Petrographic and Geochemical Evidence for Regional Fluid and Hydrocarbon Migration Associated with Late Paleozoic Tectonism: Upper Knox Group, Appalachians: ABSTRACT

Abstract

Early Ordovician, upper Knox carbonates underwent regional dissolution, replacement dolomitization, and multiple episodes of dolomite cementation during basin-wide fluid migration associated with late Paleozoic tectonism in the central and southern Appalachians. Dolomite cathodoluminescent stratigraphy can be correlated throughout a 100,000-km{sup 2} region and defines a dolomite paragenesis that can be integrated with timing of Mississippi-Valley-type mineralization, authigenic feldspar precipitation, basin-wide illitization, and hydrocarbon migration. Dissolution and fracturing of Knox host, which immediately predated and was synchronous with pervasive replacement dolomitization, resulted in solution vugs, chalky microporosity, cement-supported fabrics, and solution-enlarged, paleokarstic intraformational breccias. Oxygen isotope values ({minus}12.9 to {minus}4.5 {per thousand}) of burial dolomites in conjunction with fluid inclusions define a prograde to retrograde sequence that began during onset of thrusting, peaked following maximum deformation, and continued throughout uplift and denudation. Burial dolomites precipitated from warm (135 to 200{degree}C), saline (13.5-22 wt. % NaCl equiv.) fluids with 180 (+2 to +13 {per thousand}) and trace element compositions typical of basinal brines. The {delta}{sup 13}C values ({minus}3.8 to +3.2 {per thousand}) of all dolomite zones record an input of organic carbon. Trace element and stable isotopic regional trends indicate a southeastern fluid source from the Appalachian orogenic belt and northwesterly fluidmore » flow across the basin. Extensive microfracturing of the Knox host and fluid inclusion data indicate geopressured fluids during maximum deformation. Basin-wide migration of hydrocarbons and dolomitizing and mineralizing brines occurred in response to a large-scale, gravity-driven flow system associated with late Paleozoic deformation.« less