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Abstract

AAPG Bulletin, V. 82 (1998), No. 1 (January 1998), P. 110-146.

Architecture of Marine Rift-Basin Successions1

R. Ravnås2 and R. J. Steel3

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved
 

1Manuscript received December 14, 1995; revised manuscript received December 9, 1996; final acceptance June 16, 1997.
2University of Bergen, Geological Institute, Allegaten 47, N-5007 Bergen, Norway. Present address: Norske Conoco A.S., P.O. Box 288, N-4001 Stavanger, Norway.
3University of Wyoming, Department of Geology and Geophysics, Laramie, Wyoming 82071-3355.

We thank our fellow colleagues and research students in the Joule II: Integrated Basin Studies-Dynamics of the Norwegian Margin (IBS-DNM) project for stimulating cooperation and discussions. Constructive comments by K. T. Biddle, C. K. Morley, T. H. Nilsen, and F. Surlyk on an earlier version of the manuscript sharpened the final product and are greatly appreciated. The drafting offices at the Geological Survey of Wyoming, Statoil, Norsk Hydro, and the Geological Institute, University of Bergen, created the illustrations. This work was funded by the Research Council of Norway as part of the Joule II Research Programme (CEC Contract No. JOU2-CT92-0110). 

For clarification, the term "rift margin depositional system" as used in this paper refers to a geological feature that originates at the rift margin but may extend out into the basin.

ABSTRACT

Marine rift basins represent a continuum ranging from mixed nonmarine/marine through shallow marine to deep marine, or from partly emergent through partly submergent to completely submergent basin types. These rift basin types have strongly variable synrift sedimentary architectures because of temporal changes in relative sea level, accommodation creation, and sediment supply throughout the rift cycle. Accommodation changes are controlled mainly by local basin-floor rotation, basinwide background subsidence, and, to a lesser degree, by eustatic changes. Sediment supply determines how much of the accommodation is filled and in what manner, and is controlled by the distance to the main hinterland areas, and the size and sediment- yield potential of any local fault-block source area.

Marine siliciclastic synrift successions, whether dominantly shallow or deep marine in nature, are classified in terms of sediment supply as overfilled, balanced, underfilled, and starved. Sediment-overfilled and sediment-balanced infill types are characterized by a threefold sandstone-mudstone- sandstone synrift sediment-infill motif; the sediment-underfilled type is represented by a two-fold conglomerate-sandstone-mudstone motif; and the sediment-starved type commonly is represented by a one-fold mudstone motif. The sequential development, linked depositional systems, and stratigraphic signatures of the early synrift, the rift climax, and the late synrift to early postrift stages vary significantly between these rift basin infill types, as do the tectonic significance (timing of initiation and duration) of stratal surfaces, such as footwall unconformities, nondepositional hiatuses, and marine condensed sections. The construction of the fourfold rift basin infill classification scheme provides a first basis and a strong tool for predicting the distribution and geometry of synrift reservoir and source rock types, despite the inherent variability of the marine synrift infills. 

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