Abstract: Seismic Evidence for Low-Angle Extensional Faults Beneath Southern Railroad Valley, Nevada

J. A. Grow, C. J. Potter, J. J. Miller, K. Lund, W. J. Perry Jr.

Six seismic reflection lines in Railroad Valley near the Grant Canyon, Bacon Flat, and Sans Spring oil fields have been analyzed and integrated with drill-hole data and geologic mapping in the Grant Range Beneath the eastern side of the valley, immediately north and east of the Grant Canyon field, valley fill onlaps a planar reflecting surface that dips 10°-30° WNW. The surface undulates slightly, is gently scooped-shaped, and is not cut by significant high-angle faults within the resolution of the seismic data (<50 msec 2-way travel-time or <300 ft at a velocity of 12,000 ft/sec). The surface projects eastward and upward into the Grant Range, where it is interpreted as part of a system of west-dipping (10°-30°) attenuation faults that separate b ittlely deformed upper-plate sedimentary rocks of late Paleozoic age from lower-plate Cambrian and Ordovician sedimentary rocks. Attenuation faults in this part of the range are generally subparallel or at low angles to Paleozoic strata; both the Paleozoic rocks and the low-angle faults are deformed into a broad antiform. Reservoir blocks of the Grant Canyon and Bacon Flat oil fields are stranded hanging-wall blocks sitting on the low-angle surface. Drill holes have penetrated 2000 ft beneath the low-angle reflecting surface and have encountered granite east and northeast of the Grant Canyon field (subsurface continuation of the Late Cretaceous Troy pluton, cropping out 4 miles southeast of the Grant Canyon field) or late Paleozoic sedimentary strata that dip gently to the northwest in w lls north and northwest of the field. In two wells where formation tops were identified, the late Paleozoic section is attenuated.

In 1994, T.J. McCutcheon and W.D. Zogg interpreted four detachment faults east of the Grant Canyon field dipping 14°-29° NW and proposed that the faults rotated to lower angles by tectonic unloading of the hanging-wall blocks. Our seismic data are consistent with their observations east of the field. However, we propose that the low-angle surface (10°-20° WNW dip) north of the field, which is underlain by the late Paleozoic strata, is the erosionally denuded remnant of a major fault surface. We interpret this fault as the upper surface of a thick system of attenuation faults that, including those McCuthceon and Zogg, affect Cambrian through Oligocene strata in the Grant Range and project beneath the Grant Canyon and Bacon flat hanging-wall blocks. North of the fiel s, the upper surface of this fault system continues westward to the valley axis and appears to project beneath several large, coherent east-tilted hanging-wall blocks (also late Paleozoic) just west of the axis, including the one on which the Sans Springs oil field was recently discovered. The inferred low-angle fault system cannot actually be seen in our seismic data west of the valley axis, but this absence is probably be due to the low velocity and density-contrast between the hanging-wall and footwall blocks and multiple noise caused by a basalt flow in the upper valley fill.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada