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AAPG Bulletin
Abstract
AAPG Bulletin, V.
The role of attenuation in the formation of the Railroad Valley structural basin, east-central Nevada: Detachment control of petroleum reservoirs
Robert D. Francis,
Charles T. Walker
1Department of Geological Sciences, California State University, Long Beach, California, 90840-3902; email: [email protected]
2Department of Geological Sciences, California State University, Long Beach, California, 90840-3902; email: [email protected]
AUTHORS
Robert D. Francis received his B.A. degree in physics in 1974 and his Ph.D. in earth science from Scripps Institution of Oceanography in 1980. He worked for Getty Oil Company (merged with Texaco in 1984) from 1980 to 1987, both in the research center as an organic geochemist and in the Texas Offshore district as an exploration geologist. From 1987 to the present, Francis has been a professor in the Department of Geological Sciences at California State University Long Beach, where his research interests include the geology of the Railroad Valley and eastern Nevada; seismic mapping of the Palos Verdes 
fault
, offshore southern California; and echo sounding study of offshore hydrocarbon seeps.
Charles Trevor Walker, professor emeritus at California State University Long Beach, received his B.Sc. degree and Ph.D. from the University of Leeds in England. In 1954 he began his professional career with the Saskatchewan Department of Mineral Resources and later joined Amoco in Calgary, Alberta, where he worked as an exploration geologist. From 1960 to 1964 he worked as an Imperial Chemical Industries research fellow at the University College of Wales in Aberystwyth and Swansea. In 1964, he joined the Department of Geological Sciences, California State University Long Beach, where he taught geology until his retirement in 1992. Major themes of his research include the origin of the Millstone Grit in northern England, the Devonian stratigraphy of Saskatchewan, the use of boron in illite as a paleosalinity indicator, catastrophic phase transitions as a source mechanism for deep and intermediate focus earthquakes, and the structural geology of east-central Nevada.
ACKNOWLEDGMENTS
We thank the Nevada Bureau of Mines and Geology, University of Nevada-Reno, Reno, Nevada, for providing well data, including wire-line logs, mud logs, and geochemical data, and for allowing microscopic examination of cuttings. We also thank Eugene R. Bottoms, Stanley C. Finney, and William W. Lumsden for their critical reading of the manuscript and constructive comments.
ABSTRACT
The eastern boundary of Railroad Valley has long been considered a large displacement steep normal fault. By mapping a low-angle fault
, called the White Pine detachment (WPD), from the ranges into the valley, we show that steep normal faulting was not a significant process in the formation of Railroad Valley. The WPD or its equivalent extends throughout the White Pine and Grant ranges, as shown by mapping of previous authors. Well data show that it is also present in Railroad Valley. Our structure-contour map of the WPD, based on surface and subsurface data, shows that the WPD dips uniformly from the ranges into Railroad Valley and is not displaced significantly by high-angle faulting. Even if high-angle faulting were assumed, the maximum displacement consistent with the data would be 2000 ft (610 m), much smaller than the total structural relief of at least 16,000 ft (4880 m) between the Grant Range and the adjacent valley. Structural elevations in the valley are low because the Paleozoic section has been greatly attenuated. A related contribution to structural relief is uplifting of the ranges caused by diapiric emplacement of plutons in response to tectonic unloading.
The WPD is only one of many detachments in Railroad Valley. We propose that most or all of the oil reservoirs are in attenuated, brittle blocks between detachments. Seals are provided by detachments or by the discontinuous nature of associated fracturing. Attenuation, by juxtaposing hot infrastructure rocks with rocks that normally occur thousands of feet above the granitic and metamorphic basement, may have contributed to maturation of source rocks.
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