Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 From WILLIAMS, G. D. & DOBB, A. (eds), 1993, Tectonics and Seismk" Sequence Stratigraphy. Geological Society Special Publication No. 71, 1-13. Tectonics and seismic sequence stratigraphy: an introduction G. D. Williams Geology Department, University of Keele, Keele, Staffordshire ST5 5BG, UK Abstract: The application of sequence stratigraphic models to seismic data is restricted by the vertical and spatial resolution of the data. The fundamental stratigraphic unit of the sequence stratigraphic technique is the seismically distinguished 'sequence' although the concept of tectonically related 'mega- sequences' is useful. Basin stratigraphy is controlled to varying degrees by eustatic sea-level change (or base level in lakes), subsidence/uplift (tectonics) and sediment supply. Three main basin types, rift-, wrench-, and thrust-related basins, have distinctive gross stratigraphic architectures. Localized, tectonically controlled subsidence and uplift has a significant control on three-dimensional stratigraphic patterns. Stratigraphy is the partial record of basin evolution resulting from the interaction of a number of factors such as regional and local subsidence/uplift, sediment supply, eustacy and climatic change (Fig. 1). Basins that have a marine connection will show significant stratigraphic variations, especially in coastal deposits, that result from eustatic sea-level fluctuations. Other key factors will include the availability of sediment and the role of tectonically induced subsidence and uplift. Alternatively, land-locked or intermontane basins whose fluvial systems drain ultimately into a lake will preserve a stratigraphy dominated by local relative uplift and subsidence (local tectonics), by climatic controls and the rate of sediment supply. Tectonic activity is an important factor in controlling stratigraphy in the majority of sedimentary basins. Tectonism generates accommodation space in basins, it alters base levels and it controls source areas. When mountain ranges are generated, tectonics may even influence local climatic patterns. It is difficult to separate the relative importance of tectonics versus other factors in controlling stratigraphic architecture in basins. In this introduction, basins formed on continental lithosphere and their associated stratigraphies will be considered. Seismic resolution Increasing resolution of seismic sections invites direct comparison with stratigraphic cross-sections. The geophysical limitations of seismic data should be appreciated before interpretation is attempted. Seismic sections represent the response of the Earth to seismic waves, and because most reflections are interference composites there is no direct correspondence between seismic events and interfaces in the Earth (Sheriff 1977). The stratigraphic significance of seismic data is apparent only when tied to well data. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 2 G.D. WILLIAMS (a) (b) m Subsidence/Uplitt Subsidence/UpliR //•(regional and local) ~ (local) Is. T,G ..cl Q Is r,a .,cl I CHmI CmREI I e , CmREI I OF BASIN I I OF SIN I (Rateof Change) SedimentSupply ~_ (Rateof Changel ] SedimentSupply t / Fig. 1. Controls on the stratigraphic architecture of (a) basins with a marine connection; and (b) intermontane lacustrine basins. Seismic resolution is the factor which limits the amount of stratigraphic and structural data that can be obtained from seismic sections. Resolution is the ability to tell that more than one feature is contributing to an observed effect (Sheriff 1985). The vertical resolvable limit in seismic data is equivalent to one-quarter of the dominant wavelength. A reflector with a velocity V and a wavelet with a dominant frequency f has a quarter wavelength of V/4f. For example, a poorly consolidated sand-shale section at c. 1500 m depth would have a vertical resolvable limit of c. 7.6 m. Deep reflectors are characterized by higher velocities and lower frequency and might show a quarter wavelength thickness of c. 76m (Sheriff 1985). Many important stratigraphic observations must be made at sequences that are thinner than these values. Expanded stratigraphic packages in local depocentres may become condensed on to highs. Individual seismic reflectors in the expanded sequence may die out or interfere with other reflectors in the condensed sequence as the reflector separation reduces to less than the resolvable limit. Therefore, the resolvable limit is vitally important for stratigraphic interpretation. Spatial or horizontal resolution on unmigrated sections is described in terms of the Fresnel zone. The first Fresnel zone is the area