Stable Isotopic Evidence for Fluid Flow and Fluid/Rock Interaction During Thrust Faulting in Pumpkin Valley Shale and Rome Formation, East Tennessee

Brian K. Butler, C. S. Haase

The Pumpkin Valley Shale and the underlying Rome Formation form the lower portions of the Copper Creek and White Oak Mountain thrust sheets in east Tennessee. The Pumpkin Valley Shale consists of shale and mudstone with subordinate amounts of interbedded siltstone. The Rome Formation is composed predominantly of sandstone with interbedded shale and siltstone toward the base of the formation. The percentage of illite increases from 20% to over 80% of the bulk clay mineralogy toward the base of the section. Porosity is occluded by quartz, phyllosilicate, and calcite cements. Both formations contain calcite-filled and, less commonly, quartz-filled Alleghenian fractures and joints.

The < 2-µm material from shales and mudstones ranges from -10 to -67 ^pmil in ^dgr^Dgr and 12.8 to 16.5 ^pmil SMOW in ^dgr¹⁸O. Limestone and siderite occurrences range from 23.7 to 26.8 ^pmil SMOW in ^dgr¹⁸O and -4.2 to -2.0 ^pmil PDB in ^dgr¹³C. Calcite cements in siltstones and sandstones range from 15.2 to 25.3 ^pmil SMOW in ^dgr¹⁸O and -10.9 to -2.0 ^pmil PDB in ^dgr¹³C. Fracture-filling calcite ranges from 15.3 to 23.9 ^pmil SMOW in ^dgr¹⁸O and from -11.1 to -1.7 ^pmil PDB in ^dgr¹³C. Fracture-filling quartz ranges from 17.7 to 26.3 ^pmil SMOW in ^dgr¹⁸O.

Isotopic and textural data imply that fluid composition in the section changed with time. Early fracture-filling quartz and < 2-µm clays that formed during diagenesis equilibrated isotopically with a pore fluid that ranged from -4.5 to 5.5 ^pmil in ^dgr¹⁸O (at 100°C). Later porosity-occluding and fracture-filling calcite equilibrated with a fluid that ranged from 2.4 to 11.0 ^pmil in ^dgr¹⁸O (at < 150°C). This latter fluid is likely representative of fluids migrating through the thrust sheets during Alleghenian thrust faulting. ^dgr¹³C values are lightest in the Pumpkin Valley and indicate the oxidation of locally derived organic matter.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.