Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

Abstract

The Mendon Formation is the uppermost unit of the 3.5-3. there is an upward-deepening of the overall depositional setting recorded in the oldest upper Mendon sections, consistent with the previous interpretation that Mendon time was characterized by rifting (Lowe, 1994a;Lowe, 1999a). Younger Mendon cycles include thick, relatively ferruginous basal sections, interpreted to reflect the deepest water deposition. These sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms, reflecting gradual shoaling.This sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost Onverwacht time, before the distinct change in tectonic regime marked by impact spherule layer S2 and the onset of Fig Tree Group orogenesis and related siliciclastic deposition.

Reply to annoymous

“…≥ 100-200 m depth, marked by negligible volcanic interference (e.g., Tice and Lowe, 2006;Trower and Lowe, 2016;Konhauser et al, 2017). This interpretation is supported by the observed enrichment of Fe in the MFIF; a characteristic of relatively deeper water lithofacies (Trower and Lowe, 2016;Konhauser et al, 2017). This view is compartible with the proposition that hematite enrichment in the MFIF was under the control of photoferrotrophic biofilms (Chi Fru et al, 2013) known to thrive at lower light intensities (Kappler et al, 2005;Li et al, 2013;Konhauser et al, 2017).…”

“…The fine-grained, finely laminated nature of the MFIF, and, the lack of evidence of current or wave structures (e.g., symmetric ripples or hummocky cross-stratification), coupled to the absence of volcanogenic detrital particles and intraclast breccia structures, indicate a low energy sedimentation environment at ca. ≥ 100-200 m depth, marked by negligible volcanic interference (e.g., Tice and Lowe, 2006;Trower and Lowe, 2016;Konhauser et al, 2017). This interpretation is supported by the observed enrichment of Fe in the MFIF; a characteristic of relatively deeper water lithofacies (Trower and Lowe, 2016;Konhauser et al, 2017).…”

“…(Lowe, 1982;Mutti 1992;Talling et al, 2012;Orme and Laskowski, 2016;Shanmugam, 2016;Wang et al, 2017). Massive conglomerates containing both allochthonous sandstone clasts and intraformational andesite-dacite are interpreted as channelized submarine debris flows or slump deposits sourced from adjacent topographic highs (Lowe, 1982;Stewart and McPhie, 2006;Orme and Laskowski, 2016 Papanikolaou et al, 1990;Steele et al, 2000;Trower and Lowe, 2016;Wang et al, 2017).…”

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

“…About 100 m of the uppermost Mendon Formation was recovered. The Mendon Formation was deposited between 3.34 and 3.26 Gyr and has been studied in detail (Busigny et al, ; Galić et al, ; Hofmann & Bolhar, ; Lowe & Byerly, ; Montinaro et al, ; Philippot et al, ; Philippot, Zuilen, & Rollion‐Bard, ; Tice et al, ; Trower & Lowe, ). The sequence consists of five volcanic cycles, termed Mv1 through Mv5, which are each capped by thin chert sequences, termed Mc1 through Mc5 (Byerly et al, ).…”

“…The base of the Mc5 sequence was dated at 3.28 Gyr, and the top is marked by the S2 spherule layer that was deposited at 3.26 Gyr (Decker et al, ). Mc5 corresponds to a wide variety of chert, with mostly black chert and ferruginous gray chert, and minor amounts of black and white banded chert, banded ferruginous chert, intraclast breccia and silicified green colored ash and accretionary lapilli (Trower & Lowe, ). The Mendon Formation was deposited during a period of local volcanic quiescence with slow sedimentation rates under quiet water settings (Lowe & Byerly, ; Trower & Lowe, ).…”