Migration of orogenic fluids through the Siluro-Devonian Helderberg Group during late Paleozoic deformation: constraints on fluid sources and implications for thermal histories of sedimentary basins

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Fluid inclusion data and fracture-filling cement fabrics in deformed, but non-overthrusted, 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, two-phase fluid inclusions (liquid + vapor) which occur in annealed microfractures. Most melting temperatures (last occurrence of ice) are −20° to −25° C, which correspond to salinities of greater than 22 equivalent wt.% NaCl. Pressure-corrected liquid-vapor homogenization temperatures are 200° to 300 + °C and greatly exceed maximum paleotemperatures (≈150°C) given by conodont CAI values, vitrinite reflectance, illite crystallinity, or temperatures calculated from known sedimentary overburden. Completely cemented sandstone and limestone also have multiple crosscutting trains of secondary hydrocarbon inclusions. Hydrocarbon inclusions occur along deformation microstructures in cements. Some hydrocarbon inclusion trains crosscut cement-filled fractures. The nature of occurrence of the secondary inclusions suggests that fluids moved along intracrystalline deformation microstructures either during or after late Paleozoic deformation. Rare fractures contain transported skeletal grains, “exotic” clasts (lithologies different from wall rock), recemented clasts of fracture-filling cement, and mud. Cement clasts contain solid inclusions of mud and skeletal fragments, and indicate several episodes of particle transport, cementation, and refracturing prior to final fracture filling and cementation. These fracture fills locally crosscut stylolites. Coarse-grained, poorly sorted, “clastic fracture fills” suggest migration of rapidly moving fluids which were capable of transporting clasts through fracture conduits under deep burial conditions. Conduction calculations place upper limits on size and duration of thermal anomalies which were caused by extrabasinal fluids. These calculations indicate that hot fluid migration events most likely were short-lived. Heat transfer by extraformational fluids moving along faults and fractures may have been the mechanism by which thermal anomalies developed in the Helderberg Group. Stratigraphic constraints suggest that likely sources for the hot brines are overthrusted terranes to the east of the study area. Possible stratigraphic horizons which were sources for the overpressured, hot fluids may have been thick Cambrian shales which are important detachment horizons for major, late Paleozoic thrust sheets in the eastern Valley and Ridge province (east of the study area). Fluids may have migrated during thrusting, along thrust faults which ramp upwards from east to west into stratigraphically higher units; thrust faults ultimately die out in Devonian units. Some of the dissolved ions in the secondary two-phase inclusions may have been incorporated when shale-derived fluids moved through Silurian evaporites or Cambro-Ordovician carbonates which contain evidence of cryptic evaporites. Potential source rocks for secondary hydrocarbon inclusions may have been Lower Silurian through Lower Devonian strata, based on Lopatin modeling and assuming that migration occurred during late Paleozoic deformation. This study may help to explain many apparently anomalous geologic phenomena in deformed foreland basin sediments. It also has implications for tracking migration pathways for fluids which facilitate regional scale deformation in fold-and-thrust belts and how “tectonic fluids” may affect the rocks through which they move.