Dominion Group, and together they form the oldest preserved cratonic cover succession (Beukes and Cairncross, 1991; Paprika et al., 2021).
View in article
The Mozaan Group is ∼5 km thick and consists largely of shallow marine sandstone and shale deposited in an intracratonic sea subjected to sea-level oscillations (Beukes and Cairncross, 1991).
View in article
The presence of continental ice sheets may explain well developed sedimentary cyclicity recorded in the Pongola Supergroup (Beukes and Cairncross, 1991) due to glacio-eustatic sea-level changes.
View in article
Bindeman, I.N. (2021) Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments. Reviews in Mineralogy and Geochemistry 86, 241–290. https://doi.org/10.2138/rmg.2021.86.08
Show in context As Archaean seawater may have been lower in δ18O (e.g., Bindeman, 2021), we additionally performed computations assuming Archaean seawater with δ18O of −5 ‰, generating a MWL equation of δ17O = 0.528 × δ18O + 0.078, having higher Δ'17O values per lower δ18O in seawater (e.g., Sengupta and Pack, 2018).
View in article
The computed temperatures reflect the conditions during isotopic closure, during the last equilibrium between the weathering waters and sediment, possibly at the time of expulsion of pore waters during compaction, leading to a reduction in porosity and hydrologic impermeability (Bindeman, 2021).
View in article
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
Show in context The oxygen isotope composition of shales (and other mud-dominated rocks such as diamictites) largely reflects weathering conditions, specifically the isotopic composition of the meteoric water during formation of clays at weathering temperature, generally leading to higher than crustal δ18O values, and well above the mantle value of 5.7 ‰ (Bindeman et al., 2016 and references therein).
View in article
Oxygen isotope evolution of shales (Bindeman et al., 2016) and diamictites (Gaschnig et al., 2016) through time, and data from this study.
View in article
Average δ18O values for Mesoarchaean (3.2–3.0 Ga) shales have been reported as 9.7 ± 2.4 ‰ (Bindeman et al., 2016).
View in article
The CIA values of our samples (Table S-2) are not unlike pre- and post-2.9 Ga shales (Bindeman et al., 2016).
View in article
Bindeman, I.N., Zakharov, D.O., Palandri, J., Greber, N.D., Dauphas, N., Retallack, G.J., Hofmann, A., Lackey, J.S., Bekker, A. (2018) Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago. Nature 557, 545–548. https://doi.org/10.1038/s41586-018-0131-1
Show in context Such a procedure was used by Bindeman et al. (2018) in their global shale inversion.
View in article
Note that Delfkom Formation shales and diamictites plot around −18 ± 2.3 ‰ meteoric water and have δ18O values lower than any other shales in the geologic record (data of Bindeman et al., 2018).
View in article
de Kock, M.O., Luskin, C.R., Djeutchou, C., Wabo, H. (2021) Chapter 12 - The Precambrian drift history and paleogeography of the Kalahari craton. In: Pesonen, L.J., Salminen, J., Elming, S.-Å., Evans, D.A.D., Veikkolainen, T. (Eds.) Ancient Supercontinents and the Paleogeography of Earth. Elsevier, Netherlands, 377–422. https://doi.org/10.1016/B978-0-12-818533-9.00019-9
Show in context Palaeomagnetic data constrain the Kaapvaal Craton to mid to high latitudes at the time of deposition of the Pongola Supergroup (de Kock et al., 2021).
View in article
Eickmann, B., Hofmann, A., Wille, M., Bui, T.H., Wing, B.A., Schoenberg, R. (2018) Isotopic evidence for oxygenated Mesoarchaean shallow oceans. Nature Geoscience 11, 133–138. https://doi.org/10.1038/s41561-017-0036-x
Show in context Chemical sedimentary rocks, including banded iron formation (BIF) and stromatolitic carbonates, Mn-rich shales, and palaeosols provide an exceptional record of surface processes in the Mesoarchean and indicate shallow-marine oxygen oases under an anoxic atmosphere (Siahi et al., 2016; Eickmann et al., 2018; Ossa et al., 2018; Heard et al., 2021).
