The Sequence stratigraphic approach to the interpretation of the sedimentary section started in the mid 70's (Vail et al 1977). This and other papers from this group of Exxon geologists immediately prompted discussion on the geologic value of Sequence Stratigraphy and its inter-relationship with earlier approaches to stratigraphic interpretation. Influenced by these papers and other literature on the topic Ashton Embry and Octavian Catuneanu become prosletizers and critics of how best to apply Sequence Stratigraphy. Since then at least two groups of earth scientists developed with different perspectives on the formalization of Sequence Stratigraphy in terms of methodology, Surfaces, units and terminology.
Both groups are reviewing the Sequence stratigraphic literature and making recommendations regarding the methods and terminology of Sequence Stratigraphy. The ISSC Task Group plans to publish their results in a revised edition of the International Stratigraphic Guide (ISG). The North American Commission on Stratigraphic Nomenclature (NACSN) will evaluate feedback from both the IWGSS and the ISSC reports in preparation for an upgraded version of the North American Stratigraphic Code.
The first two posted pdf files above are circulated first drafts and outline the initial positions of the two groups and It would be misleading to suggest these are set in stone. The three pdf documents below represent the first comment from Octavian Catuneanu on the ISSC report, Ashton Emry's response and his short history of the orgin of the groups. Scientific critiques of the reports and subsequent documents from other Earth Scientists comments will be posted as text documents while longer responses and/or illustrations will be posted as pdf's. One objective of this page is that these comments should help the authors revise their manuscripts and positions.
Reply to ISSC Report
Comment on Reply
"Shoehorning in Sequence Stratigraphy and its ConSequences" by Ashton Embry
ISSC's Position on Sequence Stratigraphy
Maria Cita, Emeritus Professor of Geology University of Milano, Honorary Fellow of GSA and Honorary Member of INQUA, chair of the Italian Commission on Stratigraphy and chair of the ISSC - the Subcommission of Stratigraphic Classification of the International Commission on Stratigraphy(IUGS) has or is developing guides to LithoStratigraphy, cycloStratigraphy, chemoStratigraphy, MagnetoStratigraphy, BioStratigraphy, and ChronoStratigraphy tied to the International Stratigraphic Guide.
Maria Cita is the guiding hand behind the current International Subcommission on Stratigraphic Classification (ISSC) Task Group on Sequence Stratigraphy, and she has asked that this web site place a statement related to her position on the current debate currently ongoing on the USC web site tied to Sequence Stratigraphy. She has suggested that this statement be direct and diplomatic, while encouraging the protagonists to channel their energies into a constructive ending to the current confrontation.
Maria recognizes that there is no unique approach to Sequence Stratigraphy that fits all settings or suits every stratigrapher. Her plan is that the ISSC produce a short annotated document that considers the major points of agreement and difference between the various Sequence stratigraphic schools. Her intent is that the ISSC document provides general guidelines (not rules) that review the simple, clear, reasonable concepts of Sequence Stratigraphy for the student and seasoned professional. She would like the document to include at least three examples, from different continents and different time slices, of real world application of the various concepts to Sequence Stratigraphy so the reader can see how to apply these schemes to their own stratigraphic setting. She would like this document to evolve into something the stratigraphic community will be able to stand by and use as a guide. Her intent is that the document would acknowledge that Sequence Stratigraphy has evolved but questions of "style" and interpretation still remain!
Click the link that follows to view earlier & current ISSC Newsletters
The attached minutes are of the deliberations of the ISSC meeting at the 33rd International Congress, Oslo, Norway. They record progress in understanding among the various proponents but the issues and conclusions of the meeting participants still remain controversial and may even be incorrect (see Octavian Catuneanu's comments to these minutes below). The minutes are posted here with the intent that they might be considered as a working agenda for a discussion among the SEPM friends of Sequence Stratigraphy at the coming AAPG Denver. As indicated above Earth Scientists interested in standardizing these criteria for Sequence Stratigraphy are encouraged to a contribute constructive discourse based on the issues and conclusions of the Oslo meeting. Submissions should be sent to firstname.lastname@example.org as text or word files.
Debate - Dated and Chronological order
Zoran Kunštek wrote on Monday the 13 August at 4:48 am (See Zoran Kunštek's Image below the text box too!)
The constructive debate about sequence stratigraphy, towards its formalization is undoubtedly reasonable. It becomes very useful and practical method in geological interpretation, especially for subsurface. But, following the discussion between IWGSS and ISSC it becomes obvious that somebody ignored the most important statements of sequence stratigraphy:
Posamentier et. al, 1985: ?Sequence stratigraphic concept should be applied as tool to approach rather than as template.?
Anderton, 1985: ?Sequence stratigraphy should be considered as way of looking at and ordering geologic data, rather than end in itself.?
For the first time we have a useful method - sequence stratigraphy - that includes several tools and types of data as seismic + well-logs + stratigraphy (and not seismic vs. well-logs vs. stratigraphy – from IWGSS ?Towards the Formalisation…?) All of these data are equally important, and their integration allows us to fill in the gaps and inadequacies of each tool individually.
Sequence stratigraphy allows us to understand the interplay of sedimentation and erosion equally. Those are two opposite processes that act simultaneously. Base level changes (erosional base, Marjanac, 1996), on the other hand defines accommodation space, and their changes cause the shift of facies basinward or landward as well as changes in depositional trends.
We have to accept that some parts of the land for certain time span ?never? come under water so erosion acts continuously, simultaneously some parts of the basin have continuum in sedimentation. For that reason subareal erosion poses greater time span landward than basinward (shown by Wheeler’s diagrams).
Correlative conformity represents maximal basinward shift of base level at particular moment and it is connected with small portion of SU over the part of time that belongs to subaquateous erosion (RAV or RSmE), and sediments reworked and/or removed basinward belong to new portion of time.
MRS (TS; CTS, FS, …) is first onlap on erosional surface representing change in depositional trend and shift of base level landward. So, C.C. should be placed immediately below first onlap visible on seismic section. MFS, on the other hand, represents maximal onlap of seismic reflector (strata) on certain erosional surface (unconformity), for that reason there should be no further onlap reflectors above it, reason being obvious – change in depositional trend caused by base level shift.
