Late Diagenetic Dolomitization of Lower Ordovician, Upper Knox Carbonates: A Record of the Hydrodynamic Evolution of the Southern Appalachian Basin

Abstract

Late diagenetic dolomitization of the Lower Ordovician, Upper Knox Group in the southern Appalachian basin was closely associated with widespread secondary porosity development, hydrocarbon migration, and local Mississippi Valley-type mineralization. Regionally extensive (^sim70,000 km2), late diagenetic dolomites consist of replacement dolomites and zoned dolomite cements. Late diagenetic replacement dolomites comprise 15 to 50% of all Knox matrix dolomites. The ^dgr18O (-11.9 to -5.3^pmil), ^dgr13C (-3.8 to +0.9^pmil), and 87Sr/86Sr (0.70895 to 0.70918) values of late diagenetic replacement dolomites overlap with those of the first zone of dolomite cements (zone 2), early replacement dolomites, and Lower Ordovician arine calcites, reflecting rock buffering of initial dolomitizing fluids and extensive neomorphism of replacement dolomites by subsequent late diagenetic fluids. Nonporous to sucrosic, late diagenetic dolomites have porosities (1 to 16%) and permeabilities (0 to 1030 md) significantly greater than those of early diagenetic replacement dolomites and host limestones ( 165°C), saline (13 to 22 wt.% NaCl equivalent) basinal brines that underwent extensive fluid-rock interaction with clastics. Precipitation temperatures of late diagenetic dolomites estimated from fluid inclusion homogenization temperatures and systematic trends in ^dgr18O va ues record a regionally developed, prograde-to-retrograde thermal history. Knox late diagenetic dolomites are interpreted to record the spatial and temporal evolution of large-scale fluid flow systems that developed in response to different burial and tectonic stages of the southern Appalachian basin. The occurrence of zoned dolomite cements in tectonic fractures and breccias, and their close association with noncarbonate diagenetic minerals of Pennsylvanian to Early Permian ages, suggest that most Knox late diagenetic dolomites record deep subsurface (2 to >5 km) fluid migration in response to late Paleozoic Alleghenian tectonism (330 to 265 Ma). Late diagenetic matrix dolomites served as long-lived conduits that focused and channeled diagenetic fluids in the deep subsurface. The occurrence of bitumen in secondary porosity within late diagenetic dolomite indicates that they likely were the most viable reservoirs during hydrocarbon migration in the late Paleozoic.