Sequence Stratigraphy Template for Conceptual Models Used to
Sequence Stratigraphy Template for Conceptual Models Used to Interpret Depositional Systems - beauty but not truth? Christopher G. St. C. Kendall University of South Carolina [email protected] 803 978 7523 Sequence Stratigraphy Defined Why codify rules for sequence stratigraphic nomenclature? We use common terms and know their meaning Have a standard hierarchy of subdivisions Have a uniform methodology Guidance for revision
Teaching resource: each new generation of students & practitioners have a common understanding Sequence Stratigraphy Defined Sequence Stratigraphy A framework of genetically related stratigraphic facies geometries and their bounding surfaces used to determine depositional setting Sequence Stratigraphy Defined Sequence Stratigraphy
Terminology Stratigraphers have a position on terminology similar to the of Humpty Dumpty's from Lewis Carrolls Through the Looking Glass "When I use a word, it means just what I choose it to mean nothing more nor less". Sequence Stratigraphy Defined Talk Organization
Sedimentary Analysis & Sequence Stratigraphy History of Sequence Stratigraphy Sequence Stratigraphy Analysis Defined Geometries & Stacking Patterns System Tracts & Relative Sea Level Bounding Surfaces Conceptual Models Terminology That Should be Defined Conclusions Sequence Stratigraphy Defined
Sequence Stratigraphy The process Interpretation of the Sedimentary section involves: From oldest to youngest, identify the subdividing surfaces that envelope & enclose the discrete geometric bodies of sediment of the sedimentary section & build a template Backstrip these geometries & then use the template of surfaces to reassemble them in order of their accumulation The subdividing surfaces, geometry, lithofacies & fauna are input to a conceptual model to determine the evolving character of the depositional setting Each stratal unit is defined and identified only by physical relationships of the strata, including lateral continuity and geometry of the surfaces bounding the
units, vertical stacking patterns, and lateral geometry of the strata within the units." (Van Wagoner et al., 1990). Sequence Stratigraphy Defined al 1971 Sequence Stratigraphy Defined SB mfs mfs TS Establish a framework of genetically related stratigraphic facies geometries and their bounding surfaces to
determine depositional setting Sequence Stratigraphy Defined Barrier Coastal Elements Sedimentary Analysis Potential flow of interpretation data data Scheme used to characterize depositional
systems so they can be compared interpretation interpretation interpretation Sequence Stratigraphy Defined Power of Sequence Stratigraphy Determines depositional setting by: Establishing the order in which strata were laid down in Explains the geometric hierarchy of sedimentary strata & the elements they form
Uses the major surfaces marking breaks in the character of sedimentary section as a template for conceptual depositional models, independent of time (relative ages) Geologic time derived from: Radiometric markers Biostratigraphic markers Sequence Stratigraphy Defined Building Block Sedimentary Fill Hierarchy Sediment body characterization in order of decreasing complexity Complex Set Complex
Story Element Bed These are the fundamental components of Sequences and System tracts Sequence Stratigraphy Defined Elements Sequence Stratigraphy Defined Elements Sequence Stratigraphy Defined
Elements Sequence Stratigraphy Defined Elements Sequence Stratigraphy Defined Shelf Architectural Elements PROGRADING MARGIN ROGRADING MARGIN ONLAPPING MARGIN
PROGRADING MARGIN BED, STACKE LITHOLOGY, D BED SEDIMENTARY CYCLES STRUCTURES FAUNA Sequence Stratigraphy Defined SHELF COMPLEX MARGIN
COMPLEX ALLOCHEMS CEMENTATION & DIAGENESIS Elements Sequence Building Blocks Definition of element hierarchy Framework for systematic description & comparison of deposits of a depositional setting (fluvial, deltaic, deepwater etc) Based on physical organization of strata (geometry) & the surfaces bounding them
Genetically-related stratigraphic elements form the hierarchy Independent of type of setting Applicable at all scales & to all sedimentary systems Purpose Analysis & comparison of the elements of similar depositional systems Provide the detailed character of a sequence and/or system tract Net to-gross, aspect ratio, & connectivity Lithofacies type & aquifer/reservoir quality
