Abstract

Neoarchean deltaic deposits present in Superior Province, Canada, contain an excellent record of shallow-water siliciclastic and iron formation (IF) deposition. The deltas were fed by braided fluvial systems, which commonly resulted in the development of small, active lobes that repeatedly cycled through progradation and abandonment, although a thick progradational deltaic assemblage was also investigated. Magnetite-rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore-proximal fluvial channels and distributary-mouth bars of the braid-delta deposits. Magnetite laminae are also interbedded with some distributary-mouth sediments draping reactivation surfaces in ripples to barforms. Thicker accumulations of IF (up to 73 m) form portions of transgressive systems tracts where chemical sedimentation dominated. Offshore equivalents of these successions do not contain chemical sediments. Sedimentologic data imply that IF deposition in this well-mixed, nearshore water mass was not compatible with slow rates of constant iron-compound precipitation in the world ocean combined with siliciclastic sediment starvation. The model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore, stimulating photosynthetically induced oxidation of Fe2+. Under these conditions, elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts.

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