DOLOMITIZATION BY TIDAL-PRESSURE-DRIVEN CHEMICAL CYCLING IN ORGANIC-RICH CARBONATE SEDIMENTS

Strata-bound dolomitization must occur by alteration of initial calcium carbonate sediments during the earliest stages of diagenesis. At ambient surface temperatures, this requires recrystallization of the soft sediments in conjunction with a mechanism for the nucleation and growth of ordered dolomite crystals without an intermediate disordered dolomite phase. This, in turn, requires a mechanism that allows both dissolution of the existing calcium carbonates and deposition of alternating calcite and magnesite layers through chemical cycling of CaCO₃ and MgCO₃ saturation.

Marine surficial sediments experience pressure cycling due to the periodic rise and fall of the tide, with fluids being driven in and out of the surficial sediment in response to the tidal cycle. Where permeability of the sediments is low (e.g., in organic-rich carbonate muds), there is an appreciable lag between rise and fall of the fluid pressure in the marine water column and resulting rise and fall of the pore pressures within the surficial sediment. This lag results in periodic variations in the pressure experienced by the sediment matrix, resulting in low-level pressure solution effects during the tidal rise cycle. This pressure solution drives the initial recrystallization and cementation of shallow marine carbonates, and creates the supersaturation required for deposition of calcite layers.

Where the carbonate sediments are intermixed with organics from photosynthetic organisms, decay of the chloroplasts releases Mg into the pore fluid. The extra Mg oversaturates the system and allows deposition of MgCO₃, but only during reduction in CaCO₃ saturation (i.e., during tidal fall). The exhalation of pore fluids (e.g., H₂S and various organic acids) during tidal fall causes Eh-pH changes that facilitate deposition of MgCO₃ by effectively lowering the solubility.

Thus, ordered dolomite is created during diagenesis of surficial carbonate sediments by tidally-driven pressure differences between the marine water column and the sediment pore fluid. These pressure differences are significant only in fine grained sediments or in sediments capped by an impermeable layer (such as an algal mat), and the decaying photosynthetic organic material within the sediment is necessary for episodic MgCO₃ supersaturation required for dolomite growth.