Application Better defined depositional geometries and so depositional models More accurate aquifer/resource assessment Optimize strategy for depletion of aquifer/resource Sequence Stratigraphy Defined History of Sequence Stratigraphy
Sloss recognized intercontinental-unconformities & related them to eustasy Vail extrapolated eustatic signals to seismic sequence boundaries Jerveys simulations showed seismic sequences product of cyclic sea level (base level) change Van Wagoner & Posamentier related unconformities, transgressive surfaces & maximum flooding surfaces to boundaries of system tracts in depositional sequences responding to sea level change Hydrocarbon exploration models & production reservoir models use bounding surfaces, facies assemblages & system tracts as templates for depositional models & lithofacies geometry
predictions Sequence Stratigraphy Defined Sloss Unconformities & Eustasy After Lowell Thwaite Sequence Stratigraphy Defined 1st Seismic Sequence Interpretation Vail et al, 1976 Mesozoic through Tertiary of Offshore Morocco
Sequence Stratigraphy Defined 'Depositional' sequence Vail Jervey Posmentier Van Wagoner Sequence Stratigraphy Defined Sequence stratigraphy Analytical protocol marries rules of thumb (heuristics) to conceptual models Interprets general depositional setting of layered clastic & carbonate sedimentary strata
Models developed to interpret variety of datasets Work best with combination of outcrop, core, well log, & seismic data May work with two or more of these data sources but are least reliable for a single data source Local differences in depositional processes, topography, and tectonics and, base level position, mean a model more appropriate to one setting, than others?
Sequence Stratigraphy Defined Sequence Stratigraphys Fuzzy Rules Lithofacies geometries & confining surfaces transgress time Different geometries for a specific time interval Local variations in rates & direction of tectonic movement Local rates of sediment supply & accumulation Not all erosional or depositional surfaces are
product of base level change Depositional models successfully predict lithofacies geometry for exploration & reservoirs Sequence Stratigraphy Depositional model is Defined refined as data Sequence stratigraphic analysis Fundamental to sequence stratigraphic analysis:
Strata deposited in order Surfaces subdivide strata into packages of beds with common depositional origin Geometric arrangement of strata & packages of strata reflect Basin filling Erosional events Commonly sequence stratigraphiy applied to sediment accumulation at basin margins in continental, marginal marine, and/or down-slope
settings Fill accumulates either as: Unconfined fill over these settings Confined within erosional down-cut topography Sequence Stratigraphy Defined Unconfined geometric architecture Stacked layers tend to have three patterns basic Aggrading (lie: above one another vertically)
Prograding (inclined in order over and beside one another in a seaward direction) Retrograding (inclined in order over & beside one another in a landward direction) Sequence Stratigraphy Defined Stacking Patterns Unconfined After Van Wagoner Sequence Stratigraphy Defined Confined geometric architecture
Fill post erosional down-cutting expressed by a geometric architecture of stacked layers in either succession of : Organized offset bodies and/or Randomly ordered disorganized offset bodies Sequence Stratigraphy Defined Confined Sedimentary Fill Channels Sequence Stratigraphy Defined Stacking Pattern Rules & Problems
End member stratal stacking patterns reflect interplay of accommodation (space available for sediments to fill) & rates of sediment accumulation Local vagaries in Depositional processes Topography Tectonics & so base level position mean stacking styles of geometry may merge with one another but are tied to system tracts Sequence Stratigraphy Defined
Stacking, Surfaces & System tracts! Geometric architecture of stacked layers are inferred to be related to eustatic events : Lowstand - Progradation-to-aggradational (PA) Transgression - Retrogradational geometries (R), Highstand - Aggradational-to-progradation geometries (AP) system tracts (Van Wagoner et al., 1988; Neil and Abreu, 2008) As many as five system tracts recognized but above three are most commonly used
