Eustatic Control on Early Dolomitization of Cyclic Peritidal Carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians and Middle to Late Cambrian Bonanza King Formation, Southern Great Basin

MONTANEZ, ISABEL P., University of California, Riverside, CA

The origin of massive dolomite in ancient cyclic carbonate successions remains a poorly resolved issue reflecting the lack of modern analogs of extensive dolomitization. This paper presents evidence for extensive synsedimentary dolomitization of peritidal cyclic carbonates of the Early Ordovician Upper Knox Group, Appalachians, and of the Middle to Late Cambrian Bonanza King Formation, southern Great basin. Early dolomitization of these Cambro-Ordovician carbonates was synchronous with regressive conditions governed by superimposed sea-level oscillations (fifth-, fourth-, and third-order).

Upper Knox and Bonanza King carbonates are characterized by meter-scale shallowing-upward cycles that are extensively dolomitized. Dolomitization of these peritidal cyclic carbonates occurred in evaporatively concentrated seawater within the upper intertidal-supratidal zone of storm-recharge and reflux when tidal flats prograded seaward (for up to 95% cycle period) within each high-frequency [10(4) to 10(5)] cycle period. Early dolomite is commonly associated with mudcracked laminites, restricted subtidal facies, solution collapse breccias, and silicified evaporite nodules with relict anhydrite laths. Early dolomite clasts occur in regoliths veneering cycle tops and are reworked into transgressive limestones of the overlying cycle. Early dolomite abundance decreases with distance belo the tidal flat laminite caps.

Systematic changes in stacking patterns of cycles in conjunction with Fischer plots of the Upper Knox Group and Bonanza King Formation delineate a third-order sea-level history for each sequence, which can be keyed to depositional sequences. Early dolomite distribution shows a strong relationship to third-order sea-level events indicating that low-frequency sea-level also strongly controlled early dolomitization. Fischer plots show that thick subtidally dominated, limestone and partially dolomitized cycles formed during third-order sea-level rises and that thin peritidally dominated, completely dolomitized cycles formed during third-order sea-level falls.

Mass balance calculations suggest that pervasive dolomitization of these Cambro-Ordovician carbonates could have occurred within a few cycle periods. Dolomitization during the Cambro-Ordovician would have been several orders of magnitude more extensive than in modern supratidal flats due to lower platform slopes and long periods of progradation (several 10's k.y.), which resulted in broad dolomitization zones at the end of each high frequency sea-level fall. It is suggested that sabkha dolomitization in concert with composite eustacy (Golhammer et al., 1990) is an extremely viable mechanism for massive dolomitization of many ancient peritidal cyclic sequences.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)