I understand that sequence stratigraphy is defined as ?study of rocks relationship within chronostratigraphic framework (time table) of the repetitive, genetically related strata bounded by surfaces of erosion or non deposition, or their correlative conformity? (Van Wagoner et. al, 1988), but some could take it too seriously. We should admit that chronostratigraphic approach is rather rigid for such diachronous sequence stratigraphic surfaces, even if those surfaces have small diachronosity. Sequence and the unconformities that define it, as well as other important surfaces, posses certain time spans that need to be defined, hence geochronological approach (certain sediment package for amount of time) should be more appropriate for interpretation. Later, at the very end of interpretation we can put it in the rather rigid chronostratigraphic framework (certain time span for amount of sediments).
At the end, Embry’s simple T-R sequence is more appropriate for analysis and interpretation, rather than others with three or even four system tracts. It gives the explorer more freedom for interpretation, for nature while simple still isn’t mathematics.
Zoran Kunštek <Zoran.Kunstek@ina.hr>
Ashton Embry wrote on 2nd August 2007 at 1.02PM
Thank you very much for your comments. It is always useful to hear from a practitioner with no axe to grind. I have only a few comments to make in reply to your statements
“I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn’t it be more appropriate to use geochronology for interpretation?”
I could not agree more. The identification of specific time surfaces is always very difficult and subjective and is rarely reproducible. Our experiences over the last 100 years in lithostratigraphy and biostratigraphy have clearly shown us that chaos is the end result when you mix chronostratigraphy with material-based stratigraphy. The authors of the stratigraphic codes and guides purposefully separated chronostratigraphic practices from the material-based stratigraphic disciplines for very good reasons. Basically, almost all the problems and confusion currently associated with sequence stratigraphy are rooted in the attempted use of chronostratigraphic methods, surfaces and units in sequence stratigraphy. Unfortunately some of the strongest advocates of such a hybrid stratigraphic discipline (chrono-sequence stratigraphy) have little knowledge or understanding of the developmental struggles in stratigraphy which took place over the last 100 years.
Then, why not use term ‘retrogradational system tract’ instead of transgressive system tract for time span when basin is starving of sediment. Also, ‘progradational system tract’ instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall.
There are two reasons for preferring Transgressive ST over Retrogradational ST and Regressive ST over Progradational ST. One is that the name “transgressive systems tract” is entrenched in the literature and I don’t think it would be reasonable to change the name of this unit especially given that it is a well defined, material-based unit. Notably Galloway called the TST a Retrogradational ST unit but the name never caught on. Once you accept the name transgressive systems tract then name regressive systems tract becomes unavoidable.
The real problem lies in claims that the regressive systems tract can be subdivided into three systems tracts on the basis of the conjecture that two time surfaces can be recognized within the RST. Despite the fact no one has ever been able to recognize these time surfaces on cross sections or in outcrop or offer any reasonable guidance on how one might recognize them, the advocates of chrono-sequence stratigraphy still base their entire sequence stratigraphic classification system on these abstract entities. To me this is a classic case of a model rather than data controlling classification.
I would also note that a final phase of progradation of the shelf margin can occur during the initial phase of transgression and the early development of the TST. This does not happen often and it is often localized to areas of high supply. However, because it does happen, the start of progradation and the start of transgression do not always coincide.
“ Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression?”
The main problem with an SU-RSME-MRS boundary is that the basinward end of the SU is not co-terminus with the landward end of the RSME. Thus these three surfaces do not form one continuous boundary, a key requirement of a sequence boundary.
As discussed in the report, the SU-SR-U-MRS very often do form one continuous boundary and such a combination of surfaces represents one option for a depositional sequence boundary (defined as an SU plus correlative surfaces). Such a boundary basically separates strata deposited during regression below from overlying strata deposited during transgression and thus is a boundary between two different “genetic” deposits.
"Embry, Ashton" <AEmbry@nrcan.gc.ca>
Kunštek Zoran wrote on Jul 19, 2007 at 3:01 AM
I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn’t it be more appropriate to use geochronology for interpretation, according to no isochronous sequence surfaces? Later some can put it in to relatively rigid chronostratigraphic time span (time table).
Next question is connected to Embry (2002) descriptive term, as he mentioned in papers, ‘maximum regressive surface’ (MRS) and two system tracts that Embry proposed TST and RST. As he mentioned MRS is appropriate descriptive term. Than, why not use term ‘retrogradational system tract’ instead of transgressive system tract for time span when basin is starving of sediment. Also, ‘progradational system tract’ instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall. Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression?
Sorry for my English,
BSc Zoran Kunštek
INA d.d. SD Naftaplin
Croatia, 10 000 Zagreb
Tell. 00385 1 4592632,
Mob. 00385 98 1602965
Embry, Ashton wrote Jun 20, 2007 5:44 PM
In reply to Alex MacNeil's Comments of June 12 on “Shoehorning Essay”
It is nice to see someone commenting on my comments. That is what debate is all about. And it especially helpful to have Alex debating the subtleties of recognizing a BSFR and a CC because that is a key topic in this debate. If we can all agree how one can recognize BSFRs and CCs in our sections just like we do MFSs and MRSs we can all go away happy. Of course, if reasonable criteria are not established for recognizing and correlating these time surfaces, I suspect we will remain at a stalemate with the co-existence of a material-based approach as advocated by ISSC and a mixed time-based/material-based approach as advocated by IWGSS. I am confident that common sense will eventually win the day.
MacNeil: “Dr. Embry’s paper on “shoehorning” in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models.”
Embry: I just want to emphasize that the two examples discussed in the Shoehorning Essay were not chosen by me but were offered by the authors as good examples of studies in which BSFRs and CCs were recognized in well exposed strata. I simply commented on the problems associated with such interpretations. I do not know if these studies are representative or are “good” examples of such analysis.
MacNeil: “I find his paper to be more an example of why grey literature should be taken with a “grain of salt”
Embry: I don’t consider that Shoehorning essay as part of the literature, grey or any other colour. It is simply part of this online debate and I assume no one will ever formally reference anything on this webpage.
MacNeil: “I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption.”
