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Delineation of Proterozoic Unkar Group / Chuar Group (Grand Canyon Supergroup) Sedimentary Basin Deposits in Northern Arizona Using Gravity and Magnetic Geophysical Techniques

Mesoproterozoic Unkar Group (~1.25 to 1.1 Ga) and Neoproterozoic Chuar Group (0.9 to 0.7 Ga) sedimentary basin deposits east and southeast of the Grand Canyon were delineated using Bouguer gravity and residual aeromagnetic anomaly data, in combination with gravity profile modeling, seismic velocity analysis, preparation of geologic cross-sections, analysis of surface geologic structures, and analysis of drilling data. Delineation of Proterozoic extensional trends within this region provides insights in to Proterozoic paleogeography along the southwestern margin of Laurentia.


Two phases of Proterozoic extension resulted in the formation of two structural trends: 1) a NW-SE Unkar-related trend indicating NE-SW extension throughout the area, and 2) a predominately N-S Chuar-related trend indicating E-W extension, primarily in northern regions. All structures developed during these extensional events were strongly influenced by a more ancient structural grain comprised of multiple and parallel linear NE-oriented trends. These basement zones are interpreted to be major deep-set faults of probable Paleoproterozoic strike-slip origin.


Unkar sediments were deposited on the Vishnu terrane from ~1.25 to 1.1 Ga and were subsequently downdropped into grabens and half grabens during the first of the two extensional events. This first event was concurrent with extrusion of the Cardenas Basalt at ~1.1 to 1.0 Ga. Subsequently, long periods of erosion separated subsequent minor extensional events during deposition of Nankoweap Formation rocks. The second major phase of extension affected the region from ~0.9 to 0.7 Ga., during which time Chuar Group sediments were deposited in elongate basins created by the reactivation of older NE-trending and NW trending faults. This protracted period of extension was eventually brought to a close by uplift and subsequent extension at ~ 0.7 Ga that resulted in the deposition of the overlying Sixtymile Formation.


The Proterozoic Chuar Group has gained attention as a potential hydrocarbon source due to high TOC (total organic carbon) concentration in the 3-9% range, seen primarily within the Walcott Member of the Kwagunt Formation. This potential source rock, combined with excellent reservoir potential of the overlying Cambrian Tapeats Sandstone, with the seal consisting of the overlying Cambrian Bright Angel Shale, comprise an intriguing three part petroleum system currently being explored within southern Utah. Prospective structures include anticlines of Laramide age that have not been drilled through the Tapeats interval. A key result of this study established the presence of Chuar-age basins in regions south of the Utah state line, thereby redefining the southernmost boundary of this potential hydrocarbon play within the study area in north-central Arizona.



The following poster was presented at American Association of Petroleum
Geologists (AAPG) Annual Convention, 1999, in San Antonio, Texas:

-- Intro, Methodology, Regional Geology (18.9 Mb)
-- Gravity, Magnetics, 2.5D Gravity Modeling (8.8 Mb)
-- Paleogeographic Reconstruction and Hydrocarbon Potential (17.6 Mb)


The following paper was published in the Bulletin of the American Association of
Petroleum Geologists, vol. 87, no. 8 (2003):

Seeley, J. M., and Keller, G. R. (2003) Delineation of Subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona utilizing gravity and magnetics: Implications for hydrocarbon source potential: (8.2 Mb text) AAPG Bulletin, v. 87, no. 8, (cover) pp. 1299-1321.