Practical sequence stratigraphy for iconoclasts
- 1. PRACTICAL SEQUENCE STRATIGRAPHY for ICONOCLASTS (from Embry et al. 2007) - C.J. Modica
- 2. “Stratigraphy is the victory of terminology over common sense” - Paul Dimitri Krynine
- 3. STRATIGRAPHIC DISCIPLINES Lithostratigraphy: Since rock units are often time-transgressive this approach is inferior and, beyond simple closeology, is not very predictive or can lead to bad predictions. Chronostratigraphy: Depends on absolute age data which is often unavailable or has inadequate resolution. There are exceptions and these data can be useful for age-calibrating a sequence stratigraphic interpretation. Condensed intervals are often the best candidates for age calibration. But remember, sequence stratigraphic surfaces are NOT timelines. Sequence Stratigraphy: Unique focus on genetic surfaces that describe widespread changes in environmental conditions that tend to co-vary (SL, climate, tectonic accommodation, sediment dispersal patterns). Surfaces bound packages with relative, but not absolute, age significance. By far most predictive approach if done well.
- 4. Exxon “Slug Model” (Vail, 1987) Siliciclastic Exxon “Slug” Model note predicted net effect of eustasy: - rises are generally larger and more rapid than falls. Carbonate Exxon “Slug” Model Vail (1987) Vail (1987)
- 5. A general compulsion seems to exist to force-fit or “shoehorn” (Embry, 2007) this model onto a wide variety of very different settings, both siliciclastic and carbonate. This has been an enduring source of tremendous confusion and poor stratigraphic interpretation. This does not mean the work of Vail et al. at Exxon in the 1970’s and 80’s was without value. This work revitalized Stratigraphy in a general sense, and more specifically, demonstrated and popularized the then new discipline of Seismic Stratigraphy.
- 6. Some Common Sources of Confusion and Error: 1) Sequence boundaries are not time lines. Chronostratigraphy and Sequence Stratigraphy are not synonymous. Subaerial or erosional unconformities are rarely good low-diachroneity surfaces, even at geologic time scales (deposits on top of an erosional surface can be older than deposits underneath what is apparently the same surface -cf. Strong & Paola, 2008). Journal of Sedimentary Research, 2008, v. 78, 579–593 Research Article DOI: 10.2110/jsr.2008.059 VALLEYS THAT NEVER WERE: TIME SURFACES VERSUS STRATIGRAPHIC SURFACES NIKKI STRONG AND CHRIS PAOLA National Center for Earth-surface Dynamics (NCED), Department of Geology and Geophysics, University of Minnesota, St. Anthony Falls Laboratory, 2 at Third Avenue SE, Minneapolis, Minnesota 55414, U.S.A. ABSTRACT: Using experimental data, we show how erosional unconformities (sequence boundaries) form and evolve in response to changes in global sea level (eustasy), given passive margin style subsidence and constant conditions of supply of sediment and water. We distinguish between two types of erosional unconformities; broad planar erosional surfaces that form during relatively slow sea-level fall, and incised-valleys that form during relatively rapid sea-level fall. We find that both types of unconformities evolve continuously throughout both sea-level fall and rise, producing erosional surfaces that are highly diachronous and amalgamated. We focus mostly on the role of change in relative sea level (RSL) on the formation of incised valleys and their preservation in the stratigraphic record. We find that there is an ongoing interplay of erosion and deposition that continuously redefines the shape of an incised valley, such that valleys both narrow and widen as they deepen during RSL fall and then continue to widen and fill during RSL rise. Due to this dynamic reshaping, what is preserved in stratigraphy may resemble a valley in shape, but its geomorphic form likely never existed in the fluvial landscape. We also find that these erosional valleys tend to be most diachronous along lateral margins of valley fill in proximal areas of the basin and become somewhat younger on average landward along their axial parts. Overall, the basal erosional unconformity forms over most of the duration of the sea-level cycle, does not represent a topographic surface, and is therefore not a time line. Finally, because valleys form through a continuous process of channel incision, backfill, and channel migration (avulsion) during RSL fall, earlier fluvial fills can lie on top of the extended erosional surface, which overrides successively younger delta fronts as it develops. Thus, although locally the deposits above the unconformity are always younger than those below it, the unconformity spans so much time that some of the deposits above it end up being older than some of the deposits below it. The net result is that there are numerous, though relatively small-scale, deviations from one of the frequently quoted fundamental characteristics of a sequence boundary, which is that rocks above it be everywhere younger than rocks below it.
