Geometrical arrangements of facies or smaller-scale sedimentary cycles (`parasequences’) may be such that systems tracts can be recognized in single vertical sections at outcrop or within a borehole (Van Wagoner et al. Helland-Hansen & Gjelberg 1994 Helland-Hansen & Martinsen 1996). Within sequences, further, more subtle geometrical and facies relationships have been used to define systems tracts (Van Wagoner et al. A third important surface is the transgressive surface, which is generally taken to be the first significant marine flooding surface within the sequence. (It is the maximum flooding surface that normally defines the limits of Galloway’s (1989) genetic stratigraphical sequences: see below). The more landward portions of the maximum flooding surface may, by contrast, be hidden within a thick succession of relatively shallow marine or non-marine strata. The more distal portion of the maximum flooding surface represents, like the sequence boundary, a break in deposition or very slow sedimentation, but unlike the sequence boundary it develops at the far end of the sediment transport path as a result of sediment starvation, and may be characterised by condensed marine deposits containing an abundant pelagic fauna and well-developed early (sea-floor) authigenic mineralisation, especially with glauconite and phosphate. In addition to the sequence boundary, two other important surfaces occur within a sequence. Conversely, the distal region of the sequence boundary may be represented by the increased volume of sedimentary debris eroded from the more landward sites. The proximal region of the sequence boundary is characterized by erosion of the underlying strata and may include such features as river valleys cut into previously deposited marine strata of the underlying surface. Accommodation is a loosely-defined term meaning space available for sediment to accumulate: this space is capped by a dynamic ‘accommodation limit’, a surface which passes through the shoreline and is itself dependent on sediment supply and transport processes. 1988) the unconformity surface represents the proximal area of a sequence boundary and passes into expanded sedimentary successions in more basinal settings it develops when proximal accommodation is no longer available. In the Exxon model of sequence stratigraphy (e.g. Relative sea-level change is the net result of global sea-level change combined with local subsidence or uplift of the depositional area (Posamentier & Vail 1988).Īn important practical aspect of sequence stratigraphy is the recognition of key surfaces. The principal factor thought to govern the genesis of a depositional sequence is relative sea-level change. No temporal or thickness scale is given in this figure because sequences develop in a hierarchical fashion at a great range of scales (e.g. A generalized model of a depositional sequence, including details of the internal geometries, is shown in Fig. hillside) field exposures, or are inferred by correlations from smaller locations. The geometrical relationships are observable from seismic reflection profiles, extensive (e.g. These surfaces are defined as the sequence boundaries and the strata between them constitute a depositional sequence. The observational basis of sequence stratigraphy is the ubiquitous arrangement of strata into units bounded above and below by unconformities that can be traced out into conformable surfaces in a basinward direction. reviews by Emery & Myers 1996 Miall 1997). Sequence stratigraphy is concerned with the large-scale, three-dimensional arrangement of sedimentary strata, and the major factors that influence their geometries – such as sea-level change, contemporaneous fault movements, basin subsidence and sediment supply ( cf.
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