The isua supracrustal belt of the North Atlantic Craton (Greenland): Spotlight on sedimentary systems with the oldest preserved sedimentary structures (~3.7, ~3.75, and ~3.8 Ga)
The folded, amphibolite facies Isua supracrustal belt of the North Atlantic Craton (Greenland) contains rare low-strain lacunae that display the world's oldest sedimentary structures in dolomitic carbonates, banded iron formations, volcaniclastic sedimentary rocks, and very rare sandstones and conglomerates. U-Pb zircon geochronology shows that Isua sedimentary rocks were deposited as two unrelated assemblages of (1) ∼3810-3800 plus ∼3750 Ma rocks and (2) 3710-3695 Ma rocks that were tectonically juxtaposed in the Eoarchean. Felsic-mafic volcaniclastic sedimentary rocks are chemically immature, and were derived from volcanic sources, not from significantly older continental regions. An inferred ∼3700 Ma unconformity is marked by a basal conglomeratic lag deposit on underlying 3710-3720 Ma altered (subaerially weathered?) volcanic rocks and is succeeded by ∼3695 Ma chemical sedimentary rocks. Holistic appraisal of the Isua sedimentary units within the context of their associated volcanic sequences indicates they formed over a 100-million-year period in suprasubduction zone settings.