View in article
Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
Show in context Oxygen isotope evolution of shales (Bindeman et al., 2016) and diamictites (Gaschnig et al., 2016) through time, and data from this study.
View in article
We note that diamictites of the Mozaan Group and Afrikander Formation of the West Rand studied by Gaschnig et al. (2016) also contain low (<6 ‰) δ18O values overlapping with our data (Fig. 2).
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Gumsley, A., Olsson, J., Söderlund, U., de Kock, M., Hofmann, A., Klausen, M. (2015) Precise U-Pb baddeleyite age dating of the Usushwana Complex, southern Africa – Implications for the Mesoarchaean magmatic and sedimentological evolution of the Pongola Supergroup, Kaapvaal Craton. Precambrian Research 267, 174–185. https://doi.org/10.1016/j.precamres.2015.06.010
Show in context The Archaean Pongola Supergroup is a volcano-sedimentary succession that was deposited between 2.99 and 2.87 Ga on continental crust of the south-eastern part of the Kaapvaal Craton (Fig. 1a; Gumsley et al., 2015; Luskin et al., 2019).
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A dolerite sill dated at 2869 ± 5 Ma provides a minimum age for Mozaan Group deposition (Gumsley et al., 2015).
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Heard, A.W., Aarons, S.M., Hofmann, A., He, X., Ireland, T., Bekker, A., Qin, L. Dauphas, N. (2021) Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa). Geochimica et Cosmochimica Acta 295, 1–23. https://doi.org/10.1016/j.gca.2020.12.005
Show in context Chemical sedimentary rocks, including banded iron formation (BIF) and stromatolitic carbonates, Mn-rich shales, and palaeosols provide an exceptional record of surface processes in the Mesoarchean and indicate shallow-marine oxygen oases under an anoxic atmosphere (Siahi et al., 2016; Eickmann et al., 2018; Ossa et al., 2018; Heard et al., 2021).
View in article
Hickman, A.H. (2023) Archean Evolution of the Pilbara Craton and Fortescue Basin. Modern Approaches in Solid Earth Sciences, v. 24, Springer Nature, Cham. https://doi.org/10.1007/978-3-031-18007-1
Show in context Alternatively, climatic cooling was a global phenomenon pending verification on different crustal blocks. Stabilisation of the Singhbhum and Kaapvaal cratons at ∼3.1 Ga (Hofmann et al., 2022) and the Pilbara Craton at ∼2.9 Ga (Hickman, 2023) allowed for subaerial emergence, enhanced continental weathering, and the drawdown of atmospheric CO2, providing suitable conditions for the development of continental glaciers.
View in article
Hofmann, A., Jodder, J., Xie, H., Bolhar, R., Whitehouse, M., Elburg, M. (2022) The Archaean geological history of the Singhbhum Craton, India – a proposal for a consistent framework of craton evolution. Earth-Science Reviews 228, 103994. https://doi.org/10.1016/j.earscirev.2022.103994
Show in context Alternatively, climatic cooling was a global phenomenon pending verification on different crustal blocks. Stabilisation of the Singhbhum and Kaapvaal cratons at ∼3.1 Ga (Hofmann et al., 2022) and the Pilbara Craton at ∼2.9 Ga (Hickman, 2023) allowed for subaerial emergence, enhanced continental weathering, and the drawdown of atmospheric CO2, providing suitable conditions for the development of continental glaciers.
View in article
Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
Show in context The Pongola Supergroup is the tectonically least disturbed Mesoarchean volcano-sedimentary sequence on Earth (Luskin et al., 2019).
