Rapidity of freshwater calcite cementation—implications for carbonate diagenesis and sequence stratigraphy

Abstract

In less than 10 years, subaerially-exposed Holocene oolitic sand on Eleuthera Island, Bahamas was completely case-hardened by freshwater vadose calcite cementation. This phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains, in part, why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed. It further proves, contrary to what the Exxon conceptual sequence stratigraphic models imply, that carbonate deposits do, in fact, respond differently to subaerial exposure than siliciclastic deposits. Case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early, near-surface freshwater diagenesis helps explain why shallow-water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands. During lowstands, rapid subaerial lithification of carbonate sediments retards their reworking by wind or water. Once lithified, these deposits are further stabilized by vegetation and karstification, inhibiting their transport as unconsolidated sediment into adjacent deeper-water basins. Observations from Eleuthera Island also document the reciprocal processes associated with freshwater carbonate diagenesis. Calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore-filling, low-magnesian calcite cement. This reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10–20 cm of the subaerially-exposed body of oolitic sand. Below this case-hardened surface, oolitic sands remained unconsolidated. Only very minor amounts of calcite cement were required for effective casae hardening.