- 7. Some Common Sources of Confusion or Error: 2) Seismic reflectors are explicitly not, and contrary to Vail et al. (1977), time lines. Seismic reflectors are impedance contrasts between rocks with different physical properties (cf. Dickinson, 2003). Locally, for relatively short distances, or more rarely, for very large distances, many reflectors do exhibit low-diachroneity (usually flooding surfaces and major condensed intervals). But this should not be assumed of all seismic reflectors without sufficient knowledge of the stratigraphy.
- 8. Some Common Sources of Confusion or Error: 3) Base level or sea level moved up and down over geologic history in a cyclical fashion. However, this cyclicity was nested with multiple scales or orders. Because of this nesting we should probably not speak in terms of “highstands” or “lowstands” without reference to scale (i.e. there were typically “lowstands” within “highstands” and vice versa). > Typing certain stratal geometries or depositional facies to “Systems Tracts” will often lead to confusion and error.
- 9. Some hypothetical examples will, hopefully, illustrate some issues and possible “better practices” Siliciclastic Shelf Example Go ahead and draw or imagine the sequence boundary and systems tracts as you might tend to interpret them…
- 10. Who did something like this? this is closest to the Posamentier et al. (1988) model
- 11. this is closest to the Posamentier et al. (1988) model The Posamentier et al. (1988) Type I sequence boundary connects a point nearly equivalent to the END of base level fall with a point nearly equivalent to the START of base level fall (see Embry et al. 2007). In order to make this model defensible, a situation must be envisioned where all bypass occurred AFTER base level fall. This is only reasonable in the event of a protracted “highstand,” followed by a rapid and dramatic base level fall, followed by a protracted “lowstand.” Has this happened in the real world? Likely, but in such a case a tectonic driver would need to be invoked as opposed to a eustatic driver and this is clearly not the intention of their model. Who would do something like this? > Unfortunately, this SB violates the Law of Superposition (and that’s about as wrong as you can be in Stratigraphy)
- 12. Why it’s not just nitpicking … If, for example, you were trying to predict the T3 sand in the basin by mapping the T6 shelf edge you might make a bad prediction… Along strike, Type I SBs will tend to be very diachronous because of deltaic and basinal lobe switching and differential rates of subsidence
- 13. Who did something like this? this is the revised Type I Sequence of Posamentier & Allen (1999) Posamantier & Allen (1999) revised the “standard model” to correct the flawed Type I SB of the previous model for the reasons just shown. To achieve this they invoke a BSFR or “Basal Surface of Forced Regression” that is a through going, consistent, but largely hypothetical surface marking the start of base level fall. The main problems with this model are 1) the subaerial unconformity (SU) or offlap surface is partially WITHIN the revised sequence; 2) this surface, like the CC or “correlative conformity,” is a theoretical time surface that often has no objective rock-based criteria by which it can be recognized; and 3) Because subsidence rates can vary along strike, the BSFR can be impossibly diachronous and lead to impossible stratigraphic correlations. BSFR
- 14. Timing of BSFR can vary along strike as dynamics between sediment supply and subsidence vary…leading to incorrect sequence stratigraphic correlations
- 15. “Forced” vs “Normal” Regression:
- 16. Subaerial unconformities are often lousy SBs because they are usually composites of many erosional events that had occurred at many different times, as the above figure suggests and previous figure illustrates. Does the above cross-section imply a “forced” or “normal” regression? From Plint (2002); Late Cretaceous Dunvegan (Frontier) Fm….
