![]() He thus developed three principles that form the basis of all stratigraphy-younger layers lie on top of older layers, layers are initially horizontal, and layers continue until they run into a barrier. He also recognized that some strata contain remnants of other strata, and so must be younger. In 1669, Nicholas Steno, a Darnish geologist working in Italy recognized that strata are formed as heavy particles settle out of a fluid. Stratigraphy in one form or another has been around since the 1600s. It can therefore provide a predictive tool for determining the likely presence of source rocks, and the distribution of reservoirs and seals. An explanation of strata in terms of relative sea-level fluctuations and a combination of eustatic sea-level change and tectonic subsidence allows an understanding of why sediment packages develop where they do. The implications of sequence stratigraphy are profound. It was clear that the stratal patterns in these areas of low tectonic activity were the signatures of sea-level rise and fall. The stratal patterns were as distinctive as the biostratigraphic correlations. Moreover, it was recognized from the circum-Atlantic passive margins that the stratal pattern in one area could be correlated with others that were far distant. It was founded on the same principle as used in seismic stratigraphy, that seismic reflectors are time surfaces and that unconformites are bounding surfaces that separate strata into time-coherent packages. Sequence stratigraphy was developed from seismic stratigraphy in the 1970s, by workers in the Exxon research facility. See sequence, sequence boundary, and Haq chart. The study of rock relationships within a chronostratigraphic framework of genetically related strata bounded by surfaces of erosion or nondeposition or their correlative conformities (Van Wagoner, 1995). A scale-independent approach to methodology and nomenclature is key to a consistent application of sequence stratigraphy across the entire range of geological settings, stratigraphic scales, and types of data available.Dictionary entry for Sequence stratigraphy ( edit) ![]() The scale of the lowest rank systems tracts at any location defines the highest resolution that can be achieved with a stratigraphic study. The inability to identify the smallest sequence in every study indicates that the classification of stratigraphic cycles is best approached from the ‘first-order’ basin fill as the anchor for the definition of hierarchical ranks. In the context of seismic stratigraphy, the smallest identifiable ‘sequence’ is typically not the smallest stratigraphic cycle within the study area, but the smallest stratigraphic cycle that is above the resolution of the seismic data. The scale of the smallest stratigraphic cycle that can be identified at any location depends on the resolution of the data available. Stratigraphic cyclicity is basin-specific in terms of timing and scales, reflecting the importance of local controls on accommodation and sedimentation. Scale is a key topic in sequence stratigraphy, with implications for methodology, nomenclature, and the classification of stratigraphic cycles that develop at different scales. Beyond the purpose of the methodological workflow, the modelling and testing of the possible controls on sequence development can continue indefinitely after the construction of a sequence stratigraphic framework. The construction of a sequence stratigraphic framework follows a standard workflow guided by the observation of stratal stacking patterns at scales defined by the purpose of study or by the resolution of the data available. Stratal stacking patterns provide the key to the definition of all units and surfaces of sequence stratigraphy, in a manner that is independent of the interpretation of the underlying controls.
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