Embry: Alex and I agree that the BSFR is a time surface that separates strata deposited during base level rise below from strata deposited during base level fall above. It is well accepted that the BSFR is a conformity and Catuneanu often refers to it as a “correlative conformity”. This means there is either a very small or no time gap across the surface.
If an unconformable surface separates strata deposited during base level rise below from strata deposited during base level fall above then such a surface is referred to as the type of unconformity it is (e.g. MFS unconformity) and not as a BSFR. I would hope everyone accepts these rather straightforward constraints on a BSFR.
Given the above, then the BSFR does indeed represent the time surface at the start of base level fall with the proviso there may be an immeasurable short time lag (days?) between the start of fall and the first particle of sediment to hit the sea floor at a given locality.
As Posamentier and Morris (2000, p.39) state “this bounding surface (they are referring to the BSFR) has chronostratigraphic significance insofar as it represents the paleogeography at a moment in time” There is no doubt as to what moment in time they are referring to because on the same page they say the BSFR “represents the surface that exists at the time of initiation of sea level fall”. Notably Catuneanu also refers to the BSFR as the ‘paleo sea floor” at the start of forced regression as mentioned by Alex.
I hope Alex will give some careful thought to what a BSFR is and what it represents. It is quite clear that, by definition, it forms at the moment of start base level fall.
MacNeil: “the BSFR marks the base of sediments deposited during forced regression.”
Embry: Once it is realized that the BSFR is a conformable surface then such a statement logically leads to the conclusion that the BSFR marks the start of base level fall at the shoreline (forced regression).
MacNeil “With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18,”
Embry: There are two problems here. The main one is the surface referred to as a BSFR on the outer shelf is an unconformable MFS and not a BSFR. Marginward, MacNeil and Jones have interpreted a BSFR and show it as a dashed line downlapping onto the MFS (see their fig 18). They have not presented any evidence that at that precise horizon there is a change in sedimentary composition such that one could reasonably interpret the strata immediately below such a surface were deposited during base level rise and the strata immediately above were deposited during fall.
Almost all of the arguments on pages 349-352 deal with the deposits on the outer shelf which directly overlie the MFS. These may well be all deposited during fall but the surface at their base is an MFS not a BSFR. The only bona fide attempt at identifying a BSFR is that dashed line on Fig 18 which is underlain by “HST” and its placement is more of a guess than a well supported interpretation.
MacNeil: “This paragraph of Embry is completely incorrect – we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position”
Embry: I got out my magnifying glass and my calipers and indeed Alex is right, M+J do show a very small rise in sea level (a metre or two at best) between stage C and stage D.
It is not easy to see on their diagram that the marine carbonate sediments are onlapping the basinward end of the unconformity which must be a SR-U, given that marine strata overlie it. I will accept their interpretation that some marine carbonates onlap the very end of the unconformity although I’d feel better if this contact was described in detail and it was demonstrated beyond a reasonable doubt that the strata underlying the onlapping strata had been subaerially exposed.
Regardless, this interpretation has no bearing on their interpretation that the base of the Biohermal interval is a CC; that is, the time surface formed at the start of base level rise. Such a thin interval of onlapping strata could easily be equivalent to only the uppermost portion of the shallowing biohermal strata and could even be reasonably interpreted as being correlative with the TST interval at the top of the biohermal interval.
The important consideration here is MacNeil and Jones’ interpretation that carbonate sedimentation greatly increased at the start of base level rise (started at the CC). This is not sedimentologically reasonable. At the start of sea level fall the outer shelf was rather deep and was not the site of much in situ carbonate deposition (thin biostromal unit). As sea level fell it eventually resulted in the outer shelf becoming shallow enough to become a carbonate factory and this is marked by the initiation of the biohermal unit.
It is not reasonable to interpret, as M+J do, that sea level fell exactly to the critical water depth to initiate a carbonate factory on the outer shelf and then IMMEDIATELY started to rise. It is likely it kept falling during part or all of the biohermal growth and then eventually started to rise, probably very near the top of the biohermal unit. My experience with initial base level rise in carbonates is that it is often very fast and the start of base level rise coincides with transgression. Such a “standard” scenario is easily accommodated in the stratigraphic relationships illustrated by M+J by correlating the onlapping strata with the TST (see M+J fig 18 for how easily this can done). There is no justification for interpreting the base of the biostromal unit (or any other specific horizon within it) as start rise. A better (simpler) case can be made for start rise at start transgression but even this interpretation is somewhat speculative. The bottom line is it is impossible to identify the horizon that marks the base of base level rise with any semblance of objectivity and any attempt to do so is simply shoehorning.
MacNeil: I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The “shoehorning” argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006).
Embry: I would maintain that M+J provides a good example of shoehorning. Alex felt he had to identify a BSFR and CC (Alex was taught sequence stratigraphy by Catuneanu at U of A) and thus chose surfaces he could see and labeled them as a BSFR and a CC. Unfortunately, as discussed herein and in the Shoehorning essay, such surfaces are not reasonable representations of the time surfaces at start BL fall and start BL rise on the basis of either empirical or theoretical considerations.
If he had not been instructed to find such surfaces in order to recognize a HST, FSST and LST, I doubt if he would have made such interpretations. Such interpretations are forced on the data by the interpreter thinking that he/she needs to follow the prevailing mixed time-based/material-based approach (trying to see the unseeable). If Alex had used the material-based approach, as is done in all other stratigraphic disciplines, he would not have felt compelled to label (shoehorn) various facies changes or unconformities as time surfaces (BSFRs and CC).
"Embry, Ashton" <AEmbry@nrcan.gc.ca>
Embry, Ashton wrote Jun 20 2007 1:59 PM in reply to Mark Smith
I just read your comments on the debate website. I enjoyed your talk at the CSPG conference but did not get the chance to speak with you. I think we can profitably exchange views.
I would be interested to hear more about the surface you have interpreted as a BSFR and a picture or two and even a diagrammatic X section would be helpful for a discussion.
First of all you have decide if the recognized surface is an unconformity (significant time gap across it) or is conformable (little to no time gap across it). If the surface is conformable, you have to ask yourself "Is it reasonable to interpret this conformable facies change as a time surface which formed at the moment (and I stress moment) base level began to fall?".
