DELINEATION OF SUBSURFACE PROTEROZOIC UNKAR AND CHUAR GROUP SEDIMENTARY BASINS IN NORTHERN ARIZONA UTILIZING GRAVITY AND MAGNETICS: IMPLICATATIONS FOR HYDROCARBON SOURCE POTENTIAL

As part of an integrated geological and geophysical investigation, subsurface occurrences of Mesoproterozoic Unkar Group (~1.25 to 1.1 Ga) and Chuar Group (~0.9 to 0.7 Ga) rocks have been delineated in regions east and southeast of the Grand Canyon of northern Arizona. Bouguer gravity and residual aeromagnetic anomaly data, utilized in combination with gravity profile modeling, an analysis of seismic velocities from the PACE 1989 crossline, the preparation of conventional geologic cross-sections, and a comprehensive analysis of surface geologic structures, has identified two major Proterozoic extensional trends within the region: 1) a Mesoproterozoic NW-SE trend indicating NE-SW directed extension throughout the study area, and, 2) a Neoproterozoic, mostly N-S directed trend indicating E-W extension, evident mostly in northern regions of the study area. A more ancient, NE-trending system of basement shear zones has also been identified. These shear zones correlate with many of the exposed major fault systems of the region such as the Bright Angel and Mesa Butte fault systems. These shear zones had a profound effect on both Mesoproterozoic extensional patterns and Neoproterozoic depositional trends. Unkar Group graben and half graben formation was constrained by the preexisting NE-directed shear zones. Chuar Group basin formation was influenced by the reactivation of pre-existing NW-SE Unkar Group basin structures, the more ancient NE-trending shear zones, and by Neoproterozoic age N-S oriented extension related to the rifting of Rodinia. This superimposed network of deep set normal faults led to the development of a complex array of surface structures formed in the region during Cenozoic compressional deformation, primarily during Laramide time. By delineating Proterozoic basin structures within the region, this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the Walcott member of the Neoproterozoic Chuar Group.