Depositional Environments and Provenance of Metasedimentary Rocks in the Jack Hills Belt, Western Australia

Metasedimentary rocks in the Jack Hills Belt, Western Australia host Earth's oldest terrestrial minerals - zircons up to 4.4 Ga in age. Minimum ages of zircons in most of the Jack Hills Belt are approximately 3.05 Ga in age providing a maximum age for deposition of the sedimentary protoliths. Zircons as young as 1.6 Ga have been identified locally in metasedimentary rocks at Jack Hills. These rocks may be tectonically interleaved with Archean rocks or, less likely, the complete metasedimentary succession may be of Proterozoic age.

Zircons at Jack Hills of age > 3.05 Ga are hosted in conglomerates and associated sandstones and have been the subject of numerous geochemical and geochronological studies. None of the studies to-date has been concerned with understanding the depositional environments of these metasedimentary rocks. An intact, 320-m thick stratigraphic section that includes conglomerates and sandstones equivalent to those at the W74 discovery site of ancient zircons is interpreted to record overall progradation of the depositional environment. Facies associations are compatible with a fan delta depositional setting in which alluvial fans were sourced by proximal highlands, built directly into a lake or sea and were characterized by high gradients and high flow velocities. Fine-grained facies at the base of the section are interpreted as prodelta-delta front deposits. Overlying facies associations represent distal and more proximal alluvial fan deposits of predominantly sheet-flow origin. The succession is capped by a cross-bedded quartz arenite-gritstone unit interpreted to record transgressive tidal reworking of the fan delta. Paleocurrent modes in this facies association are northwest and southeast and may provide some constraints on the location of the provenance. A dominant mode to the southeast may indicate that sediment was derived from the northwest.