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  • 1. 1Week 6Review of week 5odefinition of faciesogeologist use of facies to build depositional systemsodiffering uses of facies in practiceTopics for Week 6osequence stratigraphyReading and reference:oMiall, Chapter 6, 6.1 to 6.4oClass handouts: Embry, A. F., 2002; Catuneanu, O., 2002

2. 2Facies Definition and Usefacies: a body of rock with specified characteristics. A distinctive rock that forms under certain conditions of sedimentation, reflecting a particular process or environment (lithofacies, biofacies, seismic facies).lithofacies: rock unit defined on the basis of distinctive lithologic features such as composition, grain size, sedimentary structures, bedding characteristics. Each lithofacies represents an individual depositional event.lithofacies associations/assemblages: groups of lithofacies characteristic of particular depositional environments 3. 3Facies Analysis within Basin AnalysisGoal is to get from facies analysis to a paleogeographic synthesis depicting an interpretation of the stratigraphic and geographic evolution of the basin through time.make use of large scale basin-fill patterns; these are depositional systemsthe difference between facies analysis and analysis of depositional systems is one of scalemapping is an essential tool to support the use of facies analysis in interpretation of depositional systems and sequences. 4. 4Facies Model: Shoreface Zone(Einsele, 2000, Fig 3.1) 5. 5Scales of Sedimentological Analysis 6. 6Summary: Facies AnalysisFacies: a distinctive rock that forms under certain conditions of sedimentation, reflecting a particular process or environment (Reading, 1986, p. 4)Facies term can be applied by geologists in differing contexts:oobservation sense for rock product (sandstone facies)ogenetic sense for the products of a process (turbidite facies)oenvironmental sense for the environment in which the rock or suite of mixed rocks was formed (fluvial facies)otectonic setting or tectofacies (post-orogenic facies)In Basin Analysis: facies description are essential for interpreting depositional systems used in sequence stratigraphy 7. 7Sequence Stratigraphy: ObjectivesDefine sequence stratigraphyDescribe key concepts and termsoWhat is a sequence?oWhy is describing basin fill in terms of sequences a useful approach? Relation to seismic stratigraphyoDevelopment of sequence concept: eustasy, base level, accomodation spaceoUnconformities and their correlative conformitiesoThe terminology of sequence stratigraphy: bounding surfaces; systems tractsoSchematic diagrams illustrating use of concepts of sequence stratigraphy 8. 8Sequence Stratigraphy for Basin Analysis(Catuneanu, 2002) 9. 9Lithostratigraphy vs Sequence Stratigraphy(Willis and Moslow, 1994))Triassic Halfway Formation, western Canada 10. 10Sequence StratigraphyA field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time.Geometry of seismic reflectors very important in unraveling sequence stratigraphy in a basinBasinwide unconformities and correlative conformable surfaces are the framework for describing sequences 11. 11DefinitionSequence stratigraphy consists of the recognition and correlation of changes in depositional trends in the rock record. Such changes, which were generated by the interplay of sedimentation and shifting base level, are now recognized by sedimentological criteria and geometrical relationships. (Embry, 2002)Think 1) surfaces 2) systems tracts and 3) sequences 12. 12Definitions: SequenceSequenceoA relatively conformable succession of genetically related strata bounded by unconformities and their correlative conformities (Vail et al, 1977)oA sequence is composed of a succession of genetically linked deposition systems (systems tracts) and is interpreted to be deposited between eustatic-fall inflection points (Posamentier, et al., 1988). 13. 