If the surface is an unconformity, you have to ask how such an unconformity was created and whether or not it is reasonable to interpret that all strata below the surface formed during base level rise and all strata above were deposited during base level fall. As long as you can defend your interpretations with data and solid theoretical reasoning then the sharks won't have a feeding frenzy on a BSFR interpretation .
You are wise to try and get some clarification at this stage of research. There is a great deal of extraneous material and innumerable, very suspect interpretations in the sequence stratigraphic literature and it is not easy for students to know what is good and what is not good.
"Embry, Ashton" <AEmbry@nrcan.gc.ca>
On Jun 18, 2007 1:27 PM Mark D Smith wrote
As a student who plans on publishing his work to the scientific community in the near future, I am finding the recent debate on sequence stratigraphy, specifically the basal surface of forced regression of great interest. Unfortunately, also being a student makes me a little hesitant to jump into the waters with the sharks and so I will keep this communication brief, and if people on either side are interested I can elaborate further.
My research examines Neoproterozoic deep-water (slope and basin facies) sedimentary rocks (Old Fort Point Formation) in the southern Canadian Cordillera and I recently presented some of the material at the 2007 AAPG and CSPG. Although the MRS and MFS can be identified, they are difficult to pinpoint where the exact surface is in outcrop (more like zones rather than surfaces). However, the BSFR (as I have interpreted it) is a sharp contact, easily recognizable in the field, possesses different facies (members of the OFP) above and below the delineated surface and can be correlated across the Windermere basin (>35,000 km2). The surface empirically exists, whether a label is attached to it or not. For those in the Alberta, Canada region, this surface can be easily viewed at the Lake Louise Ski Hill or along the Yellowhead highway about 14 km west of Jasper, Alberta.
I have numerous photos, sections and thoughts on how this surface is expressed in my research (not neccessarily sequence stratigraphy overall) and would be interested to know what people think prior to submitting my work for publication. Thank you for your time.
Mark D. Smith
Department of Earth Sciences
Marion Hall, 140 Louis Pasteur
University of Ottawa
Ottawa, ON K1N 6N5
Ph: 613.562.5800 x6729
"Look at me, I'm a grad student. I'm 30 years old and made 600 dollars last year." - Bart Simpson
On June 12, 2007 at 2:39 pm Alex MacNeil wrote
Reply to “Shoehorning in Sequence Stratigraphy”
Dr. Embry’s paper on “shoehorning” in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models. As the primary author of MacNeil and Jones (2006), I find his paper to be more an example of why grey literature should be taken with a “grain of salt”, and not generally referenced in primary research articles. I feel that he has misrepresented and ignored important parts of MacNeil and Jones (2006), and distorted definitions to create/justify his arguments. While I offer my perspectives, I encourage everyone concerned with these issues to read the references and draw their own conclusions.
1. In our paper, we identify two BSFR’s in a Late Devonian succession. All of Embry’s arguments challenging our recognition of the surface are based on his definition of a BSFR, which is not the definition found in the literature. The BSFR was defined by Hunt and Tucker (1992) as:
“a chronostratigraphic surface separating older sediments of the preceding highstand systems tract, deposited during slowing rates of relative sea-level rise and stillstand, from younger sediments, deposited during the base-level fall” (pg. 5; my italics added).
From additional discussion in Hunt and Tucker (1992) it is clear that recognition of a BSFR does not imply that overlying deposits were deposited the instant base level started to fall – the definition otherwise applied by Embry. Furthermore, Hunt and Tucker (1995) stated that the BSFR marks the “onset of falling sea-level as recorded in the rock record” (pg. 149). I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption.
Catuneanu (2002) was referenced in our paper as providing the definition for the BSFR. Catuneanu (2002) states, “The basal surface of forced regression was introduced by Hunt and Tucker (1992) to define the base of all deposits that accumulate in the marine environment during the forced regression at the shoreline” (p. 16). He goes on to state that the surface “represents the paleo-sea floor at the onset of base level fall at the shoreline”. This statement does not imply or require that deposits above the BSFR correspond to the instant that base-level started to fall, and we do not accept this assumption being applied to our study. Indeed, on pg. 340 in our paper, we state that a lag period existed between exposure of Reef Complex #1 and initiation of deposits that form the FSST – a statement ignored by Embry. Simply stating the work of Hunt and Tucker and Catuneanu, the BSFR marks the base of sediments deposited during forced regression. Yes, the surface is diachronous, but its recognition allows any geologist to walk up to the outcrop and understand that rocks above the surface were deposited at some point during the base-level fall, and are temporally distinct from underlying rocks (of deeper water origin) that belong to the preceding highstand systems tract. This can have important implications for understanding controls on the carbonate sediments that accumulated and how the system evolved.
2. With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18, which Embry fails to mention. This discussion includes reference to recent analogs in the Bahamas and Cayman Islands.
3. On page 7 of the “Shoehorning” paper, Embry states, “Notably there is NO indication of base level rise during the deposition of the biohermal deposits of the outer ramp. This is clearly indicated on the MacNeil and Jones diagram which shows NO onlap on the unconformity during the deposition of the biohermal sediments. Also note that their sea level position is at exactly the same level in the Phase D section as it is in Phase C section despite their claim of base level rise during this time. Thus, by way of their diagram, even MacNeil and Jones acknowledge that base level was not rising during the deposition of the biohermal sediment.”
This paragraph of Embry is completely incorrect – we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position only because 1) we placed its lower position using rather conservative field evidence, as discussed on pg. 335, 2) the large scale of the figure, and 3) the figure was originally drafted at a full tabloid size and scaled-down to letter size in the publication process. The yellow and light-yellow facies at the left hand side of Reef Complex #2 are onlapping reef-front and fore-slope deposits of Reef Complex #1. This was discussed in the paper and figured close-up in Figure 13, which was titled “Deposits of Reef Complex #2 onlapping margin of Reef Complex #1” – but this was not mentioned in Embry’s paper.... The reef-front of Reef Complex #1 is onlapped by restricted lagoon deposits of Reef Complex #2, resulting in a clear juxtaposition of shallow water facies against older, deep water facies. Even Esther Jamieson, in her original work at this locality for her PhD thesis in the 1960s, which was only based on weathered outcrop (not the nice roadcut that we had), noted (Fig. 9 in her thesis and page 30 in the text) that there are facies discrepancies at this locality that are not “normal”. Even though she failed to recognize the significance of the facies relationships, it was obvious that a significant stratigraphic feature is present. To state that we show “no onlap” is incorrect, and to not acknowledge the text discussions about the onlapping succession, or the corresponding, full-page figure that details the onlapping, blatantly misrepresents our work.
