Multiple overgrowths on detrital quartz sand grains in the Shakopee Formation (lower Ordovician) of Minnesota

Abstract A 2.5 ft-thick oolitic dolomite in the Shakopee Formation (Lower Ordovician) exposed near Lanesboro, Minnesota, contains scattered detrital quartz grains with multiple overgrowths. More than 75% of the quartz grains in the bed have multiple overgrowths, and as many as nine overgrowths have been found on a single quartz grain. The overgrowths are of two types, optically continuous or syntaxial and radiating fibrous or microcrystalline. Both commonly are bounded by bipyramidal faces with sharp interfacial angles. Dolomite rhombohedra, which comprise the rest of the rock, abut the detrital quartz grains but appear to "float" in the overgrowths. The multiple euhedral quartz overgrowths in the Shakopee were formed in place and are believed to result from increased porosity developed during dolomitization and from periodic supersaturation of silica-bearing groundwaters. Detrital clay-size material deposited on the euhedral surfaces during periods of stagnation or dryness forms "dust lines" separating the growth stages of the secondary quartz.