MONTANEZ, ISABEL P., and ANN B. STEFANI, Department of Earth Sciences, University of California, Riverside, CA

ABSTRACT: Sea Level Control on the Origin and Evolution of Porosity in Cambro-Ordovician Cyclic Carbonates, Knox Group, Appalachians

Recent exploration efforts in Cambro-Ordovician Knox Group carbonates have focused on karst related porosity associated with intraformational and/or post-depositional sequence boundaries that record second- and third-order relative sea level falls. Porosity development associated with sequence boundaries capping third-order depositional sequences in the Knox Group is moderate to minimal. Porosity development associated with the post-depositional Knox unconformity is stratigraphically limited to a 50 to 150 m interval, and is mostly occluded by shallow burial

cements. Porous zones (10s to 100s m thick) of local to subregional extent, however, occur throughout the Cambro-Ordovician Knox carbonates (500 to 1200 m). These zones consist of medium to very coarse crystalline dolomite with minor to significant amounts of intercrystalline and vuggy porosity. The stratigraphic and areal distribution of porous zones exhibits little relationship to intraformational sequence boundaries or to the Knox unconformity; rather, it shows a strong relationship to systematic changes in the stacking patterns of meter-scale cycles and to the distribution of early diagenetic facies.

Stacking patterns of Knox carbonates, which define third-order depositional sequences and their internal architecture, and early diagenesis of Knox carbonates, were governed by superimposed orders (fifth, fourth, and third) of sea level fluctuations. Stacks of thick, subtidal-dominated cycles define the TST's of sequences, and formed during third-order sea level rises. Stacks of thin, tidal-flat dominated cycles define HST's, and record long-term sea level falls. Syndepositional dolomitization of mud-rich, regressive cycles formed thick intervals of nonporous dolomite due to the extended period of tidal flat progradation during overall third-order sea level falls. In contrast, highly bioturbated and grainier, subtidal facies of transgressive cycles were partially to completely replace by early dolomite or remained limestone reflecting the shortened durations of dolomitization during third-order sea level rises. Early dolomite mosaics in transgressive cycles were initially more porous and served as conduits ('dolomite aquifers') for late diagenetic fluids to access limestones during burial. Limestone intervals proximal to 'dolomite aquifers' were partially replaced by porous coarse crystalline dolomite, whereas limestone intervals isolated from 'dolomite aquifers' remain undolomitized and impermeable. 'Dolomite aquifers' evolved into porous, coarse crystalline dolomites due to extensive fluid/rock interaction during burial.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.