1992; Roberts et al., 1993). ... (Kusznir et al., 1991; Roberts and Yielding, 1991). According to .... Frazer et al. (1990) have summarized Carboniferous rift, post-.
Marine and Pe~o&umGeology, Vol. 12, No. 8, pp. 881-901 1995 Elsevier Science Ltd Printed in Great Britain 0264-8172/95 $10.00+ 0.00
rglUTTERWO RTH B~]E I N E M A N N
Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea A. Nottvedt* Department of Geosciences, Norsk Hydro Research Centre, N-5020 Bergen, Norway
R. H. Gabrielsen Geological Institute, University of Bergen, A Ilegaten 41, N-5007 Bergen, Norway
R. J. Steel Department of Geology and Geophysics, University of Wyoming, PO Box 3006, Laramie, Wyoming 82071, USA Received 15 February 1995; revised 15April 1995; accepted9 May 1995 A tectonostratigraphic model for the evolution of rift basins has been built, involving three distinct stages of basin development separated by key unconformities or unconformity complexes. The architecture and signature of the sediment infill for each stage are discussed, with reference to the northern North Sea palaeorift system. The proto-rift stage describes the rift onset with either doming or flexural subsidence. In the case of early doming, a proto-rift unconformity separates this stage from the subsequent main rift stage. Active stretching and rotation of fault blocks during the rift stage is terminated by the development of the syn-rift unconformity. Where crustal separation is accomplished, a break-up unconformity commonly marks the boundary to the overlying thermal relaxation or post-rift stage. Tabular architectures, thickening across relatively steep faults, characterize the proto-rift stage. Syn-rift architectures are much more variable. Depending on the ability of the sediment supply to fill the waxing and waning accommodation created during rotation and subsidence, one-, two- or three-fold lithosome architectures are likely to develop. During the post-rift stage, an early phase with coarse clastic infilling of remnant rift topography often precedes late stage widening of the basin and filling with fine-grained sediments. Keywords: tectonostratigraphy; rift basins; North Sea
Continental lithospheric extension and basin formation have been attributed to pure shear (McKenzie, 1978), simple shear (Wernicke, 1985) and coupled simple shear/pure shear flexural deformation (Lister et al., 1986; Kusznir et al., 1991; Kusznir and Ziegler, 1992). The coupled simple shear/pure shear flexural model ascribes the fundamental deformation mechanism of crustal extension to faulting (simple shear) in the upper crust and ductile stretching (pure shear) in the lower crust. The combined thermal and elastic/isostatic response of the lithosphere to extension controls the crustal architecture and thereby the geometry of sedimentary basins (van der Beek et al., 1994). In the northern North Sea, despite the substantial amount of data available, our understanding of the lithospheric processes governing extension are strongly model-based. Although researchers agree on a model involving polyphase stretching and subsequent thermal cooling, there is considerable disagreement as to the * C o r r e s p o n d e n c e to: D r A . NC~ttvedt
deeper geometries and the relative amount, timing and nature of this long-term continental lithospheric extension. In his classical paper on pure shear rifting, McKenzie (1978) used the North Sea as one of his type examples. The symmetrical pure shear model was subsequently favoured by Giltner (1987), Badley et al. (1988) and Klemperer (1988), who proposed models which involved mid-crustal decoupling along a subhorizontal detachment. The simple shear model has been applied by other workers to explain the asymmetry of the graben system (Beach, 1986; Beach et al., 1987; Gibbs, 1987; Scott and Rosendahl, 19891) and the distribution of active seismic zones (Gabrielsen, 1988). Recent studies have suggested that elastic flexural effects on both the crustal and fault block scale are important factors in the stretching process (Marsden et al., 1990; Kusznir et al., 1991; Cloetingh and Kooi, 1992; Roberts et al., 1993). The subsidence history of extended continental margins consists of a phase of short-lived (/100 Ma)
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Tectonostratigraphy and sedimentary architecture of rift basins: A. NCttvedt et al. active stretching. Further complications arise from studies of the geometry of rift infill stratigraphy. Wedge-shaped lithosome geometry is a commonly used criterion to identify the rifting stage, particularly in reflection seismic data (Cartwright, 1987; Prosser, 1993). However, bevelling and infilling of remnant relief created during active stretching and fault block rotation commonly extend into the post-rift stage of thermal subsidence. The gross wedge geometries thus produced, particularly where there has been underfilling during rifting, are not exclusively related to the active stretching stage alone; but are composite (passive infilling, compaction) in origin. Only by detailed examination of their internal geometries may such depositional units be properly related to the structural history. These complications are, nevertheless, locally constrained. In a broad perspective, intraplate rift zones such as the North Sea palaeorift system may be subject to repeated stretching events separated by periods of thermal subsidence. In the present paper we attempt to discuss and summarize various key structural elements and tectonostratigraphic components of extensional rift basins and to evaluate how these relate in space and time. We focus particularly on the principles and style of sediment infilling during the various stages of rift evolution, with reference to the multiphase northern North Sea palaeorift system.
passive subsidence caused by thermal relaxation of the heated lithosphere (Kinsman, 1973; McKenzie, 1978). An incipient phase of either flexural subsidence (Harding, 1984; Scott and Rosendahl, 1989) or mantle plume-derived domal uplift (Morgan, 1971; Ziegler, 1982; Mohr, 1992) before major stretching of the lithosphere is similarly short-lived (