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Two groups comprise the Pongola Supergroup: the lower Nsuze Group, dominated by volcanic rocks ranging from flood basalt to rhyolite, and the upper Mozaan Group, dominated by shallow marine siliciclastic sedimentary rocks (Luskin et al., 2019).
View in article
The Archaean Pongola Supergroup is a volcano-sedimentary succession that was deposited between 2.99 and 2.87 Ga on continental crust of the south-eastern part of the Kaapvaal Craton (Fig. 1a; Gumsley et al., 2015; Luskin et al., 2019).
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(b) Stratigraphy of the Mozaan Group of the Pongola Supergroup (Luskin et al., 2019).
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Following Luskin et al. (2019) and references therein, it is subdivided into ten formations (Fig. 1b).
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For the description of the units higher up in the Mozaan Group stratigraphy, the reader is referred to Luskin et al. (2019).
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Martin, D.M., Stanistreet, I.G., Camden-Smith, P.M. (1989) The interaction between tectonics and mudflow deposits within the main conglomerate formation in the 2.8–2.7 Ga Witwatersrand Basin. Precambrian Research 44, 19–38. https://doi.org/10.1016/0301-9268(89)90074-0
Show in context Mesoarchaean diamictites of possible glacial origin were first described from the time-equivalent West Rand Group of the Witwatersrand Supergroup (Wiebols, 1955), although formation modes other than glacial processes have been proposed, such as by cohesive debris flows (Martin et al., 1989).
View in article
Nesbitt, H.W., Young, G.M. (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. https://doi.org/10.1038/299715a0
Show in context Strong downward shifts of δ18O values by several per mille in the Palaeoproterozoic and Neoproterozoic (Fig. 2) have generally been attributed to glaciations, as meteoric waters in glacial settings have very low δ18O values, and glacial rock flour contains less weathered clastic materials (Nesbitt and Young, 1982) lower in δ18O.
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Ono, S., Beukes, N.J., Rumble, D., Fogel, M.L. (2006) Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. South African Journal of Geology 109, 97–108. https://doi.org/10.2113/gssajg.109.1-2.97
Show in context In addition, muted S-MIF signatures reported from Pongola strata may hint to atmospheric oxidation and destabilisation of greenhouse methane (Ono et al., 2006).
View in article
Ossa, F.O., Hofmann, A., Wille, M., Spangenberg, J.E., Bekker, A., Poulton, S.W., Eickmann, B., Schoenberg, R. (2018) Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea. Earth and Planetary Science Letters 500, 28–40. https://doi.org/10.1016/j.epsl.2018.07.044
Show in context Chemical sedimentary rocks, including banded iron formation (BIF) and stromatolitic carbonates, Mn-rich shales, and palaeosols provide an exceptional record of surface processes in the Mesoarchean and indicate shallow-marine oxygen oases under an anoxic atmosphere (Siahi et al., 2016; Eickmann et al., 2018; Ossa et al., 2018; Heard et al., 2021).
View in article
Both formations contain shales enriched in Mn-carbonate, derived diagenetically from Mn(IV)-oxyhydroxides that precipitated in oxygenated surface waters (Ossa et al., 2018).
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A limited geochemical dataset of samples from PNG2 have been reported by Ossa et al. (2018).
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Paprika, D., Hofmann, A., Agangi, A., Elburg, M., Xie, H., Hartmann, S. (2021) Age of the Dominion-Nsuze Igneous Province, the first intracratonic Igneous Province of the Kaapvaal Craton. Precambrian Research 363, 106335. https://doi.org/10.1016/j.precamres.2021.106335
Show in context Its deposition started at 3.0 Ga in an epicontinental sea that flooded the Kaapvaal Craton, which had become a stable continent ∼100 Myr earlier, but experienced extension and subsidence (Paprika et al., 2021).
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It is lithologically and stratigraphically similar to the Witwatersrand Supergroup and underlying Dominion Group, and together they form the oldest preserved cratonic cover succession (Beukes and Cairncross, 1991; Paprika et al., 2021).