- 17. Who did this? this is the Galloway (1989) Genetic Sequence Model This model uses the MFS or “Maximum Flooding Surface” as the SB. The MFS is usually an objective, easily mapped, and relatively synchronous surface that marks the time of maximum inundation and coastline retreat. In deepwater, it is usually a strong seismic reflector and condensed section that often represents a potential source rock. On the shelf, likewise, it is often easily maped with either logs, cores, or seismic data (~downlap surface) and is also a likely place to look for source rocks. This is an excellent model for basinal silciclastic settings and probably the most commonly used in entirely deepwater successions or areas. However, none of this is necessarily true for carbonates.
- 18. Who did this? this is close to the Embry & Johannessen (1992) T-R Sequence Model This model uses the MRS or “Maximum Regressive Surface” as the SB. Two systems tracts are retained based on depositional trend: the Regressive Systems Tract (RST) and the better known TST; subdivided by the MFS. This model is probably more widely and empirically applicable than the Exxon “standard model” and may be a preferred model in shelf and slope settings. It has the advantage that the MRS can often be objectively recognized with either core, log, outcrop, or seismic data and is closer to the classical definition of a Depositional Sequence (Vail et al, 1977) in that the SB includes the subaerial unconformity (SU); whereas the Galloway Model places the SU within the sequence. However, the MRS is probably much more diachronous and less practical than the MFS in siliciclastic deep water areas due to lobe switching and bathymetric isolation of mini-basins during complex salt deformation.
- 19. Carbonate Sequence Stratigraphy:The PlotThickens… Cap Blanc Sea Cliff Exposures, Mallorca, Spain Roman era watchtower
- 20. Figures/data from Schlager et al. (1994), Highstand Shedding of Carbonate Platforms, JSR v. B64, n. 3 “Carbonate and clastic depositional systems respond very differently to sea-level change…At times of sea- level lowstand terrigenous clastics bypass the continental shelf, leading to exposure, erosion, and the development of incised valleys. Submarine canyons are deepened, and sand-rich turbidites systems develop submarine fan complexes on the slope and the basin floor. Carbonate systems essentially shut down at times of lowstand because the main “carbonate factory,” the continental shelf, is exposed, and commonly undergoes karstification. A narrow shelf-edge belt of reefs or sand shoals may occur,”… (Miall, 1997, p. 60, The Geology of Stratigraphic Sequences) WHY THE “STANDARD” EXXON MODEL IS ESPECIALLY INNAPROPRIATE FOR CARBONATE SYSTEMS… Key Concept: “Highstand Shedding”
- 21. Carbonate Platform Example (let’s call this a “distally steepened ramp”) Because shedding occurs predominantly during “highstands,” or in less ambiguous terms, when the platform is flooded or partially flooded and the “carbonate factory” is working, there is typically no “lowstand basin floor fan” as predicted by the Slug Model Give this a shot, draw the Sequence Boundary as you would… Where is the MFS? Where is the condensed interval in the basin? Are they the same?
- 22. Anyone do this? this is probably closest to the Posamentier et al (1988) model How would you recognized the “correlative conformity” in core or logs? What is fundamentally different about the facies or depositional trend (i.e. regression vs transgression) that the last few offlapping clinoforms ought to be part of an entirely different Sequence? Do the toe of slope deposits belong to a HST or LST?
- 23. Anyone do this? this is closest to the Posamentier & Allen (1999) revised model How would you recognized the “Base Start Forced Regression” or BSFR surface in core or logs? What is fundamentally different about the facies or depositional trend (i.e. regression vs transgression) that the clinoforms above the BSFR ought to be part of an entirely different Sequence? Should the diachronous toe of slope deposits be part of the HST or part of the LST? Both? Is there value to such a systems tract distinction that outweighs the potential for confusion? BSFR
- 24. From Search & Discovery article (2008): Seismic Stratigraphy – A Primer on Methodology; John Snedden and Rick Sarg Following the current Exxon model one would often end up with most or all of a progradational platform or shelf sequence in a “Lowstand Systems Tract.” Even when it is clear that sea level was rising, based on shoreline or stratal “toplap” trajectory, as in SB40 above.
