Early Archean Stratigraphy

The lithological record of the Precambrian is notorious for being incomplete. Due to this reason, stratigraphic approaches that are standard in unraveling the Phanerozoic fossil record are of no use in reconstructing the crustal, sedimentary and paleontological evolution of the early part of Earth history. “The oldest” or “the first” does not apply to the Precambrian with its vast gaps in rock successions. More so, especially with respect to the Archean record, metamorphic overprint of the original sedimentary rocks wiped out much of the ancient testimony. Lastly, the Archean worlds were so different from the modern Earth that the actualistic principle is of only to a minor degree of support. Nonetheless, some of the original information of Archean rocks remained and can be deciphered in careful and highly detailed studies. Early Archean rock assemblages include marine siliciclastics, chert and banded iron formation metamorphosed under low- to high-grade conditions. Terrestrial deposits may include fluvial conglomerates, sabkha evaporites and lacustrine material. During the 4.0 to ca. 3.6 Ga Eoarchean Era, Earth had cooled down sufficiently to allow the development of proto-continental crust. However, diversification into proto-continental and proto-oceanic crust was not advanced enough for large-scale plate tectonics that established not earlier than at the base of the Paleoproterozoic. The igneous lithology was characterized by tonalite-trondhjemite-granodiorite (TTG) suites composing an early continental crust, and ultramafic komatiites and tholeiitic basalts composing an early oceanic crust. In the Paleoarchean (3.6-3.2 Ga), proto-contiental crust thickened and stabilized and first small proto-cratons formed by continuous TTG production and intracrustal granitoid magmatism. The early Earth’s atmosphere is generally assumed to have been primarily anoxic (CO2- and CH4-rich), although very low levels of abiotic O2 may have been present. The oldest traces of life include C-isotopes and putative organic molecule associations found in possibly marine, but highly metamorphic sedimentary rocks of the 3.7 Ga Isua Greenstone Belt, Greenland. The somewhat younger ca. 3.5 Ga Paleoarchean rock suites in the Pilbara region of Western Australia and the Barberton Greenstone Belt, South Africa, are less metamorphic overprint, but experienced synsedimentary and diagenetic silicification. However, this has led to the preservation of rare, carbonaceous body fossils of prokaryotes and microbial mat textures. Microbial mats formed stromatolites in carbonate and chert lithologies as well as microbially induced sedimentary structures (MISS) in siliciclastic-evaporite paleosettings. The surprisingly high diversity of body fossils and macroscopic biosedimentary structures show that Palaeoarchaean Earth was already teeming with life. Indeed, field data suggest a close resemblance of the early biosedimentary structures to modern ones. A wide spectrum of geochemical analyses points towards Archean microbial communities colonizing terrestrial and marine environments including Bacteria and Archaea-like organisms. Over the past year, the base of the Archean Eon was ratified at 4031+- 3 Ma; Idiwhaa tonalite, Acasta River, Slave Province, Canada. The base of the Paleoarchean has been voted on and the proposal is in preparation to be submitted after the STRATI 2026 abstract submission deadline.