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AAPG Bulletin


AAPG Bulletin, V. 87, No. 8 (August 2003),

P. 1299-1321.

Copyright copy2003. The American Association of Petroleum Geologists. All rights reserved.

Delineation of subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona using gravity and magnetics: Implications for hydrocarbon source potential

John M. Seeley,1 G. Randy Keller2

1Pan-American Center for Earth and Environmental Studies, University of Texas El Paso, El Paso, Texas, 79968; [email protected]
2Department of Geological Sciences, University of Texas El Paso, El Paso, Texas, 79968


John M. Seeley is a research geologist and assistant director for the Pan-American Center for Earth and Environmental Studies at the University of Texas at El Paso. He received his B.S. (1985) and M.S. (1990) degrees from Texas AampI University and his Ph.D. (1999) from the University of Texas at El Paso. His research interests include depositional systems, sequence stratigraphy, gravity and magnetics, and tectonics.

G. Randy Keller holds the L. A. Nelson Professorship. He is also chief scientist of University of Texas at El Paso's NASA Earth Science Research Center. His research interests stress the geological applications of geophysics, and he is particularly interested in the integration of diverse data sets and the use of information technology. He is a longtime member of the AAPG, Society of Exploration Geophysicists, Geological Society of America, American Geophysical Union, and the European Association of Exploration Geophysicists.


This work was supported by the National Aeronautics and Space Administration under cooperative agreement NCC5-209 (PACES) and by the Information Technology Program of the National Science Foundation (EAR-0112968). We also gratefully acknowledge the assistance and information provided by Ben Donegan and the Arizona Geological Survey. In addition, we are grateful for the efforts of Raymond Sorenson, David A. Hyndman, and Jackie Huntoon, whose reviews and helpful comments improved this paper.


Subsurface occurrences of Mesoproterozoic Unkar Group (sim1.25–1.1 Ga) and Neoproterozoic Chuar Group (sim0.9–0.7 Ga) rocks have been delineated in regions east and southeast of the Grand Canyon of northern Arizona. Gravity and aeromagnetic data, combined with gravity modeling, an analysis of seismic velocities, geologic cross sections, and geologic structures, have identified two major Proterozoic extensional regimes in the region: (1) a Mesoproterozoic northwest to southeast trend indicating northeast-southwest–directed extension throughout the study area and (2) a Neoproterozoic, mostly north-south–directed trend indicating predominant east to west extension, evident mostly in northern regions of the study area. A more ancient, northeast-southwest–aligned 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. Unkar Group graben and half-graben formation was constrained by the preexisting northeast-directed shear zones. Chuar Group basin formation was influenced by the reactivation of northwest to southeast Unkar Group basin structures, the more ancient northeast-trending shear zones, and by Neoproterozoic age north- to south-oriented extension. This superimposed network of deep-seated normal faults led to the development of a complex array of surface structures during the Laramide. By delineating Proterozoic basin structures in the region, this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the Walcott Member of the Neoproterozoic Chuar Group.

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