I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The “shoehorning” argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006).
Alex MacNeil <email@example.com>
On Jun 8, 2007 at 12.00 pm Ashton Embry wrote
Thank you for your very thoughtful and clearly stated comments. Some of your points were emphasized in the ISSC report and I would say we are in agreement on most of the points you make. I think there is some misunderstanding on a few points and I thought it would be worthwhile if I clarified them so that everyone is clear on what was actually recommended in the ISSC Report.
"Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes".
In general this is correct with one caveat. Using time as the interpretive element is fine for chronostratigraphy. Chronostratigraphy was introduced specifically in the 1940s to take time concepts and time surfaces/units out of the material-based disciplines, which now would include sequence stratigraphy, and gather them up into one discipline. Chronostratigraphy, as you rightly note, depends on the integration of correlations of material-based horizons/boundaries/surfaces.
Basically one evaluates the diachroneity of all available material-based boundaries from the five disciplines and makes the best guestimate of the location of time surfaces. You can't actually see a time surface (it has no physical characteristics), only infer its position. Notably, each of the five material-based disciplines includes surfaces with low diachroneity and surfaces with high diachroneity. The trick is to use only those with low diachroneity (and time barriers of sequence stratigraphy) for interpreting a local chronostratigraphic framework. The available biostrat boundaries allow one to correlate the local framework to the global time scale.
"We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column."
In this case one would be mapping biostratigraphic boundaries and not time surfaces. Such boundaries often have low diachroneity so are great for interpreting time relationships by means of compensating for the diachroneity. I think it is crucial to keep the concept of a time surface completely separate from a material-based surface which essentially is never a time surface no matter how much we would like it to be.
"Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance."
You are right that various surfaces recognized in sequence stratigraphy are either (approximate) time barriers or have low diachroniety and, as emphasized in the ISSC Report, such surfaces are most valuable for correlation, framework building and unit definition
"Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.) ".
This is exactly what is done in sequence stratigraphy as was discussed in the ISSC Report. One MFS can look exactly like another and correlation lines from other material-based disciplines (bio, mag, chemo, litho) can be most useful in guiding the matching of specific sequence stratigraphic surfaces.
I might point out here that it is not acceptable to extend a sequence stratigraphic surface (the boundary of a sequence stratigraphic unit) such as an MFS along a biostratigraphic or magnetostratigraphic boundary just as we can't extend a biostratigraphic boundary (e.g. a FAD) along an MFS. A specific MFS can only be extended by correlation to MFSs at other localities with boundaries from the other disciplines sometimes guiding such correlation.
"What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterion or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother?"
Exactly!! This is chronostratigraphy and is NOT sequence stratigraphy. Thus it would be against the rules in sequence stratigraphy but completely within the rules for chronostratigraphy. As the authors of the Code and Guide emphasized, it is most important to not mix the two very different stratigraphic methods/boundaries/units. This is the basic reason why the ISSC Report recommended NOT to use the BSFR and CC which are time surfaces, for which, in John's words, "there is no physical criterion or discrete surface to map.". We could not have expressed it better!!
"Most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn’t a thing wrong with that approach."
I agree there is absolutely nothing wrong with this approach and this is exactly what was recommended and emphasized in the ISSC Report. Mapping physical surface on the basis of their characteristic physical criteria is the empirical approach we have strenuously argued for. I would emphasize that John's critical phrase is "mapping physical surfaces with physical criteria ". The physical criteria that characterize a given surface allow it to be recognized at various localities and make it available for correlation.
"It’s pretty clear that the masses do not particularly like the Ashton choices."
I think everyone feels fine about the utility of the SU, SR, MRS, MFS and RSME which are the surfaces the ISSC TG has recognized as valid surfaces of sequence stratigraphy and recommend for usage in sequence stratigraphic unit building.
What the masses don't seem to like is our "choice" to interpret that the BSFR and CC are NOT valid surfaces of sequence stratigraphy because no one has ever presented any physical criteria to allow them to be recognized in outcrop or core. Furthermore, given that they are defined as time surfaces which are independent of sedimentation (paleo-sea floors in the IWGSS report), such surfaces meet all the criteria for being chronostratigraphic surfaces.
"Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define."
Given that you have not discussed a BSFR and a CC, I would agree with this. However the debate/ disagreement between the ISSC TG and IWGSS boils down to whether or not the BSFR and CC are material-based surfaces of sequence stratigraphy or are time-based surfaces of chronostratigraphy. It simply comes down to, are there or are there not adequate physical criteria for recognizing such surfaces in well exposed strata in most geological settings. The five agreed upon surfaces meet this requirement. I have recognized and correlated thousands of such surfaces and published cross sections illustrating them. So has Octavian for that matter.
MacNaughton et al and MacNeil and Jones (and thousands of other authors) have also had no problem with delineating and correlating acceptable MFSs, MRSs, SRs and SUs. However, as discussed in the Shoehorning essay, they could not delineate and correlate a single, acceptable (meets the definition of the surface) BSFR or CC . Notably no one else has ever accomplished such a feat including Octavian. Of course many have used high diachroniety facies surfaces as shoehorned compromises which lead to not-so-great, non-actualistic interpretations.
The fact that no one has been able to recognize such surfaces in rocks underscores the lack of physical criteria required to have them accepted as bona fide, material-based surfaces of sequence stratigraphy as discussed in the ISSC Report and by John in his earlier comment. The best anyone has done is to "tag" a seismic reflector as such a surface. I would emphasize such seismic interpretations are highly equivocal and do not constitute proof of existence of a discrete, specific surface type. Such seismic tagging is simply another form of shoehorning and Octavian’s favourite seismic section, which is used repeatedly in his text book, is a classic example of this.
