Earth's oldest preserved unconformity — Prospect of a beginning in the tectono-sedimentary continental cycle?

Abstract

Earth's oldest preserved conglomerates and basaltic pillow lavas at Isua, Greenland, provide robust field evidence for deep- and shallow-water environments on our planet within the first billion years of its formation. The conglomerates represent the first Paleoarchaean archive of sub-aerial erosion and shallow water sedimentation. These ca. 3700million year old sedimentary rocks, now metamorphosed at amphibolite grade, comprise units of rounded quartz pebbles set in a sandy to muddy matrix that unconformably overlie an ophiolite sequence. The pillow lavas of the ophiolite are variolitic and essentially vesicle-free, indicative of formation in a deep water environment. Locally, an unconformity separates the conglomerates from the deformed ophiolite-related rocks; elsewhere the contact between these units is tectonic. Such field relationship between deep and shallow water environments resemble those preserved in younger orogenic belts, where obducted oceanic crust has been tectonically emplaced across terrestrial platforms or subaerial parts of forearc and backarc basins.