Ordovician Knox Paleokarst Unconformity, Appalachians

Abstract

The Ordovician Knox unconformity in the Appalachians developed in less than 10 m.y. during a time of initial collision of the passive margin and of eu- static sealevel lowering. It formed on cyclic limestones and dolomites of the 200- to 1200-m-thick Upper Knox-Beekmantown Group, and provides an example of the effects of long-term exposure on a carbonate shelf and the subsequent diagenesis related to karsting followed by deep burial. Erosional relief is over 100 m in the south. It increases over synclepositional structures and bevels down to Upper Cambrian rocks on the craton. The disconformity is virtually absent in the Pennsylvania depocenter. Paleokarst features include paleotopographic highs, sinkholes and caves that extend to over 65 m below the unconformity, and intrastratal breccias down to 300 m. Near-vertical sinkholes are filled with carbonate breccia and gravels with fine dolomite matrix. Caves are filled with breccia and laminated dolomite. Intrastratal breccias up to 35 m thick and over 200 m long contain dolomite clasts commonly with fitted fabrics, in a fine dolomite matrix. Nonluminescent calcite cements fill leached grains and intergranular spaces down to 200 m below the unconformity, and formed from slowly moving, oxidizing meteoric waters undergoing diffuse flow in an unconfined aquifer. Rapidly moving, conduit flow cave waters caused extensive dissolution of limestone beds locally forming intrastral breccias; some of these also may have resulted from dissolution in the mixing zone. During burial, compaction further fractured breccia beds, and in the Late Paleozoic, warm saline basinal brines (80°C to 200°C) caused further dissolution, dolomitization fronts surrounding breccia zones, and precipitation of saddle dolomite and associated sulfides, within permeable horizons.