A Simplified Guide For Sequence Stratigraphy:

Transcription

1 SEPM Society for Sedimentary Geology A Simplified Guide For Sequence Stratigraphy: Nomenclature, Definitions and Method Vitor Abreu President - SEPM

2 Abstract All attempts to codify Sequence Stratigraphy have failed, mostly because: - perception of cumbersome nomenclature; - conflicting or obscure causing mechanisms; - disagreement on basic definitions; - or simply that Sequence Stratigraphy is a young science Biostratigraphy has the same set of challenges as Sequence Stratigraphy: - it has a cumbersome nomenclature classification of fossils; - driving mechanisms for rate of evolution of different taxa is debatable; - there is strong disagreement as how to classify different species and genera; - and it is a relatively young science. Yet, Biostratigraphy has a set of rules and terminology (code) followed by all. Biostratigraphy is codified as a method, not as a science, based on simple criteria that can be directly observed from available data. Implications for interpretation, in terms of causal mechanisms, follow after initial interpretation, and are not part of the code. Why do Sequence Stratigraphers have such difficulty agreeing on basic rules for identifying surfaces and systems tracts based on direct observational criteria?

3 At First, A Contradiction Conflicting Nomenclature: Highstand Transgressive Lowstand Why Not: Highstand Midstand(!?!) Lowstand Or Lower Regressive Transgressive Upper Regressive??

4 A Bit of History 70 s Peter Vail Exxon Eustasy Bob Mitchum Basic Observations 80 s Posamentier Van Wagoner 80 s To Today Plint Tucker Catuneanu Community Galloway Uliana M Blum Embry

5 Introduction The close association between base-level changes, the formation of surfaces, and specific stratal stacking that define systems tracts is at the heart of the confusion. Highstand and Lowstand conflict with terms that are related to shoreline translation, or attributes that can be directly observed from the geologic record, such as "transgression", "regression", "progradation", and retrogradation". We propose a back-to-basics approach, emphasizing five key observations that can be made from any geologic data: lithofacies, lithofacies association, vertical stacking, stratal geometries, and stratal terminations Key observations shoreline position and shoreline trajectory Terms like highstand, lowstand, and falling stage should be replaced by observation-based terms like "aggradation-progradation", "progradationaggradation", and "degradation", respectively. Finally, much basic research remains to be done on the relations between stratal stacking and various controls, and on the formation and chronostratigraphic significance of key surfaces that demarcate changes in stacking.

6 Definitions: Parasequence and Sequence Shoreline Trajectory Sequence Parasequences Parasequence A relatively conformable succession of genetically related beds and bedsets bounded by surfaces of flooding, abandonment, or reactivation and their correlative surfaces (modified from Van Wagoner, 1988). Sequence a relatively conformable succession of genetically related strata bounded by unconformities and their correlative surfaces (modified from Mitchum, 1977).

7 Definitions: Stratigraphic Surfaces Sequence Boundary a regional unconformity and its correlative surface characterized by a downward shift in coastal onlap and across which distinctive changes in vertical stacking occur. It is the surface that commonly separates parasequences with aggradationalprogradation-degradation stacking from those with progradation-aggradation or retrogradation stacking.

8 Definitions: Stratigraphic Surfaces Transgressive Surface (Maximum Regressive Surface) a regional surface characterized as the first parasequence boundary atop the basinward-most position of the shoreline. Parasequence stacking patterns across this surface commonly change from progradation-aggradation to retrogradation. AKA: Maximum Regressive Surface (Embry,1995) or Maximum Progradation Surface (Emery and Myers,1996).

9 Definitions: Stratigraphic Surfaces Maximum Flooding Surface (Maximum Transgressive Surface) a regional surface characterized as the first parasequence boundary atop the landward most position of the shoreline. Parasequence stacking patterns across this surface commonly change from retrogradation to aggradation-progradation-degradation. In seismic, this surface can be identified as the shelfal downlap surface.

