Alternating bands of hard and soft limestone in a 14-ft (4 m)-thick lens at the base of the marine Waikawau Sandstone (Lower Miocene), West Auckland, are clearly secondary in origin since in places they cut across primary sedimentary structures such as current bedding. The only significant differences between the two lithologies are the calcium carbonate content, the development of glauconite, and the packing of bioclasts. It is inferred that segregation of calcium carbonate from an initially homogeneous, sandy biocalcarenite (62% CaCO3), to form hard bands with an average of 75% CaCO3and soft bands with an average of 42% CaCO3, took place in early diagenesis, penecontemporaneously with the development of glauconite. No other elements appear to have migrated with the calcium carbonate. Calcite was deposited as spar cement in hard bands, whereas calcareous bioclasts in soft bands were partially dissolved and pore space was drastically reduced. Segregation probably occurred because the pH in the interstitial fluids fluctuated above and below 7.8, in a mildly reducing environment. It is inferred that the differential cementation occurred no more than 6 ft (2 m) below the sea bed.