GEOPHYSICAL MAPPING OF IGNEOUS INTRUSIONS IN THE EASTERN UINTA MOUNTAINS, NORTHEASTERN UTAH

Ritzma (1983) mapped sparse exposures of dioritic-gabbroic dikes of Cambrian – early Silurian age intruding the Neoproterozoic metasandstones and metaconglomerates of the Uinta Mountain Group in the eastern Uinta Mountains of northeastern Utah. The strike of the dikes was N68W, roughly equal to the strike of the anticlinal arch of the Uinta Mountains. The above observations are difficult to reconcile with the conventional wisdom that (1) the early Paleozoic in Utah was dominated by shallow marine sedimentation along a passive margin (2) the anomalous east-west orientation of the Uinta Mountains was a result only of the rotation of the Colorado Plateau or the stress patterns that accompanied the Laramide Orogeny during the Tertiary Period. The objective of this study was carry out geophysical mapping of the igneous intrusions in the vicinity of Chepeta Lake, which are mostly buried by glacial till, in order to gain greater insight into the possibility of Paleozoic or older structural control on the orientation of the Uinta Mountains. The objective was addressed through ground profiling of magnetic susceptibility and the total magnetic field, ground profiling and sounding using electrical resistivity, induced polarization and self-potential, and small-scale surficial and bedrock mapping. The magnetic profiles showed that the intrusions could be traced along strike 1600 m past the limit of exposure to the northwest, after which the intrusions were offset 300 m by a possible right-lateral strike-slip fault perpendicular to the strike of the intrusions. Magnetic modeling showed that the dikes had a very shallow dip to the south, roughly equal to the regional dip of the Uinta Mountain Group, so that the intrusions should be regarded as sills, rather than dikes. All intrusions were accompanied by electrical resistivity lows to the north, which was consistent with fracture zones in the advance of propagating sills. Locations and orientations of slickensides as well as resistivity lows were consistent with the hypothesized strike-slip fault. A strong negative correlation between resistivity and self-potential in the vicinity of the fault suggested sulfide mineralization along the fault, which was visible in groundwater-fed ponds lying along the fault. Further results and interpretation will be reported at the meeting.