Tidal-Flat and Shallow Subsurface Evaporative Dolomitization

Isabel P. Montanez, J. Fred Read

The term "reflux dolomitization" generally connotes a silled lagoon undergoing evaporite precipitation, beneath which dense brines move down section, dolomitizing subtidal facies. Ramps lacking evaporite lagoons in semiarid settings subjected to low-amplitude sea level oscillations (up to 100,000-year periods), however, show much early dolomite formed by gravity-driven brines. Cycle progradation starts after transgression, and accompanies shoaling of the inner ramp. As tidal flats prograde, laminites on flats become dolomitized as a result of storm-flooding by marine waters that infiltrate the flats, become evaporatively concentrated, and undergo capillary rise. These waters have elevated Mg/Ca largely due to CaCO₃ and minor CaSO₄ precipitation. As f ats prograde (reaching widths of up to 300 km in 100,000 years), the supratidal surface (brine factory) enlarges, at the same time undergoing gradual subsidence, allowing it to track sea level fall and decrease in slope (to a few centimeters/1,000 years). Thus, vast areas are formed for evaporation of storm-recharge waters that infiltrate previously dolomitized laminites on interiors of flats. The resultant large volumes of dense brines move downdip through subtidal sediments beneath flats, causing widespread shallow subsurface dolomitization, which decreases toward younger parts of flats. Older dolomites in interiors of flats become more coarsely crystalline and obscure primary fabric. The tidal flats prograde out from cratonic shorelines and arches, and migrate out into depocenters on he ramp or toward the regional ramp margin; intensity of dolomitization decreases in the direction of progradation. Thus, in depocenters and along the outer ramp, sections have abundant subtidal limestone preserved. These types of dolomite should have ^dgr¹⁸O values enriched 2-4 ^pmil relative to carbonate precipitated from marine waters. However, because of the relative instability of these disordered Ca-rich dolomites, their isotopic and trace-element chemistries may be modified later by isotopically light diagenetic fluids. These dolomites are the dominant type in the Knox Group.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.