Grainstone
“Grain-supported limestone with no mud”
Grainstone is a type of limestone in which sand-sized carbonate grains — ooids, peloids, and shell fragments — are in direct contact with each other, with crystalline calcite cement filling the pores instead of lime mud. This grain-supported texture indicates deposition in high-energy environments where waves and currents winnowed away fine sediment.
calcite grains (ooids, bioclasts, peloids), sparry calcite cement, minor quartz, dolomite (if altered)
Bahamas (modern shoals), Persian Gulf, Great Barrier Reef, Salem Limestone (Indiana, USA), Jurassic limestones (England)
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Grainstones are among the best petroleum reservoir rocks because their lack of mud matrix preserves excellent pore space between grains — many of the world's major oil fields in the Middle East produce from ancient grainstone reservoirs.
Test Your Knowledge
What does the absence of lime mud (micrite) between grains in a grainstone indicate about its depositional environment?
Related Specimens
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