"While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary."
Actually you might do well to begrudge Henry such a sequence boundary simply because it has no physical characteristics which allow it to be recognized and correlated. Even Henry acknowledges such a lack of physical attributes. I think it is most important to understand that Henry Posamentier sees sequence stratigraphy as equivalent to chronostratigraphy. Here are a few quotes from Henry: “The essence of sequence stratigraphy involves the recognition of time synchronous surfaces”; “The correlative conformity represents a surface that exists at the time of initiation of sea-level fall”; “The correlative conformity may have no obvious diagnostic attributes”.
Henry and Octavian represent the view that sequence stratigraphy includes chronostratigraphy and that is the basic differences between their mixed time-based/material-based approach and the material-based approach of the ISSC TG. Given that you appear to be on the material-based only side of the street, you might want to more closely examine what Henry proposes as a sequence boundary.
"That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins."
Here is where I think there is some significant misunderstanding of the recommendations of the ISSC Report. The Report emphatically recommends that a sequence/ sequence boundary be completely generic and that workers are free (encouraged) to define as many specific types of sequence boundaries as their data allow. The generic definition offered is that a sequence boundary is a specific type of unconformity and its correlative (i.e. physically join together) surfaces. Doesn’t get any simpler or flexible than this! Two common types of sequence boundaries were discussed in the Report and the definitions of these two types were taken straight from the literature (Van Wagoner et al. 1988 and Galloway, 1989).
One was a depositional sequence boundary defined as a subaerial unconformity and its correlative surfaces. The other was an R-T or genetic stratigraphic sequence boundary defined as an unconformable MFS and its correlative surfaces. It was further noted that other types of sequence boundaries could be defined on the basis of other specific types of unconformities and their correlative surfaces.
Also, it is even possible to have different sub-types of a given type of sequence boundary. For example we noted that very often a MRS and SR-U are correlative (join together) with an SU and thus represent a one possible combination of surfaces for a bona fide depositional boundary. If other workers demonstrate that other types of material-based surfaces correlate to an SU, they could establish other combinations for a depositional sequence boundary.
We believe it would be inflexible if only one specific combination of correlative surfaces were allowed for a given type of sequence boundary as advocated by Catuneanu (2006) and the IWGSS report. It is the specific unconformity type which defines a given type of sequence boundary NOT the correlative surfaces. We believe this is about as flexible as it gets and certainly matches/exceeds the flexibility of the IWGSS proposal.
"Codifying any of these surfaces as the “true bounding surface” just means that most of the remaining sequence-stratigraphic community will simply ignore the code."
No argument here and I hope you agree that it is worthwhile to try to reach consensus on specific names for specific types of boundaries and units to help communication. We suggested commonly used names for the two types of sequence boundaries most often employed in sequence stratigraphy. We leave it to others to name other types of sequence boundaries.
"Likewise, the other forms of stratigraphy don’t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code."
As was noted in the ISSC Report, we used the biozone concept as the guide to how a sequence can be effectively defined. In both cases the definitions are generic and modifiers allow specific types to be defined - specific types of fossil change (e.g. assemblage biozone) or unconformity types (depositional sequence). As mentioned earlier, this system allows new types of biozones/sequences to be defined as required.
"The individual worker is given enormous latitude as to what can be lumped into a lithostratigraphic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy?"
I hope you can now see we are not being picky but are being extremely flexible. All we ask is that material-based surfaces be used as they are in every other stratigraphic discipline except chronostratigraphy. Unfortunately a few of our most vociferous detractors have completely misrepresented the ISSC recommendations in various posts so I can understand why John had a wrong and negative impression of our recommendations for defining sequence boundaries.
"While it has difficulties too, a more inclusive approach like that favored by Octavian."
I would note that Octavian's approach is less inclusive. It does not offer a generic definition for a sequence. It does not allow different sub-types for a given sequence boundary. The biggest problem of course is that Octavian encourages the use of featureless time surfaces for the definition of different types of sequence boundaries. This last problem of course leads to all the problems discussed in the Shoehorning essay and is the source of the “difficulties” that John alludes to.
I would again reiterate it is hard to imagine how one could come up with a more flexible and pragmatic approach than that recommended in the ISSC Report. A worker can mix and match as he/she pleases with the only caveat being that all surfaces have to be material-based.
" I can think of no processes that demands shoehorning more than codification."
There is no doubt that guidance or a codification in an official publication of an international stratigraphic commission could potentially lead to major shoehorning IF the guidance/codification is based on a non-material entity. It is critical to note that main problem which leads to shoehorning is not where the recommendation is but the nature of the recommendation. Once you recommend a non-material surface for a boundary, people are going to have to use "compromise" material-based surfaces to compensate.
Thus, even though neither NACSN or ISSC have not offered any recommendations on sequence stratigraphy, there is MAJOR shoehorning going on simply because a couple of abstract time surfaces have become entrenched in the literature and textbooks on sequence stratigraphy.
Sedimentary geologists are pressured to identify these entrenched, abstract surfaces (peer pressure) and, as discussed in my recent essay, this has lead to a lot of most unfortunate shoehorning and non-actualistic interpretations of sedimentary processes and depositional histories. "Ulrich-ism" reigns supreme right now with many people interpreting diachronous facies contacts as time surfaces.
"The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code."
Not really. As long as we recommend, material-based surfaces to define units there will be no need to shoehorn. I would note that there is no noticeable shoehorning in the other material-based stratigraphic disciplines. When the time surfaces are eliminated, everyone will be able to recognize the accepted surfaces and, in most instances, there will be agreement on their placement. I am sure, if I took John, Octavian and Ron up one of my sections, we would all agree on the approximate placement of the MRS, MFS, SU and SR-U. We might haggle over a few tens of centimetres for the exact placements and such is interpretation. The same goes for a well log section although there may be some slightly larger variation in placement choices due to the lower resolution. However we could still have a good discussion as to which picked surface was the best one.
Most importantly, none of us would be able to point to a BSFR or a CC in the best exposed section even though such time surfaces would be there in theory. Each of us might shoehorn such surfaces in at very different horizons and there would be no way to discuss which is a better shoehorned pick. And that is the basic difference between the empirical surfaces which need no shoehorning and the unrecognizable time surfaces which do.