View in article
The Mozaan Group overlies the Nsuze Group disconformably, the latter as young as 2954 ± 8 Ma, the youngest age of the correlative Dominion Group (Paprika et al., 2021).
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Sengupta, S., Pack, A. (2018) Triple oxygen isotope mass balance for the Earth’s oceans with application to Archean cherts. Chemical Geology 495, 18–26. https://doi.org/10.1016/j.chemgeo.2018.07.012
Show in context As Archaean seawater may have been lower in δ18O (e.g., Bindeman, 2021), we additionally performed computations assuming Archaean seawater with δ18O of −5 ‰, generating a MWL equation of δ17O = 0.528 × δ18O + 0.078, having higher Δ'17O values per lower δ18O in seawater (e.g., Sengupta and Pack, 2018).
View in article
Sheppard, S.M.F., Gilg, H.A. (1996) Stable isotope geochemistry of clay minerals: “The story of sloppy, sticky, lumpy and tough” Cairns-Smith (1971). Clay Minerals 31, 1–24. https://doi.org/10.1180/claymin.1996.031.1.01
Show in context The δD values of our samples are similar to those of average modern shales and crustal fluids (Sheppard and Gilg, 1996).
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Siahi, M., Hofmann, A., Hegner, E., Master, S. (2016) Sedimentology and facies analysis of Mesoarchaean carbonate rocks of the Pongola Supergroup, South Africa. Precambrian Research 278, 244–264. https://doi.org/10.1016/j.precamres.2016.03.004
Show in context Chemical sedimentary rocks, including banded iron formation (BIF) and stromatolitic carbonates, Mn-rich shales, and palaeosols provide an exceptional record of surface processes in the Mesoarchean and indicate shallow-marine oxygen oases under an anoxic atmosphere (Siahi et al., 2016; Eickmann et al., 2018; Ossa et al., 2018; Heard et al., 2021).
View in article
Surma, J., Assonov, S., Staubwasser, M. (2021) Triple Oxygen Isotope Systematics in the Hydrologic Cycle. Reviews in Mineralogy and Geochemistry 86, 401–428. https://doi.org/10.2138/rmg.2021.86.12
Show in context Triple oxygen isotope diagram showing shale-water fractionation lines connected to various parental meteoric waters (MWL, present day meteoric water line; Surma et al., 2021) that participated in weathering.
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von Brunn, V., Gold, D.J.C. (1993) Diamictite in the Archean Pongola Sequence of southern Africa. Journal of African Earth Sciences 16, 367–374. https://doi.org/10.1016/0899-5362(93)90056-V
Show in context The Pongola Supergroup contains the oldest purported glacial deposits (von Brunn and Gold, 1993; Young et al., 1998).
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Stratigraphically resolved sampling of diamictite and shale from the Delfkom Formation was conducted in the Klipwal Mine area (Fig. 1a; von Brunn and Gold, 1993).
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The composition of diamictite has been described by von Brunn and Gold (1993) and Young et al. (1998), and its matrix is compositionally similar to the shale (Fig. S-3).
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Diamictites of the Mozaan Group were first described by von Brunn and Gold (1993).
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Wiebols, J.H. (1955) A suggested glacial origin for the Witwatersrand conglomerates. Transactions of the Geological Society of South Africa 58, 367–382. https://journals.co.za/doi/10.10520/AJA10120750_2097
Show in context Mesoarchaean diamictites of possible glacial origin were first described from the time-equivalent West Rand Group of the Witwatersrand Supergroup (Wiebols, 1955), although formation modes other than glacial processes have been proposed, such as by cohesive debris flows (Martin et al., 1989).
