Cyclic tidalites and seismites at a submarine hydrothermal system for a 2450 Ma banded iron formation, Hamersley Basin, Western Australia

Abstract The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.