"By the way, I noticed Octavian’s e-mail has somehow fallen from the mailing list."
Good catch. It would be nice if Octavian could supply a few examples of where he has identified and correlated BSFRs and CCs in rocks (outcrop, logs) so we get beyond the “do as I say, not as I do” problem. He certainly has published lots of cross sections with MFSs, MRSs. SRs and SUs on them. The fact that his book lacks a single stratigraphic cross section which illustrates the delineation and correlation of BSFRs and CCs is telling.
Ashton Embry: AEmbry@nrcan.gc.ca
On Jun 8, 2007 at 1:07pm John M Holbrook wrote
Please to feel free to post these comments and the Ashton response. My comment pertained mostly to the e-mail exchanges, but Ashton points out that other people are involved in the debate who are not on the e-mail listing. Please to shift these on over.
I have been following the sequence-stratigraphic debate between Aston and what appears to be the sequence-stratigraphic world at large with great interest. I would like to humbly offer some comments regarding points that seem to have emerged from the debate.
1) Aston does have a valid point.
The stratigraphic code is built on the core principle that stratigraphic units should be defined based solely on “objective” and “reproducible” descriptive criteria. Each form of stratigraphy is defined based on the criteria that are used to define boundaries/units. The main issue with codification of sequence stratigraphy from the beginning has been that we are accused of imposing interpretive criteria for defining key units and boundaries. Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes.
2) Time as the force behind defining criteria
While it might be bad form to use a time-interpretation as criteria do define a boundary, it is perfectly legitimate to define a boundary based on observable criteria that has time significance. Case in point, biostratigraphy. We all recognize this as the most fundamental means of defining “time-rock” units, and commonly define time-lines in the rock record using this method. We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column. We also know enough about evolution to know that species dispersion and extinction are diachronus events. Yet, we have long felt comfortable basing our original concept of geologic time, and the fundamental process of mapping time-rock units, based on biostratigraphic criteria. Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance. It is perfectly legitimate to deliberately choose to map these surfaces because of the expectation that they will yield time/event-significant information. This is indeed one of the more powerful aspects of sequence stratigraphy. Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.). What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterian or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother.
In short, mixing time and physical criteria to define units is far from unprecedented; it is in fact the norm. What we are charged to do, however, is clearly define the physical criteria we use to define boundaries, and state the time significance of this choice separately. Though we commonly appear to “define” boundaries by such time/event criteria as first episode of relative sea-level drop, most all of us are actually defining boundaries by such physically mapable criteria as landwardmost surface of downstep in coastal onlap. While it isn’t so hard to find a person who violates from time to time, most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn’t a thing wrong with that approach.
3) Surfaces surfaces everywhere!
Most of the disagreements seem to stem from the fact that we each prefer to map different surfaces from a wide selection of possible surfaces. This is usually either because these are the surfaces that are easiest to map given the data and settings in which we tend to work, or because these are the surfaces that yield the information we prefer to know. It’s pretty clear that the masses do not particularly like the Ashton choices. While I’m thinking I might find more support for my choices, I would hardly expect them to be universally accepted as the pure surfaces either. Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define. That’s why we like to use them. That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins. Codifying any of these surfaces as the “true bounding surface” just means that most of the remaining sequence-stratigraphic community will simply ignore the code. This is destine to generate a lot of confusion. Likewise, the other forms of stratigraphy don’t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code. The individual worker is given enormous latitude as to what can be lumped into a lithostratigrahic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy? While it has difficulties too, a more inclusive approach like that favored by Octavian makes a certain since if we are to bother with codification at all.
I can think of no processes that demands shoehorning more than codification. The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code. How would science have been served by codifying Vail, et al 1977?
By the way, I noticed Octavian’s e-mail has somehow fallen from the mailing list. I added it back in my listing.
"Holbrook, John M" <firstname.lastname@example.org>
On June 6th 2007 William Helland-Hansen wrote
Guidelines for Sequence Stratigraphy
– Comment to the ongoing debate
Dear Sequence Stratigraphers
There are a few points that I feel is important to communicate after having read most of the ongoing dialogue on the formalization of Sequence Stratigraphy:
Without rephrasing the definition of sequence stratigraphy, this discipline basically deals with unconformities (with an unconformity defined as “a surface of erosion or nondeposition that separates younger strata from older rocks and represents a significant hiatus”, Mitchum, 1977) and the sedimentary deposits between the unconformities – all put together in a genetic and chronostratigraphic framework.
Unconformities include levels of erosion, non-deposition and very slow deposition that can mark facies-belt jumps - or major environmental shifts - across surfaces or condenced intervals. They are caused by base-level changes and brought about most commonly by allogenic forcing mechanisms, but also by large scale autogenic forcing mechanisms (e.g. delta-lobe shifting).
Deposits between unconformities consist of sediment and minor breaks (diastems) (e.g. common channel erosion, base of storm bed, base of turbidite etc) not caused by base-level changes (Embrys WTFC). These between-unconformity successions can be analyzed in terms of Walther’s law and are deposited in a response to a particular trend of environmental change – a migration trajectory of the depositional system.
The systems tract concept, as currently applied, should be avoided in formalization attempts because it anticipates a succession in depositional trends. For example: 1) Vertically expanded successions of alternating regressive and transgressive strata without intervening base-level falls and 2) laterally expanded successions of alternating normal and forced regressions, without intervening transgressions, are both common in the stratigraphic record. These situations are not included in the proposed schemes. Furthermore, neither of the drafts from the two groups propose how the stratigraphic problems of sequences delineation in continental and deep-marine successions should be tackled.
Formalization, if any, should be on the “tool box” level (cf. earlier comment by Ole Martinsen) – without any pre-conceived succession of sedimentary trends and tracts: What kind of breaks do we have, and what kind of trends do we have between the breaks? If we can really define all these as “concrete” – without forcing successions of events or trends into one universal template – then we may be a step further. Meanwhile, the concept of allostratigraphy plus the original definition by Mitchum 1977 (“.. a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities”) seem to work just fine.
