Marine Vadose Beachrock Cementation by Cryptocrystalline Magnesian Calcite--Maui, Hawaii

Abstract

ABSTRACT Active precipitation of high-magnesian cryptocrystalline calcite cement (15.5 mole % MgCO3) in the intertidal zone on the north shore of Maui, Hawaii, results in formation of an extensive beachrock exposure at Hookipa Park. Framework grains in the beachrock are coarse algal, molluscan, and intraclast-bearing sands. Meniscus fabric is dominant in the cement, indicating that most lithification occurs in the vadose zone. The cryptocrystalline cement is generally brownish gold in color, but very thin, clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement, especially in upper horizons of the beachrock. Peloidal texture is scarce in the cement but is present in a few samples. Silt and fine-sand-sized skeletal fragments have been trapped in he evolving cement, resulting in cement-supported fabric. Hollow rims of microcrystalline calcite occupy some pore spaces. These hollow rims may have precipitated at the air-water interface of bubbles that formed in pores, and they may therefore be additional evidence of cementation in the vadose zone. Interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of CO2 degassing in the formation of beachrock cement. When chemical and tidal cycles fluctuate with the same period, PCO2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand. Increasing values of PCO2 most often occur during semi-daily rising tides and decreasing values of PCO2 occur during falling tides. Saturation ratios with respect to calcite vary in direct proportion to PCO2. Saturation levels generally increase during rising tides and decrease during falling tides. T ese data suggest that CO2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach.