Ferruginous banding in Sydney sandstone

Abstract

Spectacular ferruginous (goethite-rich) banding is a common feature of Triassic sandstone in the Sydney Basin, southeast Australia. The banding is of two broad types: (1) intersecting sets (implying water composition in which goethite is stable) and (2) trough-like sets that truncate and obliterate previously formed banding (implying water composition destabilising goethite, probably owing to increased acidity and/or reductive power). The following two mechanisms could be responsible for the formation of banding during or after permeation of sandstone by water. The pre-nucleation (Ostwald) model involves a supersaturation–nucleation–depletion cycle, during which meteoric water reacts with siderite cement and takes iron into solution, until becoming sufficiently supersaturated to precipitate a goethite-rich band; then the water permeates further into the sandstone, dissolving more iron until the supersaturation is large enough for another episode of precipitation. The post-nucleation (self-organisation) model involves the development of bands from randomly dispersed nuclei, small particles dissolving and large particles growing, to minimise the total surface free energy. The trough-type of banding cannot be explained by either of these processes, because ferruginous bands are inferred to be dissolved, not deposited, during advance of the water. An alternative mechanism is the evaporation–precipitation model, which involves precipitation caused by evaporation of retreating water, rather than precipitation from advancing water. The festoon patterns of trough-type band sets resemble strand line patterns in sand left by backwash and could be the result of spasmodic reduction in the volume of rainwater. The typical iron-rich margins of the troughs could reflect evaporation during prolonged stagnation, but transitions from iron-rich borders to transgressive veins, as well as later-thickened borders, suggest continued penetration of iron-rich groundwater after formation of the banding. KEY POINTS Some bands intersect (implying water composition in which goethite is stable). Trough-type bands obliterate previously formed banding (implying water composition destabilising goethite, owing to increased acidity and/or reductive power). Possible banding processes are the pre-nucleation (Ostwald) model and the post-nucleation (self-organisation) model, each with some problems. Banding (especially trough-type) can also be formed by evaporation–precipitation during water retreat.