Sequence Stratigraphy Defined System Tracts - Reviewed Sequence Stratigraphy Defined Unconfined 3-D facies assemblages of common genetic origin with unconfined geometric architecture are interpreted to form during phases of relative sea-level cycle (Posamentier, et al, 1988) System tracts defined by: Bounding surfaces
Position within a sequence Geometric stacking pattern Lowstand - Progradation-to-aggradational (PA) Transgression - Retrogradational geometries (R), Highstand - Aggradational-to-progradation geometries (AP) system tracts (Van Wagoner et al., 1988; Neil and Abreu, 2008) As many as five system tracts recognized but above three are most commonly used Sequence Stratigraphy Defined Surfaces, System tracts & Sequences
Role of conceptual depositional models: System tracts are inferred to be related to accommodation generation and fill. This relationship is explained using conceptual models Sequence Stratigraphy Defined Bounding Surfaces Identification & interpretation of bounding surfaces that separate layered clastic and carbonate sedimentary strata into genetically related packages, system tracts, a key to Sequence
Stratigraphy Maximum Flooding Surfaces (MFS) Transgressive Surfaces (TS) (maximum regressive surfaces & transgressive ravinement surfaces) Sequence Boundaries (SB) (subaerial unconformities, regressive surfaces of marine erosion & correlative conformities). Sequence Stratigraphy Defined Conceptual Models Three conceptual models commonly used
in sequence stratigraphy are separated from each other on basis of bounding & subdividing boundaries of system tracts: 'Depositional' sequence of Vail et al 1971 'Genetic Stratigraphic' sequence model of Galloway, 1989 'Transgressive-Regressive' (T-R) sequence model of Embry and Johannessen, 1992 Sequence Stratigraphy Defined 'Depositional' Sequence (Vail et al 1971) Bounded by subaerial unconformities &
marine correlative conformities Bounding surface assumed genetically connected with stratigraphic hiatus associated with subaerial unconformity that is progressively younger in seaward direction No matter systems-tract terminology used bounding surfaces of this sequence enclose: Forced regressive Normal regressive (lowstand and highstand) Transgressive sedimentary packages Sequence Stratigraphy Defined
'Genetic Stratigraphic' Sequence Galloway, 1989 Bounded by maximum flooding surfaces. These surfaces interpreted to form as sea level rises rapidly, sediment supply slows & organic matter accumulates sequestering radioactive materials Sequence encloses Forced regressive deposits Normal regressive (lowstand and highstand) deposits Transgressive deposits Sequence Stratigraphy Defined
'Transgressive-Regressive' (T-R) sequence (Embry & Johannessen, 1992) Composite sequence boundary includes subaerial unconformity & marine portion of maximum regressive surface Normal' & 'forced' regressive deposits are included within the 'regressive systems tract' Sequence Stratigraphy Defined Boundaries & Conceptual Models
After Catuneanu, 2007 Sequence Stratigraphy Defined Sequence Stratigrap hy Model Tree After Catuneanu, 2002 Sequence Stratigraphy Defined
Application & nomenclature of conceptual models subject to debate Nomenclature of systems tracts & sequence stratigraphic surfaces Surfaces that should be ranked as 'sequence boundary Framework of surfaces, systems tracts, selection of sequence boundaries may vary with approach, available data sets, & depositional setting Good news!! Sequence stratigraphic method determines depositional
setting of section Identification of genetic units & bounding surfaces mostly model independent. For both non-marine and marine systems, no matter characterization of 'sequence boundary', these boundaries envelope sedimentary section as throughgoing physical surface from basin margin to depocenter Sequence Stratigraphy Defined Caveats
Sequence stratigraphic surfaces serve, at least in part, as boundaries between different genetic types of deposit Not all data lend themselves to identification of all sequence stratigraphic surfaces Not all sequence stratigraphic surfaces are present in every depositional section Depositional settings of nearshore to just offshore generate an array of depositional and erosional surfaces, many of incorporated into sequence stratigraphy In contrast, within either fluvial and/or deepwater systems, conditions favor formation of fewer key bounding sequence stratigraphic surfaces Sequence Stratigraphy Defined
