Petrologic and geochemical evidence for water-rock interaction in the mixed carbonate-siliciclastic Nolichucky Shale (Upper Cambrian) in East Tennessee

Author

J. Lincoln Foreman

Date of Award

5-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Gary K. Jacobs, Steve Driese, Claudia Mora, Kula Misra, Bruce Robinson

Abstract

Intraclastic limestone, thin fossiliferous limestone, and oolitic limestone interbedded with fissile shale are characteristic of the Nolichucky Shale (Upper Cambrian) in the Whiteoak Mountain thrust sheet, eastern Tennessee. These lithologies are interpreted as slope and basinal units deposited in water depths that may have exceeded 250-300 m. Intraclastic limestone debris flow deposits and skeletal and oolitic calcarenite turbidites indicate that nearly all carbonate is allochthonous, derived from up-slope and platform shallow-water environments. Storms were a dominant factor in initializing down-slope transport from shallower-water settings. Polymictic limestone conglomerate and shallow-water-derived calcarenite composed of ooids, peloids, and fossils, indicate that depositional slopes were steep enough at times to permit considerable transport (in excess of 20 km) from up-slope areas. During times of increased siliciclastic sedimentation, carbonate production was suppressed and the basin margin had a gentle ramp morphology. Algal and oolitic shoals built up close to sea level when siliciclastic input was reduced, resulting in the progradation of carbonate facies into the basin. Depositional slopes at the basin margin were steepened to the point that gravity sediment flows were effective in transporting shallow water carbonate lithologies into the basin. A variety of ooid cortical fabrics are preserved in shallow-water and deep-water oolitic limestone from the Nolichucky Shale. Petrographic and geochemical evidence suggest that these ooids were originally high-magnesium calcite fibrous radial ooids. Unequivocal evidence for originally aragonitic and bimineralic ooids is lacking. The ooids are now low-magnesium calcite ooids with approximately 1.5 mol% MgCO₃. Iron and manganese concentrations in these ooids are 2-3 orders of magnitude greater than predicted and observed for unaltered Modem marine calcite. The elevated iron and manganese concentrations in the Nolichucky ooids reflect stabilization and progressive alteration in the presence of pore fluids modified by interaction with interbedded shale. Fibrous radial ooids with relatively good original fabric preservation and sparry radial ooids with poor original fabric preservation commonly occur in the same thin sections. This can be explained by different degrees of water-rock interaction which was probably controlled by small-scale variations in microporosity and permeability. Increased water-rock interaction resulted in aggrading neomorphism of fibrous radial ooids to sparry radial ooids and associated increases in Fe²⁺ and Mn²⁺ in the sparry radial ooids. The Nolichucky Shale from the southern Appalachian foreland thrust system experienced a three-stage history of calcite vein formation from oldest to youngest: bed-normal, bed-parallel, and bed-oblique fracturing, respectively. Freezing temperatures from primary fluid inclusions in calcite veins demonstrate that all veins precipitated from Na-Ca- Cl brines with ca. 27 wt% NaCl equivalent salinities. The Whiteoak Mountain thrust sheet contains four coeval bed-normal vein sets trending 015°, 055°, 090°, and 320°. These pre-tectonic veins are cut by syn-tectonic bed-parallel slickensided veins. Ruid inclusion thermometry from calcite veins indicates precipitation at 80-110 °C for all bed-normal veins. These temperatures correspond to burial depths of 2.4-3.6 km, which were most likely attained during the late Mississippian to early Pennsylvanian. At that time, horizontal stress directions were controlled by both Alleghanian and Ouachitan orogenic stresses. The interaction of these two stress fields may account for the simultaneous development of the four coeval bed-normal sets. In this model, the 055° and 320° sets formed parallel and perpendicular to the Alleghanian front, respectively, whereas the 015° and 090° sets formed sub-perpendicular and sub-parallel to the Ouachitan front, respectively. Subsequently, bed-parallel veins formed during thrusting at 110 °C and 3.6 km burial depth, indicating that local thrust-wedge thickness did not exceed the original maximum burial thickness. Isotopic compositions from interbedded limestone and authigenic carbonate in calcareous shale show a parallel decrease in σ¹⁸O and σ¹³C with increasing depth in the Nolichucky Shale, indicating extensive water-rock interaction between these lithologies during authigenic calcite precipitation. Prior to fracturing, fluid isotopic compositions, and carbonate minerals precipitated from them, were dominated by local rock compositions, reflecting a stratified fluid system and lateral fluid flow. In contrast, isotopic compositions from calcite veins in the Nolichucky Shale have a fairly constant composition with depth. Bed-normal fracturing enabled cross-formational fluid flow along open fractures, vertical mixing of vein-forming fluids, and possible extraformational introduction of Na-Ca-Cl brines into the Nolichucky Shale. During calcite vein precipitation, there was little isotopic exchange between fluids and adjacent wall-rocks resulting in vein calcites that are not in isotopic equilibrium with the adjacent wall-rocks. Cross-formational fluid flow along open fractures occurred before local Alleghanian thrusting because the vein calcites are deformed by Alleghanian twinning and are offset by bed-parallel calcite veins that have Alleghanian displacements. Pressure-corrected homogenization temperatures for the bed-normal and bed-parallel calcite veins indicate that they formed at near maximum sedimentary burial depth of 3.6 km in advance of thrusting. These veins lack evidence for inclusion deformation caused by increased burial, which suggests that thrusting did not subject the Nolichucky Shale to a maximum burial depth in excess of 3.6 km. Homogenization temperatures of primary fluid inclusions in a single bed-oblique calcite vein average 157 °C and pressure-corrected temperatures exceed 200 °C. These temperatures are significantly greater than maximum burial temperatures of 110-150 °C calculated for the Nolichucky Shale. These elevated temperatures indicate that during later thrusting, fluids hotter than the ambient rock temperature migrated through the Nolichucky Shale. However, these hotter fluids were unable to re-equilibrate older, lower temperature fluid inclusions or reset time-temperature indicators including conodont color alteration indices, illite crystallinity values, and clay-mineral assemblages. These hot fluids may reflect large-scale fluid migration out of the Alleghanian hinterland driven by tectonic overthrusting.

Recommended Citation

Foreman, J. Lincoln, "Petrologic and geochemical evidence for water-rock interaction in the mixed carbonate-siliciclastic Nolichucky Shale (Upper Cambrian) in East Tennessee. " PhD diss., University of Tennessee, 1991.
https://trace.tennessee.edu/utk_graddiss/11105