1、Basic terms of Sequence StratigraphyAccommodation. Another term for relative sea-level. Can be thought of as the space in which sediments can fill, defined at its base by the top of the lithosphere and at its top by the ocean surface. Eustatic Sea Level. Global sea level, which changes in response t
2、o changes in the volume of ocean water and the volume of ocean basins. Highstand Systems Tract. Systems tract overlying a maximum flooding surface, overlain by a sequence boundary, and characterized by an aggradational to progradational parasequence set.Lowstand Systems Tract. Systems tract overlyin
3、g a type 1 sequence boundary, overlain by a transgressive surface, and characterized by a progradational to aggradational parasequence set. Marine Flooding Surface. Surface separating younger from older strata, across which there is evidence of an abrupt increase in water depth. Surface may also dis
4、play evidence of minor submarine erosion. Forms in response to an increase in water depth.Parasequence. Relatively conformable (that is, containing no major unconformities), genetically related succession of beds or bedsets bounded by marine-flooding surfaces or their correlative surfaces. Paraseque
5、nces are typically shallowing-upward cycles. Parasequence Set. Succession of genetically related parasequences that form a distinctive stacking pattern, and typically bounded by major marine flooding surfaces and their correlative surfaces. Parasequence set boundaries may coincide with sequence boun
6、daries in some cases. See progradational, aggradational and retrogradational parasequence sets.Relative Sea Level. The local sum of global sea level and tectonic subsidence. Locally, a rise in eustatic sea level and an increase in subsidence rates will have the same effect on accommodation. Likewise
7、, a fall in eustatic sea level and tectonic uplift will have the same effect on accommodation. Because of the extreme difficulty in teasing apart the effects of tectonic subsidence and eustatic sea level in regional or local studies, sequence stratigraphy now generally emphasizes relative changes in
8、 sea level, as opposed to its earlier emphasis on eustatic sea level. Sequence. Relatively conformable (that is, containing no major unconformities), genetically related succession of strata bounded by unconformities or their correlative conformities. Sequence Stratigraphy. The study of genetically
9、related facies within a framework of chronostratigraphically significant surfaces.Shelf Margin Systems Tract. Systems tract overlying a type 2 sequence boundary, overlain by a transgressive surface, and characterized by a progradational to aggradational parasequence set. Without regional seismic con
10、trol, most shelf margin systems tracts may be unrecognizable as such and may be inadvertently lumped with the underlying highstand systems tract as part of one uninterrupted progradational parasequence set. If this occurs, the overlying transgressive surface may be erroneously inferred to also be a
11、type 1 sequence boundary.Systems Tract. Linkage of contemporaneous depositional systems, which are three-dimensional assemblages of lithofacies. For example, a systems tract might consist of fluvial, deltaic, and hemipelagic depositional systems. Systems tracts are defined by their position within s
12、equences and by the stacking pattern of successive parasequences. Each sequence consists of three systems tract in a particular order. For a type 1 sequence, these are the lowstand, transgressive, and highstand systems tracts. For a type 2 sequence, these are the shelf margin, transgressive, and hig
13、hstand systems tracts.Transgressive Systems Tract. Systems tract overlying a transgressive surface, overlain by a maximum flooding surface, and characterized by a retrogradational parasequence set.Type 1 Sequence Boundary. Characterized by subaerial exposure and associated erosion from downcutting s
14、treams, a basinward shift in facies, a downward shift in coastal onlap, and onlap of overlying strata. Forms when the rate of sea-level fall exceeds the rate of subsidence at the depositional shoreline break (usually at base level or at sea level). Note that this means that if such changes can be ob
15、served in outcrop and the underlying strata are marine, then the boundary is a type 1 sequence boundary. Type 2 Sequence Boundary. Characterized by subaerial exposure and a downward shift in onlap landward of the depositional shoreline break (usually at base level or at sea level). Overlying strata
16、onlap this surface. Type 2 sequence boundaries lack subaerial erosion associated with the downcutting of streams and lack a basinward shift in facies. Forms when the rate of sea-level fall is less than the rate of subsidence at the depositional shoreline break. Note that the lack of a basinward shift in facies and the lack of a relative fall in sea level at the depositional shoreline break means that there are essentially no criteria by which to recognize a type 2 sequence boundary in outcrop.
