Beachrock, consisting of carbonate-cemented algal pisoliths and gastropod shells, forms a resistant unit 15 to 25 cm thick along the eastern shore of Ore Lake in southeastern Michigan. The cement occurs as thin crusts of highly acicular, low-magnesium calcite crystals. These crystals are actually aggregates of many small fibrous crystallites, each having the form of an elongate trigonal prism, An orderly arrangement of these crystallites forms a prismatic crystal with many small elongate triangular intercrystallite spaces. Crystals lining large pores consist of crystallites exhibiting either rhombohedral or flat terminations, whereas terminations in small voids are entirely rhombohedral, Scanning electron microscopy shows that in large pores the ends of the flat crystals define smooth curved surfaces around the void margins. These surfaces record the position of a meniscus surrounding air bubbles trapped within the larger pores, demonstrating cementation with the vadose zone. Crystal growth by elongation of the c-axis ceased upon coming in contact with the air phase. Smaller voids were completely filled with fluid, and in these areas uninhibited growth of calcite occurred. The acicular prismatic crystal morphology resembles that found in low-magnesian calcites from speleothems, suggesting that it may be the result of calcite precipitation from extremely low-salinity water. Mg/Ca ratios of beachrock pore waters are low, demonstrating that even though these crystals exhibit an anomalous habit for low-magnesian calcite, concentration of magnesium ions was not responsible for the lateral inhibition of crystal growth, and that "magnesium poisoning" of lateral crystal faces did not play a significant role in the formation of these acicular cements. Ore Lake beachrock resembles its marine counterparts in occurrence, rapidity of cementation, and general fabric, suggesting a similar origin. Lake waters and inflowing meteoric ground waters at Ore Lake are chemically similar and evaporation rates are negligible. Thus, the occurrence of these cements and an observed CO 2 loss from the groundwater favor degassing of groundwater as a general mechanism for beachrock formation, rather than the mixing of meteoric and saline waters or evaporative concentration.

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