10 Surface definitions, translation terms, and recognition criteria. Surface Translation Terms Primary Recognition Criteria Secondary Recognition Criteria (based on limited available data) Maximum Flooding Surface MFS Maximum Transgressive Surface MTS Atop maximum landward position of the shoreline Downlaps s,o. Turn around in stacking pattern from retrogradation to aggradation or progradation w,c,o. Transgressive Surface TS Maximum Regressive Surface * MRS Atop maximum basinward position of the shoreline Surface beneath first backstep (landward step) of shelf-slope break s. Turn around in stacking pattern from progradation or aggradation to retrogradation w,c,o. Sequence Boundary SB Sequence Boundary SB s=seismic; w=wells; c=core; o=outcrop. Beneath abrupt basinward shift in shoreline Surface beneath first increase in accommodation after progradation or degradation s. Break in shoreline trajectory S s. Truncation and/or toplap below, onlap above s. Abrupt occurrence of proximal facies over distal facies w,c,o. *sensu Embry, 2002

11 Definitions: Systems Tracts Lowstand Systems Tract (PA) a linkage of contemporaneous depositional systems (Brown and Fisher, 1977), characterized by a progradational to aggradational stacking of parasequences and bounded by a Sequence Boundary (SB) at the base and a Transgressive Surface (TS or MRS) at the top. Transgressive Systems Tract (R) a linkage of contemporaneous depositional systems (Brown and Fisher, 1977), characterized by retrogradational stacking of parasequences and bounded at the base by the TS (or MRS) and at the top by the Maximum Flooding Surface (MFS or MTS). Highstand Systems Tract (APD) a linkage of contemporaneous depositional systems (Brown and Fisher, 1977), characterized by an aggradational to progradational to degradational stacking of parasequences, bounded at the base by the MFS (or MTS) and at the top by the SB.

12 System Tract definitions, stacking patterns, and recognition criteria. Systems Tract Stacking Pattern Bounding Surfaces Accommodation/ Sediment Supply Trend Highstand Systems Tract HST Aggradation to Progradation to (possible) Degradation A-P-(D) Above: SB Below: MFS (MTS) Decreasing, at increasing rate Transgressive Systems Tract TST Retrogradation R Above: MFS (MTS) Below: TS (MRS) Rapidly increasing, to a maximum Lowstand Systems Tract LST Progradation to Aggradation P-A Above: TS (MRS) Below: SB Increasing, at increasing rate

13 Method The sequence stratigraphy method can be summarized in 4 steps: 1. Define lithofacies and vertical lithofacies successions to identify vertical stacking trends and stratal terminations 2. Use vertical stacking patterns and stratal termination patterns to define 3 surfaces: Sequence Boundary Transgressive Surface (Maximum Regressive Surface) Maximum Flooding Surface (Maximum Transgressive Surface) 3. Use Surfaces, vertical stacking, and stratal geometries to define 3 systems tracts: Lowstand Systems Tract (PA) Transgressive Systems Tracts (R) Highstand Systems Tract (APD) 4. Use Systems Tracts and Surfaces to define depositional sequences Then: make maps, find resources, seek to explain by various mechanisms,

14 Defining Surfaces: Well Logs Step 1: a) Identify bedset stacking patterns; b) Interpret high- and moderate-confidence candidate flooding surfaces; c) Identify parasequence stacking patterns; d) Select datum on candidate maximum flooding surface above interval of interest.

15 Defining Surfaces: Well Logs HST TST (R) LST (P-A) HST (A-P-D) Step 2: Correlate lower-confidence bedset surfaces and sequence boundary as constrained by flooding & bedset surfaces, truncation & onlap, and reservoir quality changes, and connect lithofacies within parasequences. Step 3: Use parasequence stacking patterns & stratal geometry to interpret systems tracts, using criteria on previous page. Note the relation of rock properties (e.g., porosity) to systems tracts.

