Regional dolomitization of Early Ordovician, Upper Knox Group, Appalachians

TR Number



Journal Title

Journal ISSN

Volume Title


Virginia Polytechnic Institute and State University


The Early Ordovician, Upper Knox Group consists of meter-scale shallowing-upward cycles that were deposited on a low-sloping ramp. Cycles formed in response to short term (<100 k.y.) eustatic sea-level fluctuations and typically have well developed tidal flat caps. Cycles are bundled into five transgressive-regressive sequences which correspond to third order (1-10 m.y.) sea-level fluctuations defined by Fischer plots. The Upper Knox Group is 90% dolomite of which greater than 75% predates Middle Ordovician, Knox Unconformity development.

Early dolomitization occurred penecontemporaneously with tidal flat progradation during fifth-order (up to 100 k.y.) sea-level falls as indicated by: abundant dolomite in cycles with well-developed tidal flat caps and scarce dolomite in cycles with no or thin laminite caps; decrease in dolomite abundance with distance below tidal flat caps; dolomitized cycles decrease basinward; and dolomite clasts veneer cycle tops and the Knox Unconformity surface. Third-order sea-level fluctuations also strongly controlled early dolomitization as indicated by Fischer plots; limestone, subtidal-dominated cycles correspond to third-order sea level rises and completely dolomitized, peritidal-dominated cycles correspond to third-order sea level falls.

"Early" dolomite was metastable and its geochemical composition was modified during initial stabilization by marine brines during progradation of each cycle, and by mixed fresh/marine waters of the Knox aquifer associated with unconformity development. Much "early" dolomite however, remained metastable into the deep burial environment where it was replaced and overgrown by burial fluids as suggested by: covariant trends between crystal size, mole % CaCO₃, Sr²⁺, Mn²⁺ and δ¹⁸O; similar regional trends defined by stable isotope values of "early" dolomites and burial dolomites; and water-rock modeling of trace element and stable isotopic trends. Trace element and stable isotope compositions of least-altered "early" dolomite however, record a memory of a precursor evaporative dolomite.

Cathodoluminescent dolomite stratigraphy defines five generations of burial dolomite that can be correlated over 100,000 km². Burial dolomites postdate a regional dissolution event attributed to migration of organic acid-rich fluids through the Knox carbonates. Regional dolomitization occurred coeval with Late Paleozoic deformation and was closely associated with MVT mineralization and hydrocarbon migration. The δ¹⁸O values and trace element contents of burial dolomites in conjunction with fluid inclusions, suggest that burial fluids were warm (135 to 200°C), saline (13 to 22 wt. % NaCl equiv.), ¹⁸O-enriched (+2 to +9 % SMOW) fluids with geochemical compositions similar to present day basinal brines. Mn²⁺ and Fe²⁺ contents of the dolomites suggest a redox control over Mn and Fe fluid chemistry, and in conjunction with regional δ¹³C trends, likely record precipitation from organic acid-rich fluids. Regional trace element and δ¹⁸O trends record a basinal fluid source and regional northwestward flow. Stable isotope values of burial dolomites and fluid inclusions from dolomites and associated minerals, define a prograderetrograde sequence that formed during basinwide, gravity-driven fluid flow which developed in response to Late Paleozoic thrusting and uplift.