13Definition: Base Level, Accommodation SpaceBase Level:o(Cross, 1991) a surface of equilibrium between erosion and depositiono(Schumm, 1993) an imaginary surface to which subaerial erosion proceeds. It is effectively sea level, although rivers erode slightly below it.osequence stratigraphy is concerned with base level fluctuations at the shorelineAccommodation space:o(Jervey, 1988) the space made available for potential sediment accumulation where in order for sediments to be preserved, there must be space available below base level (the level above which erosion will occur). 14. 14Base Level(Catuneanu, 2002) 15. 15EustasyGlobal sea level and its variations. Changes in sea level can result from movement of tectonic plates altering the volume of ocean basins, or when changes in climate affect the volume of water stored in glaciers and in polar icecaps. Eustasy affects positions of shorelines and processes of sedimentation, so interpretation of eustasy is an important aspect of sequence stratigraphy.(http://www.glossary.oilfield.slb.com) 16. 161stand 2ndOrder Cycles of Sea Level Change(Vail et al, 1977) 17. 17Base Level: Eustasy, Tectonics, Sedimentation(Catuneanu, 2002) 18. 18Base Level Changes and Transgression/Regression(Catuneanu, 2002) 19. 19Scenarios for Relative Sea Level Rise(Catuneanu, 2002) 20. 20Scenarios for Relative Sea Level Fall(Catuneanu, 2002) 21. 21Response: Depositional Architecture(from http://strata.geol.sc.edu) 22. 22Transgression/Regression Cycle ResponseMFS: maximum flooding surface(Catuneanu, 2002) 23. 23Example Bounding Surfaces: Lowstand Shoreline(Posamentier et al, 1992) 24. 24Normal vs Forced Regression(Posamentier et al, 1992) 25. 25Rate of Base Level Change and SedimentationFR: forced regressionNR: normal regression(Catuneanu, 2002) 26. 26Bounding SurfacesSequences and systems tracts are bounded by key stratigraphic surfaces that signify specific events in the depositional history of a basinomaximum flooding surface, subaerial unconformity, othere are differing models of sequence stratigraphy that use different bounding surfaces to define sequencesSequences correspond to full stratigraphic cycles of changing depositional trendsofull cycle of systems tracts in response to base level change 27. 27Reflector Geometry From Seismic Stratigraphy(Mitchum et al, 1977) 28. 28Surfaces of Sequence Stratigraphysubaerial unconformityshoreface ravinementounconformableoconformableregressive surface of marine erosionmaximum regressive surfacemaximum flooding surfacestart of base level fallstart of base level rise 29. 29Bounding Surfaces (Embry, 2002) (Embry, 2002) 30. 30Bounding Surfaces and Changing Base Level(Embry, 2002) 31. 31Defining Sequences: which boundary to useType 1 Sequence (Posamentier et al, 1988)ounconformity + start of base level fallType 2 Sequence (Posamentier et al, 1988)ounconformity + start of base level riseGenetic Stratigraphic Sequence (Galloway, 1989)omaximum flooding surfacesTransgressive-Regressive Sequence (Embry and Johannessen, 1992)ounconformity + shoreface ravinement unconformable + maximum regressive surfaceUse depends on problem being investigated, recognition of changes in rock record 32. 32Sequences, Systems Tracts and Sea Level(Miall, Fig 6.5) 33. 334 Different Definitions of Sequences(Embry, 2002) The main issue: what is the correlable conformity to the well known subaerial unconformityT-RS transgressive- regressive sequenceT1DS type 1 depositional sequenceT2DS type 2 depositional sequenceGSS genetic stratigraphic sequence 34. 34Definitions: Systems TractSystems Tract:oGenetically associated stratigraphic units that were deposited during specific phases of the relative sea-level cycle (Posamentier, et al, 1999). These units are represented in the rock record as three- dimensional facies assemblages. They are defined on the basis of bounding surfaces, position within a sequence, and parasequence stacking pattern (Van Wagoner et al., 1988).oA linkage of contemporaneous depositional systems, forming the subdivision of a sequenceoInterpreted based on stratal stacking patterns, position within the sequence and types of bounding surfacesoThere are numerous systems tracts in use in sequence stratigraphy. It is important to know how systems tracts are defined and the bounding surfaces used. 35. 35Sequence Stratigraphy: continued next week