sequence stratigraphy

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Contents : 

EaES 350-12 1 Contents Introduction Unconsolidated clastic sediments Sedimentary rocks Diagenesis Sediment transport and deposition Sedimentary structures Facies and depositional environments Glacial/eolian/lacustrine environments Fluvial/deltaic/coastal environments Shallow/deep marine environments Stratigraphic principles Sequence stratigraphy Sedimentary basins Models in sedimentary geology Applied sedimentary geology Reflection

Sequence stratigraphy : 

EaES 350-12 2 Sequence stratigraphy Sequence stratigraphy constitutes a ‘minor revolution’ in the Earth sciences, and has certainly revitalized stratigraphy Sequence stratigraphy highlights the role of ‘allogenic’ (or external) controls on patterns of deposition, as opposed to ‘autogenic’ controls that operate within depositional environments Eustasy (changes in sea level) Subsidence (changes in basin tectonics) Sediment supply (changes in climate and hinterland tectonics)

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Sequence stratigraphy : 

EaES 350-12 4 Sequence stratigraphy Accommodation refers to the space available for deposition (closely connected to relative sea level in shallow marine environments); however, application of this concept to subaerial environments is problematic An increase of accommodation is necessary to build and preserve a thick stratigraphic succession; this requires eustatic sea-level rise and/or basin subsidence (i.e., relative sea-level rise), as well as sufficient sediment supply The subtle balance between relative sea-level change and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur

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Sequence stratigraphy : 

EaES 350-12 9 Sequence stratigraphy A depositional sequence is a stratigraphic unit bounded at its top and base by unconformities or their correlative conformities, and typically embodies a continuum of depositional environments, from updip (continental) to downdip (deep marine) A relative sea-level fall on the order of tens of meters or more will lead to a basinward shift of the shoreline and an associated basinward shift of depositional environments; commonly (but not always) this will be accompanied by subaerial exposure, erosion, and formation of a widespread unconformity known as a sequence boundary Sequence boundaries are the key stratigraphic surfaces that separate successive sequences

Sequence stratigraphy : 

EaES 350-12 10 Sequence stratigraphy Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller-scale relative sea-level fluctuations Systems tracts are the building blocks of sequences, and different types of systems tracts represent different limbs of a relative sea-level curve Falling-stage (forced regressive) systems tract Lowstand systems tract Transgressive systems tract Highstand systems tract The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward facies successions, or by parasequence stacking patterns

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Sequence stratigraphy : 

EaES 350-12 12 Sequence stratigraphy Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller-scale relative sea-level fluctuations Systems tracts are the building blocks of sequences, and different types of systems tracts represent different limbs of a relative sea-level curve Falling-stage (forced regressive) systems tract Lowstand systems tract Transgressive systems tract Highstand systems tract The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward facies successions, or by parasequence stacking patterns

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Sequence stratigraphy : 

EaES 350-12 15 Sequence stratigraphy Maximum flooding surfaces form during the culmination of sea-level rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the transgressive and highstand systems tracts In the downdip realm (deep sea), where sedimentation rates are very low during maximum flooding, condensed sections develop

Sequence stratigraphy : 

EaES 350-12 16 Sequence stratigraphy In a very general sense, relative sea-level fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased deposition in downdip settings (e.g., submarine fans) Relative sea-level rise will lead to trapping of sediment in the updip areas (e.g., coastal plains) and reduced transfer of sediment to the deep sea (pelagic and hemipelagic deposition; condensed sections)

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Sequence stratigraphy : 

EaES 350-12 18 Sequence stratigraphy Clastic environments Relative sea-level fall in clastic environments commonly leads to fluvial incision into offshore (shelf) deposits, usually associated with soil formation (paleovalleys with interfluves) Relative sea-level rise causes filling of paleovalleys, commonly with estuarine or even shallow marine deposits Submarine fans and associated high aggradation rates in the deep sea occur especially during late highstand and lowstand, when sediments are less easily trapped updip of the shelf break

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Sequence stratigraphy : 

EaES 350-12 20 Sequence stratigraphy Carbonate environments Relative sea-level fall in carbonate environments can lead to the development of karstic surfaces (dissolution of limestones) or evaporites (e.g., sabkhas), depending on the climate Highstands generally expand the area of the carbonate factory (drowning of shelves) and vertical construction of reefs, as well as accumulation of other carbonates is enhanced Extreme rates of relative sea-level rise can lead to the drowning of carbonate platforms