16 Accommodation Succession A resulting motif in a depositional succession starting with negative accommodation rates on the shelf, to maximum accommodation and to negative again is of: progradational to aggradation (PA or LST), followed by retrogradation (R or TST), followed by aggradation to progradation to degradation (APD or HST). - ( Transgressive Stacking) ( Highstand Stacking) ( Lowstand Stacking) Depositional Sequence Neal and Abreu, 2009

17 Experimental Stratigraphy Sequence Stratigraphic methods are amenable to experimental stratigraphy because stratal terminations are geometric, scale-independent features. Stratigraphic section comes from the Experimental EarthScape Facility (XES) located at Saint Anthony Falls and was recently published (AAPG Bulletin) Martin et al., 2009

18 Experimental Stratigraphy Interpretation applying the sequence stratigraphic method

19 Experimental Stratigraphy Interpretation applying the sequence stratigraphic method

20 Quaternary Example Coastal onlap APD PA SB Modified from Roberts (2005)

21 Sequence Boundary Expresssion Coastal onlap relatively conformable APD unconformable PA SB Uninterpreted and interpreted seismic line through the Lagniappe delta. LST, lowstand systems tract; HST, highstand systems tract; sb, sequence boundary. Modified from Roberts et al. (2004) and Sydow and Roberts (1994)

22 Sequence Sets and Composite Sequences modified from Mitchum and Van Wagoner, 1991 Composite Sequence: A relatively conformable succession of one or more sequence sets overlain by a regional drape complex and bounded by regional (100's to 1000's of km2) unconformities or their correlative conformities (called composite sequence boundaries; Mitchum and Van Wagoner, 1991).

23 Pelotas Basin, South Atlantic (Brazil) Modified from Abreu (1998) Nested Stratigraphic Hierarchy of the Neogene (Pelotas Basin, Brazil) km R-SS Middle to Late Miocene APD-SS Pliocene multiple Early Miocene APDSS Termination Early to Middle Miocene PA-SS RSS Sequence Boundary (SB) PASS Composite SB Multiple

24 Sequence Sets and Composite Sequences Kunin and Segalovich,

25 Sequence Sets and Composite Sequences West Siberia Kunin and Segalovich (1996) Kunin and Segalovich,

26 Sequence Sets and Composite Sequences PASS Composite Sequence Boundary Modified from Kunin and Segalovich, 1996 APSS 26

27 So, Where is Sea Level in all of this??? In the minds of men and women!! Or, better sea level changes cannot be directly observed from the geologic record. Shoreline trajectory can. Fluvial Floodplain Coal Delta Front Prodelta Shelfal Muds HST - APD PA Basiward shift in coastal onlap CSB Modified from Li and Bhattacharya, 2013 Relative movement of sea level is not used and is very misleading to define systems tracts and surfaces. For example, all Systems Tracts are at least, in part, deposited during a relative rise of sea level.

28 Conclusions: Objective Criteria Offlap break observed in seismic profiles is interpreted as the position of the shoreline and vertical stacking of lithofacies in cores, well-logs and outcrops indicate changes from proximal to distal shelfal environments. Systems Tract bounding surfaces are defined at the changes in direction of a shoreline trajectory during an accommodation succession. Transgressive Surface (or Maximum Regressive Surface) is defined at the basinward-most position of the shoreline within an accommodation succession Maximum Flooding Surface (or Maximum Transgressive Surface) is defined at the landward-most positions of the shoreline during an accommodation succession, The Sequence Boundary is defined by the basinward shift in coastal onlap High-resolution, Quaternary inner-shelf to slope data sets and tank experiments clarified these processes and are of unique importance to further advance the understanding of sequence stratigraphic architecture prediction

29 Conclusions 2: A Practical & Useful Approach Understanding sequence hierarchy is an important part of seismic sequence stratigraphy at several spatial and temporal scales Placing your observations into a stratigraphic hierarchy will put local interpretations into a regional context for prediction away from well control Regional thinking is required even for prospect-scale (or outcrop-scale) problems Regional mapping guided by composite surfaces key for play elements presence and distribution prediction Our method: Successions of accommodation change and sediment fill, observed from stratal patterns and facies stacking relative to key bounding surfaces, not defined by time duration or relative sea level position, placed into a hierarchy framework, & calibrated with age control