Why codify rules for sequence stratigraphic nomenclature? We use common terms and know their meaning Have a standard hierarchy of subdivisions Have a uniform methodology Guidance for revision Teaching resource: each new generation of students & practitioners have a common understanding Sequence Stratigraphy Defined Terms to Define
Surfaces Separate layered clastic and carbonate sedimentary strata into genetically related packages, system tracts System Tract Specific three-dimensional facies assemblages of common genetic origin, associated with unconfined geometric architecture & interpreted as formed during phases of a relative sea-level cycle Conceptual Models Explains configuration of surfaces and geometries
that occur in layered sedimentary record in terms of: Basinal setting Rates of sediment supply & erosion (accumulation) Eustasy Tectonic movement Sequence Stratigraphy Defined Terms to Define - Surfaces Separate layered clastic & carbonate sedimentary strata into genetically related packages, or system tracts Subaerial unconformity
Correlative conformity (sensu Posamentier, and sensu Hunt and Tucker; the former have renamend Posamentiers CC as the basal surface of forced regression). (Two different surfaces). Regressive surface of marine erosion Maximum regressive surface Maximum flooding surface Transgressive ravinement surfaces Sequence Stratigraphy Defined Terms to Define Systems Tracts
Specific three-dimensional facies assemblages of common genetic origin, associated with unconfined geometric architecture & interpreted as formed during phases of a relative sea-level cycle Falling Stage Systems Tract (FSST) Lowstand Systems Tract (LST) Transgressive Systems Tract (TST) Highstand Systems Tract (HST) Regressive System Tract Low-accommodation systems Tract
High-accommodation Systems Tract Forced regression Normal regression Transgression Sequence Stratigraphy Defined Terms to Define Conceptual Models Models that explain surfaces & geometries of layered sediments in terms of basinal setting, rates of supply & erosion eustasy,model and tectonics (accumulation), Depositional sequence
Genetic Stratigraphic sequence model Transgressive-Regressive sequence model Sequence Stratigraphy Defined Definitions Points to consider Definition Alternate terms Origin of Feature Absolute time or Fuzzy time significance if any? Sequence Stratigraphy Defined
"What's in a name? That which we call a rose. By any other name would smell as sweet. WHAT, YOU THINK IM AN IDIOT? OF COURSE I WROTE DOWN WHAT WENT WHERE. OH.. WAIT IS THAT AN H OR AN I
Shakespeare's Romeo and Juliet (II , ii, 1-2) Sequence Stratigraphy Defined Sequence Stratigraphy The fundamental key to sequence stratigraphic interpretation is the surfaces that subdivide the sedimentary section. Unfortunately the nomenclature of each of these sequence stratigraphic surfaces is constantly changing as our understanding of sedimentary systems & their interpretation improves
Though changes in nomenclature are well intentioned they add to the confusion to a scientific methodology that is already overlaid with complex multi-syllable terminology A surface can be given a name that has been used before for a different surface. The innocent, not knowing the terminology has been changed, may lack understanding of the reason for the change, & feel that they are going stark raving mad Sequence Stratigraphy Defined Sequence Stratigraphy Conclusions Subdivision & interpretation of sedimentary record using a framework surfaces seen in outcrops, well logs, & 2-D and 3-D seismic. Include:
Surfaces of erosion & non-deposition (Sequence Boundaries, Forced Regression Erosion Surface, Regressive Surface of Marine Erosion) Flooding (Trangressive Surfaces [TS or Max Regressive Surface] &/or maximum flooding surfaces [mfs] Ravinement Surfaces [RStransgressive ) This template is used to predict the extent of sedimentary facies geometry, lithologic character, grain Sequence size, sorting & reservoir Stratigraphy Defined
Which way now? Paul Dimitri Krynines alleged mantra was that: Stratigraphy is the triumph of terminology over common sense Our collective hope is to circumvent this cynical position & propose a more consistent & practical set of terminology! Sequence Stratigraphy Defined Sequence Stratigraphy Defined
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