Abstract

Abundant and delicately preserved hopper shaped calcite pseudomorphs after halite occur in the supratidal-intertidal Joachim Dolomite (Middle Ordovician) of north Arkansas. The wispy intergrowth of matrix and hopper crystals and downward preferred hopper orientation suggest that the halite grew within the sediment from upward moving phreatic marine water by an evaporative pumping mechanism. The molds are filled with two generations of cement, a thin crust of euhedral, or limpid dolomite lining the hopper mold walls with monocrystalline or mosaic calcite spar filling the remaining pore space. Sr (super 2+) concentrations for the limpid dolomite, calcite spar and dolomicrite are 424, 848, and 60 ppm, respectively. When compared to the Sr (super 2+) concentrations obtained from Holocene and Pleistocene calcites and dolomites of subtidal, supratidal, and meteoric origins, Joachim calcite and limpid dolomite appears to be in equilibrium and both are interpreted as having formed from mixed meteoric-marine water. Low Sr (super 2+) concentration in the dolomicrite indicates that it has undergone extensive diagenesis by low Sr (super 2+) waters. A reconstruction of the sequence of diagenetic events based on petrographic and geochemical analyses is as follows: Stage 1 . Dolomitization of tidal flat carbonates and halite hopper growth. Stage 2 . Tidal flat progradation and influx of mixed meteoric-marine water which dissolved the halite and precipitated both dolomite and calcite. Early dolomite matrix began to convert to dolomicrite of present form and porosity is reduced. Stage 3 . Meteoric water completely dominated subsequent diagenesis by completing the dolomicrite conversion and eliminating the remaining porosity.

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