Abstract

Archean iron formation (I.F.) typically occurs over 5-15-m intervals within predominantly arenitic sedimentary rocks in the Beardmore-Geraldton greenstone belt. The I.F. consists primarily of dark, laminated magnetite (chemical component) with common intercalations of siltstone/sandstone (clastic component) which range in thickness from sub-millimeters to tens of centimeters. Sedimentary structures in the latter beds, which include graded bedding parallel lamination, and load casts, indicate deposition of clastic strata, together with some magnetitic material, by turbidity currents. Within intervals of alternating magnetite, siltstone, and sandstone beds, the clastic component may coarsen and thicken upward over a scale of meters. Such sequences eventually pass vertically into a 100% clastic succession. These features suggest progradation of submarine-fan, clastic-sediment lobes (with a minor magnetite component) into an environment where essentially iron-rich sediment (the magnetite precursor) was otherwise being deposited as background chemical rainout. Variations in coarseness and thickness of clastic beds in different sections within the I.F. are interpreted as reflecting proximal to distal locations relative to channel-feed systems on the fan. On the outcrop scale, the background (clastic-free) chemical sediment appears as blackish, laminated magnetite. Thin sections, however, reveal that it actually consists of regular sub-millimeter alternations between magnetite and chert. These magnetite-chert microlaminations are analogous to those of the Hamersley Basin in Western Australia (Trendall and Blockley 1970) and similarly may reflect annual varvings.

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