Bioclastic, medium-grained sand beds in the Chesapeake Group formations (Miocene) of Maryland and Virginia, contain abundant intergranular dolomite and calcite cement. The sand beds are dominated by fairly well-sorted subrounded quartz grains (mean grain size approximately 250 µm), molluscan-shell debris, and variable amounts of mud and iron oxides that coat many of the grains. The surfaces of the quartz grains surrounded by carbonate cement are etched and pitted, indicating high pH conditions within the diagenetic environment. Initially abundant primary porosity and permeability may be completely filled with diagenetic equant calcite spar and/or rhombic dolomite. However, shell material may be completely dissolved by this process, creating a conspicuous secondary po osity.

Dolomitization appears to be stratigraphically controlled by impermeable silty clay layers, above and below the sandy beds, which confine the diagenetic fluids to these zones. Though calcitization appears to be more variable, it also seems to be stratigraphically controlled.

Preliminary ¹³C and ¹⁸O results indicate that the dolomite and calcite cements are in isotopic equilibrium, and a ¹⁴C-age determination suggests that dolomitization began less than 30,000 yr ago. The abundant presence of limpid dolomite suggests a fresh or brackish water origin for these cements, and the presence of ferroan dolomite may indicate an organic influence in the dolomitization process.

The confinement of diagenetic fluids in thin (1-2 m) permeable sand layers may induce the concentration of the necessary available cations to initiate dolomitization.

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