Electric Log Analysis Of Precambrian Igneous And Metamorphic Rocks In The St. Francois Mountians, Missouri

Abstract

Electric log, core, and optical petrographic analysis of a metamorphosed and deformed PreCambrian-aged rhyolite effusive event identified 2 litho-facies and effected characterization of the rock deformation. Single-point resistance (SPR) and spontaneous potential (SP) electric logs identified the ubiquitous clay seams and localized dikes. The igneous/metamorphic rocks had naturally elevated gamma emissions; consequently, natural gamma (NG) logging was useless in the identification of clay seams. However, NG logs proved useful in the identification of dikes, which had readings of less than 150 cps. The SPR/SP suite also identified intrusive black zones, characterized as martite by thin section analysis, within the rhyolite. Acoustic televiewer (AT) logs identified a conjugate shear fracture set with the main fracture set bearing a strike of N35W and dip 80NW or SE. The minor fracture set trends N35E and dips 80NE or SW. Thirty percent of the fractures logged are horizontal suggesting a vertically upward stress relief consistent with granite emplacement. Porous zones within the rhyolite porphyry and contiguous with fractures appear to be the result of subsurface potassium feldspar phenocryst weathering and erosion. These zones are invisible to SPR, SP, and NG tools. Much of the feldspars within the rhyolite porphyry show weathering to kaolinite. Erosion and mobilization of the kaolinite is putatively considered to be the causative agent for the deposition of clay in the horizontal fracture sets.