Syndepositional hydrothermalism selectively preserves records of one of the earliest benthic ecosystems, Moodies Group (3.22 Ga), Barberton Greenstone Belt, South Africa

Abstract

The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most non-silica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also – within the resolution of radiometric age data – contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the “just right” degree of heating and very early hydrothermal silicification.

A comment on “Metamorphic origin of anastomosing and wavy laminas overprinting putative microbial deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt)”

“…The BGB, the Moodies Group, and its carbonaceous laminae are complexly metamorphosed (e.g., Byerly et al, 2019;Heubeck, 2019;Reimann et al, 2021); in addition, most outcrops are at least surficially weathered. These conditions do not necessarily erase the multiple lines of mesoscale evidence supporting the synsedimentary and biogenic origin of the laminations, well documented in the literature cited by Saitoh et al (2021).…”

“…A potential driver not mentioned of this heating is the 15 km long and up to 500 m-thick, Moodies-age Lomati River Sill (LRS), stratigraphically about 1 km below the investigated section (Lowe et al 2012); Heubeck and Lowe 1994). Reimann et al (2021) documented the consequences of thermal alteration in the LRS halo, including a well-exposed dike stockwork intruding unconsolidated, in part biolaminated Moodies sands in correlative strata only 2.5 km from the section investigated by Saitoh et al (2021). This stockwork may have its equivalent in a dike network visible on Google Earth images cross-cutting this section.…”

“…The precipitation of sericite from solution into fluid-filled pores is rather rare; much more common is its origin during low-grade metamorphism of clays at around 300°C. Its origin in Moodies sandstones as an alteration product of metastable volcanic rock fragments, shale clasts and sedimentary (pseudo-)matrix is extensively documented (Heubeck and Lowe, 1999;Sleep and Hessler, 2006;Heubeck, 2019;Reimann et al, 2021). Stage 4 shows the unexplained reappearance of the "mud matrix/organic matter (microbial mat?)"…”

Reassessing evidence of Moon–Earth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)

“…Moodies Group strata consist mostly of quartz‐rich sandstones, with subordinate siltstones, conglomerates and shales as well as minor BIF (banded iron formation), jaspilite, and volcanic rocks which were largely deposited in fluvial, shoreline, tidal and deltaic settings (Eriksson, 1977, 1979; Heubeck & Lowe, 1994a, 1994b; Heubeck et al ., 2013, 2016; Homann et al ., 2015; Nabhan et al ., 2016; Heubeck, 2019; Stutenbecker et al ., 2019; Reimann et al ., 2021). Deep‐water, below‐wave base deposits, mostly in prodelta‐facies, are common but poorly exposed; alluvial environments are restricted to the base and the top of the unit.…”

“…There are no indications of major scours or of erosional downcutting. The geometry of the abundant sedimentary structures and thin‐section petrography show that compaction was minor and that carbonate, clay (now sericite) and silica cementation occurred in many places early‐diagenetically (Heubeck & Lowe, 1999; Reimann et al ., 2021).…”