CEMENT LOADING: A CARBONATE RETROSPECTIVE

Marine carbonate cementation involves the reorganization and/or addition of internal carbonate material to an unconsolidated marine carbonate sediment on its way to becoming a limestone. "Cement loading" is a term introduced here to describe a sequence of effects and consequences of progressive local marine cementation from hardly noticeable incipient cementation in unconsolidated oozes to mega-breccia detachment from carbonate bank margins.

In deep water surrounding the Bahama Banks we see the effect of incipient cement altering the acoustic velocity within unconsolidated peri-platform oozes. This results in a fine-parallel seismic sequence produced by alternating beds of soft, bank-derived aragonitic beds from banktop shedding during high sea-level stages and slightly firmer calcitic pelagic beds associated with low sea-levels. Upslope, progressive cementation continues as observed via direct observations from the submersible, "Alvin." Prior to extensive hardground formation, the effects of soft-sediment brecciation, firm-ground and nodule formation can be seen. Under the effect of flow, lithified build-ups of coral encrusted hardgrounds grow into crudely layered mounds called, "lithoherms." Ultimately, large sections of carbonate bank margins break off into deep water as a result of the localized over-weighting of the margin due to the localization and enhancement of cementation there. All this appears as a progression of cement overweighting, or loading, of the original deposit.

These peri-platform cements, down to at least 1000 m., reveal a petrographic para-genetic sequence from loose micrite (paste) to a clotted micro-structure, to distinct peloids with micro- and then macro-crystalline overgrowths ending in macro-crystalline cavity filling.. These Mg-calcite cements need an external source of ions provided by diffusion and/or advection of seawater. Advection is enhanced by pressure gradients set up by bottom water flow over topographic irregularities. This process suggests the coarse bedding seen in the lithoherms may be due to diagenetic as well as sedimentary episodes. Lastly, recent work by others reveals intra-crystalline humates within the micritic cements suggesting an intimate association of organic and inorganic reactions in the cementation process.