Setting and origin for problematic rocks from the >3.7 Ga Isua Greenstone Belt, southern west Greenland: Earth’s oldest coarse clastic sediments

Abstract

Whether or not coarse detrital sedimentary rocks occur within the >3.7 Ga Isua Greenstone Belt (IGB), southern west Greenland, has been debated for some time. Repeated, regional metamorphic, deformational, and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition. Rocks here interpreted as meta-conglomerate crop out in a fault-bounded structural domain that is lower in strain relative to adjacent domains. The meta-conglomerate has a strike length of ∼1 km and is ∼10 m thick. Bed thickness ranges from 10 cm to more than 1 m, and beds may be either framework- or matrix-supported. A poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta-chert, BIF, and a variety of mafic volcanic rock fragments are set into a matrix of biotite+quartz+garnet schist; clast compositions indicate reworking of adjacent stratigraphic units. A lack of structural similarity in framework clasts, range of grain sizes, range of rounding, and polymict composition demand a primary sedimentary origin for the deposit. Inferred depositional processes include traction and debris flow, which would be consistent with subaerial or shallow subaqueous environments, although the limited extent of the meta-conglomerate warrants interpretational caution.