Effects of Grand-Cycle Cessation on the Diagenesis of Upper Cambrian Carbonate Deposits in the Southern Appalachians, U.S.A.

ABSTRACT The vertical transition from the mainly subtidal alternating shale and carbonate units (or grand cycles) of the Conasauga Group (Middle to Upper Cambrian) to the peritidal dolostone of the Knox Group (Upper Cambrian to Lower Ordovician) marks a major change in the early Paleozoic passive-margin sedimentation of the southern Appalachians. The grand cycles represent an interplay between intrashelf shale basin and carbonate-platform deposition. The end of grand-cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost Conasauga Group strata--the Maynardville Formation. The Maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone. This change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the Maynardville and, in conjunction with the changes in the regional facies distribution at the end of grand-cycle deposition, extended its influence upon the burial diagenesis of these deposits. The subtidal deposits of the Maynardville contain a variety of calcite cements. Dolomite is not abundant in these deposits, where it occurs mainly as a fine-crystalline ferroan phase associated with argillaceous layers and pressure-dissolution features. The local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits, diagenesis of clay minerals, and pressure dissolution of carbonates. Ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite. The subtidal deposits also contain saddle dolomite cement associated with Mississippi Valley-type (MVT) minerals, which indicate the involvement of externally derived fluids during late burial diagenesis of these strata. The peritidal deposits, on the other hand, are extensively dolomitized. Fine-crystalline dolomite most likely represents early diagenetic replacement of tidal-flat carbonate sediment. Coarse-crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines. Fenestrae, desiccation cracks, and evaporite-dissolution voids are occluded with dolomite cement, which is commonly complexly zoned. Zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement. Saddle dolomite in pore centers, dissolutional voids, and tectonic fractures formed during late burial from warm fluids associated with the migration of MVT brines. The effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal Maynardville and the underlying Conasauga Group carbonate deposits, and between the peritidal deposits of the upper Maynardville and the rest of the overlying Knox Group. The depositional environments and early diagenesis also affected the burial diagenesis. The changes in regional facies distribution, caused by carbonate platform progradation at the end of grand-cycle deposition, resulted in a decrease in the proportion of siliciclastic components. This decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits. The Maynardville is conformably overlain by carbonate deposits of the Knox Group. This allowed the Maynardville-Knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the Ordovician foreland (Sevier) basin to the southeast during the conversion from a passive into a convergent continental margin. The Nolichucky Shale separates the Maynardville-Knox succession from the carbonate deposits of the underlying Conasauga Group, which were, in contrast, affected by the migration of burial fluids from the Cambrian intrashelf shale basin to the west.