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Sequence stratigraphy : 

EaES 350-12 24 Sequence stratigraphy Sequence-stratigraphic concepts contain numerous pitfalls! Variations in sediment supply can produce stratigraphic products that are very similar to those formed by sea-level change Sea-level fall does not necessarily always lead to the formation of well-developed sequence boundaries (e.g., fluvial systems do not always respond to sea-level fall by means of incision); sequence boundaries may therefore be very indistinct and difficult to detect Allogenic incision is easily confused with autogenic scour

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Sequence stratigraphy : 

EaES 350-12 30 Sequence stratigraphy Sea-level change Causes of relative sea-level change (amplitudes ~101-102 m) Tectono-eustasy (time scales of 10-100 Myr) Glacio-eustasy (time scales of 10-100 kyr) Local tectonics The time scales of these controls have given rise to the distinction of eustatic cycles of different periods First-order (108 yr) and second-order (107 yr) cycles (primarily tectono-eustatic) Third-order (106 yr) cycles (mechanism not well understood) Fourth-order (105 yr) and fifth-order (104 yr) cycles (primarily glacio-eustatic)

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Sequence stratigraphy : 

EaES 350-12 32 Sequence stratigraphy Sea-level change The global sea-level curve for the Mesozoic and Cenozoic contains first, second, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization Conceptual problems: the role of differential local tectonics is extremely difficult to single out Dating problems: correlation is primarily based on biostratigraphy that typically has a resolving power comparable to the period of third-order cycles

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Sequence stratigraphy : 

EaES 350-12 35 Sequence stratigraphy Seismic stratigraphy Seismic reflection profiling forms the basis of seismic stratigraphy, which in turn has been the foundation for the development of sequence stratigraphy The technique is based on contrasts in acoustic impedance between different materials; reflections of sound or shock waves occur at transitions between different types of sediment or rock v=sonic velocity; =sediment or rock density

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Sequence stratigraphy : 

EaES 350-12 38 Sequence stratigraphy Seismic stratigraphy A seismic section consists of a large number of vertical traces; acoustic impedance contrasts that can be correlated between large numbers of traces constitute reflectors Seismic reflectors are often believed to approximate isochronous surfaces that may be relevant in a sequence-stratigraphic context The vertical resolution of seismic profiling has increased considerably over time, and is now on the order of 101 m, but depths and thicknesses have to be derived from two-way travel times which may occur with the aid of geophysical logs 3D seismic imaging is becoming increasingly important

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Sequence stratigraphy : 

EaES 350-12 42 Sequence stratigraphy Seismic stratigraphy A seismic section consists of a large number of vertical traces; acoustic impedance contrasts that can be correlated between large numbers of traces constitute reflectors Seismic reflectors are often believed to approximate isochronous surfaces that may be relevant in a sequence-stratigraphic context The vertical resolution of seismic profiling has increased considerably over time, and is now on the order of 101 m, but depths and thicknesses have to be derived from two-way travel times which may occur with the aid of geophysical logs 3D seismic imaging is becoming increasingly important

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Sequence stratigraphy : 

EaES 350-12 47 Sequence stratigraphy Seismic stratigraphy A seismic section consists of a large number of vertical traces; acoustic impedance contrasts that can be correlated between large numbers of traces constitute reflectors Seismic reflectors are often believed to approximate isochronous surfaces that may be relevant in a sequence-stratigraphic context The vertical resolution of seismic profiling has increased considerably over time, and is now on the order of 101 m, but depths and thicknesses have to be derived from two-way travel times which may occur with the aid of geophysical logs 3D seismic imaging is becoming increasingly important

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Sequence stratigraphy : 

EaES 350-12 49 Sequence stratigraphy Cyclostratigraphy Subtle changes in the earth’s orbital parameters cause variations in the distribution of solar radiation, known as Milankovitch cycles Eccentricity (~100 kyr) Obliquity (~40 kyr) Precession (~20 kyr) When Milankovitch cycles produce sufficiently large climatic changes, they may leave an imprint in the stratigraphic record (e.g., sapropels in deep marine deposits) Beware of the ‘magic number’ syndrome!

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