Abstract

Great Salt Lake ooids consist of a nucleus grain coated by layers of aragonite nannograins and minor impurities. Most nannograins are elongate parallel to their c-axis and individual layers are composed of grains which are tangentially, normally (radially) or randomly oriented with respect to the ooid surface. This orientation may be determined petrographically or with a scanning electron microscope. Evidence from the Great Salt Lake and other ooid-forming environments that relates ooid-forming processes to ooid structure generally falls into three categories: 1) randomly oriented aragonite and organic influences; 2) tangentially oriented aragonite and the role of agitation; and 3) radially oriented aragonite and "normal" crystal growth. Living microorganisms play a destructive, not constructive role in ooid growth and are not present in most Great Salt Lake ooids. Organic matter in both marine and Great Salt Lake ooids may play an important role as an adhesive during cortex accretion. Although the general physical conditions of ooid formation are known, detailed knowledge of the mechanism of ooid growth is lacking. Few hypotheses account for the tangential orientation of nannograins in the ooid cortex and those that do evoke a modification of Sorby's "snowball" hypothesis. Some "snowballing" of Great Salt Lake ooids is indicated by clays in the ooids, which match the detrital clays in the lake. Radial grains in Great Salt Lake ooids range in length from 1 to 100 microns. They display petrographic relationships that are ambiguous relative to their origin, but a magnesium-rich zone around large radial grains suggests these result from an early reorganization of a precursory material. Large radial grains are interpreted as secondary features, albeit syndepositional. These grains are the result of recrystallization only if the postulated earlier precipitate was calcium carbonate. Ooids with large-scale radial fabric, like those of the Great Salt Lake, are characteristic of hypersaline and freshwater environments. The radial fabric renders Great Salt Lake ooids weaker than tangential Bahamian-type ooids of the same size and shape. This produces broken ooids in Great Salt Lake ooid sands. Syndepositionally broken ooids are associated with several evaporitic ancient oolites but are very rare in normal marine oolites. Broken ooids are considered significant indicators of unusual salinity if they comprise more than 1% of the grains in an oolite.

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