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

I. P. Montanez, J. Fred Read

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-km² region and defines a dolomite paragenesis that can be integrated with timing of Missippi-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. Burial dolomite cements fill intergranular voids in replacement-dolomite mosaics, solution voids, and tectonic fractures.

Oxygen isotope values (-12.9 to -4.5^pmil) 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°C), saline (13.5-22 wt. % NaCl equiv.) fluids with 180 (+2 to +13^pmil) and trace element

compositions typical of basinal brines. The ^dgr¹³C values (-3.8 to + 3.2^pmil) 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 fluid 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.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990