ABSTRACT

The Lower Ordovician Upper Knox Group is characterized by shallowing-upward peritidal cycles that are extensively dolomitized to form massive dolomite. Most (85%) replacive dolomite is stratigraphically controlled, composed of very fine to medium crystalline, planar to nonplanar (early) dolomite composed of Zones 1, 2 and 3 dolomites. Remaining dolomite occurs as medium to coarsely crystalline, nonplanar (late) replacive dolomite (Zones 2 and 3 late dolomites) that selectively replaces limestone associated with stylolites and fractures or forms massive dolomite fronts. Complexly zoned (Zone 2 to 6) saddle dolomite cements make up less than 5% of all dolomite and primarily fill solution voids and fractures. Early dolomites are near stoichiometric and, relative to Early Ordovician marin carbonates, are similar to or significantly depleted in ¹⁸O and Sr, slightly to significantly enriched in Mn and Fe, and have similar ⁸⁷Sr/⁸⁶Sr values. Relative to early dolomites, late (Zones 2 and 3) dolomites are depleted in ¹⁸O and St, enriched in Mn and Fe, and have similar ⁸⁷Sr/⁸⁶Sr values.

Early dolomites are interpreted to have formed from modified seawater during seaward progradation of tidal flats within each high-frequency (fifth- and fourth-order) cycle duration. Late dolomites, based on their texture and geochemistry, formed as a result of recrystallization of early dolomites, as direct replacement of limestone at elevated temperatures, and as cement in fractures and secondary solution voids. Covariant trends between textures and geochemical compositions for early dolomite indicate that presentday compositions of Knox early dolomite record a history of progressive diagenetic modification (stabilization or recrystallization), involving varying degrees of changes of textural and geochemical compositions during multiple episodes of dolomitization. Early dolomites wit the most extensively altered textures are more stoichiometric, more depleted in ¹⁸O and Sr, and more enriched in Mn and Fe relative to texturally less-altered early dolomite.

Initial stabilization may have occurred syndepositionally in modified seawater. Continued stabilization of early dolomite likely occurred in fresh or mixed waters of a Middle Ordovician meteoric aquifer associated with Knox unconformity development. Final stabilization occurred by deep burial brines at elevated temperatures as indicated by 1) extensive replacement and overgrowth of early dolomite by late dolomites: 2) a progressive increase in crystal size and abundance of nonplanar crystal boundaries with increase in replacement by late dolomites; and 3) the similarity of the geochemical compositions of texturally most-extensively altered early dolomites and late dolomites.