- 25. How about one of these? Can we conclude that application of one of these two models, in lieu of the Exxon model(s), approaches “better practice?” Which one in this case? How would the MFS be recognized in the basin? Does it correspond to the condensed section(s)? Is it possible that this approach is a “best practice,” in general, for most? all? carbonate platforms? Galloway (1989) SB (MFS) Embry & Johannessen (1992) SB (MRS)
- 26. Miocene Progradational Carbonate Platform, Mallorca, Balearic Islands, Spain
- 27. Real-world example of response of a Miocene carbonate platform to base level change. From Luis Pomar’s work on world-class sea cliff exposures: Llucmajor Platform, Mallorca, Spain “keep up” reef growth at platform margin; no retrogradation or backsteps… no TSTs? or only TSTs and RSTs?
- 28. “The outcrops and modeling also Indicate that the bulk of the carbonate production and platform progradation occurs during the transgressive and highstand periods rather than during periods of falling sea level. Condensed sections characterize lowstands and not transgressions.” Bosence, Pomar, Waltham, & Lankster (1994); Computer modeling a Miocene carbonate Platform, Mallorca, Spain BSFR MFS MRS “…the majority of the sequence Stratigraphic definitions of Van Wagoner et al. (1988) and Sarg (1988) do not apply …Our conclusions also illustrate the dangers of identifying systems tracts purely from stratigraphic geometries when the architecture of the entire platform and its slope are not understood.”
- 29. Toplap (i.e., shoreline) Trajectory Mapping: Tool to Extract Base Level History from Progradational Carbonate Platform Architectures – Permian (Guadalupian) San Andres Fm. Example: May work best in tropical and sub-tropical carbonate platforms that are much more resistant to erosion; thereby potentially preserving upsteps (normal regressive phases) and downsteps (forced regressive phases). Chemical erosion (karst) or mechanical erosion in certain kinds of mixed or over-steepened systems might compromise this approach to varying degrees. Interpreted from 2D seismic line presented by Permian Growth Team
- 30. Give this a shot… this geology should look familiar… Any guess as to where this drawing is taken from (geologic and geographic location)?
- 31. This is the T-R Sequence Model bounded by MRSs (Maximum Regressive Surfaces) In carbonate systems the initial flooding surface (or MRS) often is or is nearly the same as the MFS! This is what Shlager termed the Type III or “drowning unconformity.” In a rimmed carbonate shelf “keep up” system the MFS, MRS, and SU (Type I SB) can all collapse into a single horizon (reef never drowns), and systems tractology likewise collapses.
- 32. COLLAPSED SLIGO PLATFORM MARGIN; PAWNEE FIELD, S. TEXAS -Modica & Katz (2008) Lime mud filled fractures strongly suggest that platform margin collapse occurred during relative SL highstand, not relative lowstand conditions…
- 33. Would not want to replace one dogma with another. Perhaps the real lesson is that the best practice depends on the geologic setting and data set and limitations… …I hope you enjoyed Practical (Iconoclastic) Sequence Stratigraphy Siliciclastic Systems Carbonate Systems Shelf / Platform Top Shelf / Ramp Margin Slope / Basin Floor T-R or Exxon T-R, maybe Exxon T-R, maybe Exxon T-R Galloway or T-R T-R General recommendations (not rules)… Some variation of the T-R Model is the “better practice” in carbonate and mixed systems in general, mainly because it focuses on DEPOSITIONAL TRENDS, without any requirement for deductions about absolute SL position …BUT… Conclusions