Evidence for Post-Cementation Migration of High-Temperature, High-Pressure Fluids--Siluro-Devonian Helderberg Group, Central Appalachians: ABSTRACT

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Fluid inclusion data and fracture-filling cements in carbonate and siliciclastic rocks of the Siluro-Devonian Helderberg Group, central Appalachians, indicate post-cementation, late Paleozoic migration of high-pressure, high-temperature fluids. Void-filling quartz and calcite cements contain secondary, 2-phase fluid inclusions that give freezing temperatures of -20 to -25°C (-4 to -13°F) (salinity > 22 wt.% NaCl). Homogenization temperatures are 200 to 300 + °C (392 to 572°F) (temperatures calibrated and pressure corrected) and greatly exceed maximum paleotemperatures (120 to 160°C; 248 to 320°F) given by conodont color-alteration index values or calculated from known sedimentary overburden. High homogenization temperatures suggest rapid movement of metamorphic fluids so that ambient burial temperature was not raised for long enough periods of time to affect conodont CAI values. These fluids probably came from Blue Ridge-Piedmont thrust sheets that were undergoing metamorphism during late Pal ozoic deformation. These fluids migrated more than 75 km (47 mi) during thrusting. Well-cemented sandstone and limestone have multiple crosscutting trains of secondary hydrocarbon inclusions. Some trains crosscut cement-filled fractures. Hydrocarbons also occur as thin films along cement crystal boundaries and as secondary inclusions trapped along calcite deformation twins. These inclusions indicate geopressured fluids moved along intercrystalline boundaries and along deformation twin planes in calcite under deep burial conditions either during or after deformation. Rare fractures contain transported skeletal grains, exotic clasts, recemented clasts of fracture-filling cement, and mud. Cement clasts contain included mud and skeletal grains, and indicate several episodes of particle transport, cementation, and refracturing prior to final fracture filling. Primary(?) 2-phase fluid inclusions in vein-filling calcite give homogenization temperatures of 120 to 150°C (248 to 302°F). Coarse-grained clastic fracture fills indicate migration of rapidly moving fluids capable of transporting clasts through fracture conduits under deep burial conditions. End_of_Article - Last_Page 471------------