LATE DIAGENETIC DOLOMITIZATION OF LOWER ORDOVICIAN, UPPER KNOX CARBONATES - A RECORD OF THE HYDRODYNAMIC EVOLUTION OF THE SOUTHERN APPALACHIAN BASIN

Late diagenetic dolomitization of the Lower Ordovician, Upper Knox Gro up in the southern Appalachian basin was closely associated with wides pread secondary porosity development, hydrocarbon migration, and local Mississippi Valley-type mineralization. Regionally extensive (approxi mately 70,000 km2), late diagenetic dolomites consist of replacement d olomites and zoned dolomite cements. Late diagenetic replacement dolom ites comprise 15 to 50% of all Knox matrix dolomites. The deltaO-18 (- 11.9 to -5.3 parts per thousand), deltaC-13 (-3.8 to +0.9 parts per th ousand), and Sr-87/Sr-86 (0.70895 to 0.70918) values of late diageneti c replacement dolomites overlap with those of the first zone of dolomi te cements (zone 2), early replacement dolomites, and Lower Ordovician marine calcites, reflecting rock buffering of initial dolomitizing fl uids and extensive neomorphism of replacement dolomites by subsequent late diagenetic fluids. Nonporous to sucrosic, late diagenetic dolomit es have porosities (1 to 16%) and permeabilities (0 to 1030 md) signif icantly greater than those of early diagenetic replacement dolomites a nd host limestones (1 to 6% and 0.0 to 0.04 md, respectively). Five ge nerations of dolomite cements can be correlated regionally; zone corre lations are interpreted to have time significance and to record region ally extensive diagenetic events. The deltaO-18 (-12.4 to -3.0 parts p er thousand) and Sr-87/Sr-86 (0.70885 to 0.71000) values, and Sr (18 t o 147 ppm), Mn (63 to 1069 ppm), and Fe (109 to 8452 ppm) contents of zoned dolomites, in conjunction with fluid inclusion data, indicate th at late diagenetic dolomites precipated from hot (80 to >165-degrees-C ), saline (13 to 22 wt. % NaCl equivalent) basinal brines that underwe nt extensive fluid-rock interaction with clastics. Precipitation tempe ratures of late diagenetic dolomites estimated from fluid inclusion ho mogenization temperatures and systematic trends in deltaO-18 values re cord a regionally developed, prograde-to-retrograde termal history. Kn ox late diagenetic dolomites are interpreted to record the spatial and temporal evolution of large-scale fluid flow systems that developed i n response to different burial and tectonic stages of the southern App alachian basin. The occurrence of zoned dolomite cements in tectonic f ractures and breccias, and their close association with noncarbonate d iagenetic minerals of Pennsylvania to Early Permian ages, suggest that most Knox late diagenetic dolomites record deep subsurface (2 to >5 k m) fluid migration in response to late Paleozoic Alleghenian tectonism (330 to 265 Ma). Late diagenetic matrix dolomits served as long-lived conduits that focused and channeled diagenetic fluids in the deep sub surface. The occurrence of bitumen in secondary porosity within late d iagenetic dolomites indicates that they likely were the most viable re servoirs during hydrocarbon migration in the late Paleozoic.