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SEQUENCE STRATIGRAPHY 171 Tectonics Figure Interaction between subsidence and multiple orders of eustatic cyclicity, and the effect on sequence stratigraphy Third , fourth , and fifth order eustatic changes combine with subsidence to produce a relative sea level curve, which controls the character of the sequence expression Section I reflects de position during a period of lower accommodation related to the third order sea level fall; points and identify fourth order sea level falls corresponding to sequence boundaries (SB) Section II reflects deposition during a period of greater accommo dation related to the third order sea level rise; points a and b designate fourth order sea level rises corresponding to para sequence boundaries (PSB) (Adapted from Van Wagoner JC, Mitchum RM, Campion KM, and Rahmanian VD (1990) Siliciclastic sequence stratigraphy in well logs, cores, and outcrops American Association of Petroleum Geologists Methods in Exploration Series 7: 55.) into an area, and is capped by a flooding surface after sedimentation shifts to another lobe and the abandoned lobe sinks through subsidence and compaction In a carbonate system, upward growth of carbonate sediments normally produces an upward shoaling pattern until all the available accommodation space is used; carbonate production is then shut off until subsidence provides space for carbonate growth to resume These successions may occur repeatedly at scales of tens of thousands of years, similar to the time scales of higher order sea-level cycles Thus, differentiating autocyclicity from allocyclicity in high-resolution sequence analysis may be difficult Tectonics is another major factor that contributes to accommodation; subsidence creates space for sediment accumulation, and uplift takes it away Subsidence patterns vary according to tectonic setting In an extensional basin, subsidence may be a response to lithospheric thinning and cooling following rifting Subsidence due to extensional faulting is typically greatest in the early phases and decreases with time In a foreland basin, subsidence may be a response to lithospheric flexure due to loading in the adjacent fold-thrust belt Subsidence in foreland basins commonly accelerates over time as the load on the lithosphere increases during thrusting, followed by post-orogenic rebound In cratonic basins, subsidence may be due to a subtler regional warping Tectonics is considered to be generally too slow a process to cause third- and higher-order sequences Certainly, movement on individual faults can be significant at shorter time scales However, overall, changes in rates of basin subsidence tend to occur on a time scale of tens of millions of years, which is a broader period than third-order sea-level variations As a result, tectonics exerts a broader-brush influence on the nature of expression of the sequences, upon which higher-frequency sea-level events are superimposed (Figure 9) Sediment Supply Sediment supply controls how rapidly the accommodation space is filled, and the balance between accommodation and sediment supply controls sediment stacking patterns and thus the expression of sequences In a setting with a high input of sediments, such as a delta, sedimentation may fill the available space in the two systems tracts with lower accommodation, the lowstand wedge and the highstand systems tract As a result, these intervals will be characterized by aggradational and progradational packages The sequence boundary reflects low accommodation relative to sediment input Retrogradation of the depositional system will probably occur only at the peak of transgression, and condensed sections will be developed only in offshore locations In settings with a low influx of sediments, accommodation may exceed the ability of the sedimentary system to fill it In such cases, only during the part of the sequence when sea-level is falling and accommodation is lowest will sediment advance basinwards The transgressive and highstand systems tracts will probably be characterized by sediment starvation and represented as a condensed section Climate changes arising from Milankovitch cycles can exert a strong influence on sediment supply
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