View in article
Young, G.M., von Brunn, V., Gold, D.J.C., Minter, W.E.L. (1998) Earth’s Oldest Reported Glaciation: Physical and Chemical Evidence from the Archean Mozaan Group (∼2.9 Ga) of South Africa. The Journal of Geology 106, 523–538. https://doi.org/10.1086/516039
Show in context The Pongola Supergroup contains the oldest purported glacial deposits (von Brunn and Gold, 1993; Young et al., 1998).
View in article
The composition of diamictite has been described by von Brunn and Gold (1993) and Young et al. (1998), and its matrix is compositionally similar to the shale (Fig. S-3).
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They reported the presence of a highly varied suite of extra-basinal clasts, some of which being striated and faceted, and argued for their emplacement by gravity flows derived from highland glaciers. Young et al. (1998) noted moderate CIA values (Chemical Index of Alteration, 66.8 on average) and high Fe-oxide contents in diamictites and interbedded shales.
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Zeh, A., Wilson, A.H. (2022) U-Pb-Hf isotopes and shape parameters of zircon from the Mozaan Group (South Africa) with implications for depositional ages, provenance and Witwatersrand–Pongola Supergroup correlations. Precambrian Research 368, 106500. https://doi.org/10.1016/j.precamres.2021.106500
Show in context The youngest concordant detrital zircon date of 2903 ± 14 Ma from a sandstone of this formation provides a maximum depositional age (Zeh and Wilson, 2022).
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Supplementary Information
The Supplementary Information includes:
Download Tables S-1 to S-3 (xlsx)
Download the Supplementary Information (PDF)
Figures
Figure 1 (a) Simplified geological map showing the distribution of the Nsuze and Mozaan groups of the Pongola Supergroup and its location within the Kaapvaal Craton. (b) Stratigraphy of the Mozaan Group of the Pongola Supergroup (Luskin et al., 2019
Luskin, C., Wilson, A., Gold, D., Hofmann, A. (2019) The Pongola Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton Stabilization. In: Kröner, A., Hofmann, A. (Eds.) The Archaean Geology of the Kaapvaal Craton, Southern Africa. Springer Nature, Cham, 225–254. https://doi.org/10.1007/978-3-319-78652-0_9
). Distribution of δ18O in shales and diamictites with depth in PNG cores and surface samples from Klipwal Mine area. KM, Klipwal Member.
Figure 2 Oxygen isotope evolution of shales (Bindeman et al., 2016
Bindeman, I.N., Bekker, A., Zakharov, D.O. (2016) Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Earth and Planetary Science Letters 437, 101–113. https://doi.org/10.1016/j.epsl.2015.12.029
) and diamictites (Gaschnig et al., 2016Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., Beck, M.L. (2016) Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochimica et Cosmochimica Acta 186, 316–343. https://doi.org/10.1016/j.gca.2016.03.020
) through time, and data from this study. Stippled lines point to Palaeo- and Neoproterozoic as well as Permo-Carboniferous glaciations.
Figure 3 Triple oxygen isotope diagram showing shale-water fractionation lines connected to various parental meteoric waters (MWL, present day meteoric water line; Surma et al., 2021
Surma, J., Assonov, S., Staubwasser, M. (2021) Triple Oxygen Isotope Systematics in the Hydrologic Cycle. Reviews in Mineralogy and Geochemistry 86, 401–428. https://doi.org/10.2138/rmg.2021.86.12
) that participated in weathering. Note that Delfkom Formation shales and diamictites plot around −18 ± 2.3 ‰ meteoric water and have δ18O values lower than any other shales in the geologic record (data of Bindeman et al., 2018Bindeman, I.N., Zakharov, D.O., Palandri, J., Greber, N.D., Dauphas, N., Retallack, G.J., Hofmann, A., Lackey, J.S., Bekker, A. (2018) Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago. Nature 557, 545–548. https://doi.org/10.1038/s41586-018-0131-1
). Curved arrow indicates the effect of subtracting a detrital component in the samples (with crustal δ18O of +6.5) to compute a “pure weathering product” (Table S-3).