Abstract

The Early Ordovician Upper Knox Group is characterized by stacked meter-scale peritidal cycles that repeat at high frequencies (104 to 105 yr). Stacking patterns (stratigraphic trends in lithofacies and cycle thickness) of meter-scale cycles define five depositional sequences that, in conjunction with Fischer plots, delineate five long-term relative sea-level fluctuations during Upper Knox deposition. Intrabasinal and interbasinal correlation of Upper Knox Fischer plots suggests that the third-order sea-level events were eustatic. Meter-scale peritidal cycles likely formed in response to high-frequency, fourth-and fifth-order, eustatic sea-level fluctuations superimposed on these third-order sea-level events.

Upper Knox cyclic carbonates are extensively dolomitized; as much as 85% of all dolomite is stratiform and consists of early dolomite exhibiting minor to extensive modification by burial dolomite. Synsedimentary dolomitization likely occurred in modified sea water during tidal-flat progradation governed by high-frequency sea-level events. This is suggested by the common association of dolomite with mud-cracked laminites and silicified evaporite nodules, the systematic decrease in dolomite abundance below laminite cycle caps, and the presence of dolomite clasts in regoliths veneering high-frequency cycle tops or in transgressive limestones of the overlying cycle. Dolomite distribution within depositional sequences shows a strong relationship to third- and fourth-order eustatic sea-level events, indicating that long-term eustasy also strongly controlled early dolomitization of Upper Knox carbonates.

Mass-balance calculations show that the proposed sabkha model of dolomitization in concert with composite eustasy could generate stratiform dolomite of considerable vertical and lateral extent in peritidal cyclic carbonates. This reflects the duration of progradation and supratidal exposure (104 to 105 yr) available for dolomitization to proceed, and the broad zone of active dolomitization that would develop during continued progradation throughout each cycle period.

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