Primary features in cherty iron-formations

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A variety of primary textures and structures are recognized within beds of cherty iron-formation. These include granules and oolites, nodules, microbreccias, scour-and-fill structures and ripple marks. Syneresis cracks, contraction cracks and polygonal patterns occur in the oolites, nodules and thin laminated beds. Compaction features suggest the physical nature of the various siliceous sediment components at the time of deposition. Penecontemporaneous deformation features include intraformational breccias and corrugations, flowage folds and flow-cast structures. Many of these features in cherty iron-formations are similar to those found in carbonate beds and fine-grained elastic sediments that passed through a hydroplastic stage. Some internal structures can be used for determining stratigraphic tops of beds. Small folds formed by slumpage and flowage of beds are difficult to distinguish from folds related to major imposed structures, but in contrast their plunge directions and deformation features do not form consistent structural patterns and flow folds are usually isolated between relatively undeformed beds. These primary features leave no doubt that the cherty iron-formations were deposited as chemical precipitates and that most beds passed through a hydroplastic stage as the siliceous gels became dehydrated and compacted. Algoma-type formations associated with volcanic rocks have features indicative of deposition in eugeosynclines, and primary features in Superior-type formations indicate depositional conditions found in neritic zones of continental shelves.