Setting and origin for problematic rocks from the > 3.7 Ga Isua GreenstoneBelt, southern west Greenland: Earth's oldest coarse elastic sediments

Whether or not coarse detrital sedimentary rocks occur within the > 3.7 Ga Isua Greenstone Belt (IGB), southern west Greenland, has been debated for s ome time. Repeated, regional metamorphic: deformational, and metasomatic ev ents have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition. Rocks here interpret ed 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 similar to 1 km and is similar to 10 m thick. Bed thickn ess ranges from 10 cm to more than 1 m, and beds may be either framework- o r matrix-supported. A. poorly sorted and variably rounded polymict assembla ge of framework clasts consisting of meta-chert, BIF, and a variety of mafi c volcanic rock fragments are set into a matrix of biotite + quartz + garne t schist; clast compositions indicate reworking of adjacent stratigraphic u nits. A lack of structural similarity in framework clasts, range of grain s izes, range of rounding, and polymict composition demand a primary sediment ary origin for the deposit. Inferred depositional processes include tractio n and debris how, which would be consistent with subaerial or shallow subaq ueous environments, although the limited extent of the meta-conglomerate wa rrants interpretational caution. (C) 2000 Elsevier Science B.V. All rights reserved.