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AAPG Bulletin, V.
Emplacement and evolution of the Mahogany salt body, central Louisiana outer shelf, northern Gulf of Mexico
1Rowan Consulting Inc., 1633 D 4th St., Boulder, Colorado, 80302; email: [email protected]
2Geo-Logic Systems, LLC, 3434 47th St., Boulder, Colorado, 80301; email: [email protected]
3T. H. Huxley School, Imperial College, RSM Building, Prince Consort Rd., London SW7 2BP, United Kingdom; email: [email protected]
4PEMEX-Exploracion y Produccion, Domingo Sabio 350-9, Arboledas del Sur, Tlalpan, Mexico, D.F., 14380; email: [email protected]
Mark Rowan received a B.S. degree from Caltech (1976), an M.S. degree from the University of California-Berkeley (1982), and a Ph.D. from the University of Colorado (1991). He has worked as an exploration geologist with Sohio, a consultant with Geo-Logic Systems and Alastair Beach Associates, and a research professor at the University of Colorado. He is now an independent consultant specializing in structural geology and salt tectonics and the instructor for AAPG's salt tectonics school.
Bob Ratliff received his Ph.D. in structural geology from the University of Colorado in 1992. He was one of the first employees of Geo-Logic Systems (GLS) in 1987 and from 1992-1998 was assistant director of the CogniSeis Development Boulder Center, where he was responsible for the technical content and marketing of the GeoSec, GeoSec3D, and GeoStrat software packages. He rejoined GLS in 1998 and is currently director of Product Development and active in research on the geometry, kinematics, and interpretation of rock deformation.
Bruce Trudgill is a graduate in geology from Aberystwyth University in Wales and gained a Ph.D. from Imperial College in London studying aspects of basin inversion. Subsequently, he worked for Amerada-Hess in the United Kingdom and studied fault linkages in southeast Utah as a research fellow at Imperial College. In 1994, he joined the Energy & Minerals Applied Research Center (EMARC) group at the University of Colorado, working on salt tectonics in the Gulf of Mexico. Since January 2000 he has been a lecturer in geology and geophysics in the T. H. Huxley School at Imperial College, London. His main research interests are centered on the evolution of sedimentary basins through time, salt tectonics, growth of fault systems, and geomorphological responses to tectonic activity.
Jaime Barceló Duarte received his Ph.D. in geological sciences from the University of Texas at Austin in 1983. Jaime was the chairman of the Exploration Department at the Universidad Nacional Autonoma de Mexico until 1997. He is currently with PEMEX, where his main activities deal with sedimentological and stratigraphic modeling of siliciclastics in the Gulf of Mexico.
We thank Davis Ratcliff and Diamond Geophysical for the three-dimensional seismic data. We also acknowledge Phillips, Anadarko, BP Amoco, Unocal, Mobil, and Paleo-Data, Inc. (Art Waterman) for well and biostratigraphic data; Western Geophysical for additional seismic data; and Landmark and CogniSeis (now Paradigm Geophysical) for their software. Helpful comments from Bert Bally, Wayne Camp, James Emme, and Cindy Yeilding improved the manuscript, for which we are grateful. The research was carried out at the Energy & Minerals Applied Research Center (EMARC) at the University of Colorado, where Rowan and Trudgill were funded by an industrial consortium consisting of Agip, Amerada-Hess, Amoco, Anadarko, BHP, BP, Burlington, Canadian Oxy, CNG, Conoco, Enterprise, Exxon, Marathon, Maxus, Mobil, Occidental, Phillips, Shell, Texaco, Union Pacific, Unocal, and Vastar. We thank Paul Weimer and David Knapp for their contributions within EMARC.
We used three- and two-dimensional seismic data, well and biostratigraphic constraints, and structural restorations to evaluate the evolution of the Mahogany salt body and surrounding strata on the central Louisiana outer shelf of the northern Gulf of Mexico. The history of emplacement and subsequent modification of the Mahogany salt body is divided into six stages: (1) growth of a basinward-leaning salt ridge sourced from a deep allochthonous salt sheet prior to 7.5 Ma; (2) loading-induced evacuation of a large volume of salt from the sheet and ridge through a narrow feeder, resulting in rapid salt flow and consequent radial growth of a bulb-shaped salt stock between 7.5 and 4.3 Ma; (3) gravitational collapse of the inflated salt stock after depletion of the deep source layer, leading to basinward extrusion of a subhorizontal salt tongue and its rafted overburden between 4.3 and 3.65 Ma; (4) burial of the composite salt body during a period of slow sedimentation between 3.65 and 1.95 Ma; (5) basinward translation of the overburden resulting from gravity gliding/spreading on the upper slope between 1.95 and 0.5 Ma, causing extension, reactive diapirism, and matching contraction; and (6) cessation of lateral translation once the shelf margin prograded past the salt body, and subsequent loading of the salt driving active diapirism in the footwalls of normal faults. The Mahogany salt body provides an excellent case study that illustrates both the value and limitations of published simple models for allochthonous salt, and the reconstructed evolution yields insights into the complex interactions between salt deformation and sedimentation. The results also suggest that the Mahogany salt body did not influence the trap style of the subsalt Mahogany field or hydrocarbon migration into the pay sands but that it did affect sediment transport pathways and, to a lesser degree, reservoir facies distribution.
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