THE ROLE OF OCEAN TIDES IN RAPID DIAGENESIS OF SHALLOW MARINE CARBONATES

Although generally recognized as a major player in the movement of nutrients onto and off of shallow shelves and platforms, tidal controls on pressure and chemical variations in the surficial sediment layer of carbonate banks have been largely ignored. However, this surficial sediment layer, where much of carbonate diagenesis takes place following deposition of shallow marine carbonates, experiences significant variations in pressure and chemistry on a semidiurnal basis as the tides rise and fall in the overlying water column.

Where the sediment is highly permeable, the water depth variation is accompanied only by a minor fluctuation in pore pressure that reflects the variable depth of the overlying water column, with very little variation in pore water chemistry. In contrast, where the sediment has a low permeability, the pore pressure lags relative to the tidal height, creating temporary high matrix stresses during tidal rise and over-pressured pore fluids during tidal fall. The higher matrix stresses cause localized pressure solution and higher carbonate concentrations in the pore fluids leading to grain overgrowths and cementation, resulting in the rapid stabilization of carbonate sediments. The over-pressured pore fluids are partially exhaled into the overlying water column, enabling water exchange with the adjoining sea water that provides a periodically cyclic environment in which the production of new and otherwise difficult-to-nucleate minerals, such as dolomite, is possible. Thus, rapid diagenesis of shallow marine carbonates may be largely a function of the tidally influenced environment in which they are deposited.