of a reflector from which the reflected energy reaches the detector within a half cycle so that interference is constructive. The dimension of the Fresnel zone depends on the seismic frequency, with higher frequencies generating smaller Fresnel zones for a given reflector. Spatial resolving power deteriorates with depth. A deep reflector must have larger areal extent than a shallower reflector to produce the same effect on a seismic section (Sheriff 1977). With migrated seismic data, Fresnel zone size is not an important consideration. Migration works properly only on primary reflection and diffraction energy and it smears out noise to give migration 'smiles'. Migration of noise is one of the factors that limits spatial resolution on migrated sections. Sequence stratigraphic concepts Sloss (1950) first defined a 'sequence' as an unconformity-bounded stratal unit. This led to the recognition of six 'sequences' on the North American craton ranging in age from late Precambrian to Holocene (Sloss 1963). Mitchum et al. (1977) provided the Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 INTRODUCTION 3 present definition of a sequence as 'a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities and their correlative conformities'. The 'sequence' of Mitchum et al. (1977) incorporates a much smaller time range than that of Sloss (1963) and, therefore, the six sequences of the North American craton could be multiply sub- divided. Sloss' sequences were renamed 'super-sequences'. Vail et al. (1977a,b) proposed that eustacy is the predominant driving mechanism for the generation of sequences. This has been the subject of much recent debate (e.g. Galloway 1989) and the concept is addressed by the majority of authors in this volume. Seismic sequence analysis introduced by AAPG Memoir 26 (Payton 1977) was further advanced via seminal articles by Jervey (1988) and Posamentier et al. (1988). A key element in such analysis is the recognition of seismically distinguished sequences bounded by chronostratigraphically significant surfaces of erosion, non- deposition or their correlative surfaces. A sequence boundary is a single widespread surface that separates all of the rocks above from all of the rocks below the boundary and it forms independently of sediment supply. A sequence boundary is commonly marked by significant erosional truncation beneath and onlap above. A major research effort by the Exxon Group during the 1970s and 1980s led to the publication of the Exxon Cycle Charts (Fig. 2), (Vail et al. 1977b; Haq et al. 1987, 1988) which record global sea-level fluctuations through Phanerozoic time. The role of tectonics versus eustacy in the formation of sequence boundaries has been widely debated (e.g. Bally 1982; Thorne & Watts 1984; Cloetingh 1986; Hubbard 1988; Sloss 1988). Hubbard (1988) described 'megasequence boundaries' in the Santos, Grand Banks and Beaufort basins that related to folding and faulting of underlying strata at the various stages of basin evolution. Hubbard described stages of rift onset, syn-rift faulting and rift termination with reference to basin megasequences. Megasequence boundaries were shown to have an average cyclic frequency of c. 50 Ma in contrast to sequence boundaries which have a frequency range of 10-15 Ma. Cloetingh (1986) proposed that temporal stress variations in the lithosphere of a few hundred bars can explain stratigraphic changes at passive margins and intra- cratonic basins. A stress change of 1 kbar would produce an apparent sea-level change of > 50 m and such stress changes could occur episodically over a few million years. Therefore, rapid sea-level changes on the Exxon Cycle Charts could represent plate tectonic reorganization of lithospheric stress fields (Bally 1982). Factors controlling basin stratigraphy Sea-level change Evidence for the relative change in sea-level relies on the fact that regional cycles determined on different continental margins are synchronous and the magnitude of relative sea-level change is similar (Vail et al. 1977a,b). The global cycle charts of Vail et al. (1977b) show cycles of three orders of magnitude. Two first-order cycles in the Phanerozoic are present: the older from Precambrian to early Trias (300Ma duration); and the younger from mid-Trias to the present (225 Ma duration). There are fourteen second-order cycles from 10-80 Ma duration and over eighty third- order cycles with durations of 1-10 Ma (Fig. 2). Van Wagoner et al. (1990) have Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 4 G.D. WILLIAMS •j m l RELATIVE CHANGES OF SEA LEVEL ,..,= , 2 ~ RISING FALLING (-~. "= 1.0 .5 0 (/1 O-- I I I I I I I I I I I Q Td ~" \ 'm2.3 TId2.2 ~ -x..~