May June 6th 2007
William Helland-Hansen, University of Bergen
On June 3rd 2007 Ashton Embry wrote:
Reply to Alex MacNeil
It was a very nice surprise to see that our debate attracted comments from a member of Gen Y, the generation that represents the future of our discipline. It was also very encouraging that Alex addressed the main issue of the debate in his incisive comments and I thank him for reminding me of another example of an attempted application of the seven-surface/four systems tract approach to a real world succession. Such real world examples are completely lacking in Catuneanu (2006) and the draft IWGSS report and they are undoubtedly the acid tests for the acceptance or rejection of such an approach.
Below are replies to some of Alex’s specific comments.
“In several parts of the ISSC draft paper (e.g., page 44), the authors assert (rather strongly) that no one has ever presented any defining characteristics of a BSFR or demonstrated such a surface in core or outcrop. Before making such strong assertions, the authors should have taken the time to familiarize themselves with the recent literature on sequence stratigraphy. The MacNeil and Jones (2006) paper in Sedimentology documented the BSFR in outcrop and core, and undermines some of the key arguments made in the ISSC paper.”
Actually we make the point that no one has ever presented any suitable/acceptable defining characteristics for a BSFR or a CC. Many people have used various types of real stratigraphic surfaces that have observable characteristics as “compromises” for the two time surfaces. However, in every case we looked at, such “compromise surfaces” were not time surfaces (paleo sea floors) or even reasonable approximations.
I may be slowing down in regards to remaining current with the sequence stratigraphic literature but I try to read as many papers as I can. I hope Alex will be happy to hear that I read his paper soon after it was published. I cut my stratigraphic teeth on Devonian reefal carbonates almost 40 years ago you never forget your first love. I downloaded the MacNeil and Jones sequence stratigraphy paper as soon as I became aware of it. I was flattered that they referenced my very old papers from the early 70s and even used those less than euphonious, lithologic terms (e.g. bindstone) I dreamed up while lying in a hospital bed in 1969.
As is discussed in the “Shoehorning and its Consequences” post, I found that the surfaces MacNeil and Jones labeled as BSFR (2 surfaces) and CC (1 surface) do NOT fit the definition of a BSFR and a CC. The compromise surfaces labeled as BSFR and CC are an unacceptable attempt to recognize such surfaces. I will not discuss the details of these surfaces in this reply and the reader is referred to ‘Shoehorning and its Consequences” for a detailed discussion of the MacNeil and Jones’ attempt to recognize a BSFR and a CC.
“Likewise, the biotic composition of the biostrome is consistent with an intermediate water depth (ironically, I can reference some of Embry’s original work from the Arctic on suggested absolute water depths for Devonian carbonate facies to support the intermediate water depth) between deeper water, nearly-drowned carbonates of the TST and HST and shallow water, high-energy carbonates of the ensuing LST.”
I am pleased my past work helped Alex in facies interpretation. There is no doubt MacNeil and Jones have identified a nice shallowing upward succession from the basal TST to the top of the high energy carbonates which, as interpreted, are capped by a well documented MRS. The question becomes how one can recognize the time surfaces at the start and end of base level fall in such a shallowing upward succession.
MacNeil and Jones have used within trend facies boundaries to do so and, as discussed in the “Shoehorning” post, their rationalizations that such facies changes occur at the instant that base level starts to fall or starts to rise are not convincing. In fact they demand very unlikely and non-actualistic sedimentological processes and events. I can understand why they chose such facies contacts as their best guess as to the placement of the time surfaces because the facies contacts were the only recognizable surfaces present. In other words, these surfaces were the only “compromise surfaces” available to label as a BSFR and a CC.
“In carbonate systems with ramp-type geometries, the Basal Surface of Forced Regression can be identified as the surface that separates depth-controlled biofacies, and biofacies controlled or influenced by trophic resources, if the change in base-level shifted nutrient regimes…. Note that sediment shed in the HST has a specific grain size range (for review see MacNeil and Jones 2006) and changes in grain size can be used to constrain the shift from normal to forced regression.”
The basic, underlying assumption here is that such changes (e.g. in nutrient regimes, grain size, etc) happen over a large area at the instant when base level starts to fall. Thus it is assumed that such changes mark a paleo-seafloor (time surface) at the start of BL fall. This assumption of very significant changes occurring over the basin at the moment of start base level fall is not reasonable or actualistic. There is no doubt changes occur as base level falls but the preserved record of such changes (i.e. the recognized facies change) varies in time and space. Thus labeling such a facies/ material-based change as recognized at various localities as a time surface is not realistic or acceptable.
“Delineation of the BSFR with these empirical criteria is consistent with the type of data that Embry et al. argue (e.g., last paragraph on page 40) can be used to define the MRS and MFS. As such, to argue that there is no empirical data that can be used to identify a BSFR invokes a double standard and is incorrect”
This is fundamentally not so. The BSFR is defined as a time surface whereas the empirical surfaces such as the MRS and MRS are defined by observable characteristics. The difference between the two types of definitions is profound.
MacNeil and Jones see a change in trend from a shallowing to a deepening in their shallow water carbonate rocks and, as described in the ISSC report, such characteristics are sufficient to delineate an MRS. This is all well and good and I doubt if anyone would dispute their placement of the MRS in this instance.
The BSFR is the time surface that marks the beginning of base level fall. MacNeil and Jones have not demonstrated that the observable facies change they observed (at the base of the biostrome) formed at the instant that base level started to fall. As argued in “Shoehorning”, it is reasonably certain that the recognized facies change did NOT form at the moment of the start of base level fall but most likely occurred AFTER base level began to fall. Thus, in contrast to their MRS, their BSFR is highly disputable. The reason for this is there are no noticeable changes in sedimentation over most of the basin at the instant base level starts to fall and consequently there are no reasonable criteria for delineating such a time surface.
The bottom line is that MacNeil and Jones have failed to provide “empirical data” that can be used to identify a “BSFR” (the paleo-seafloor at the instant when base level starts to fall). All they have done is to try to rationalize that the facies change they do see formed at the moment that base level started to fall. As noted earlier, such a rationalization “does not hold water”.
“It is not very “real-world” to limit sequence stratigraphic interpretations and systems tract definition to whether or not a specific surface is identifiable across a study region – a criterion argued for throughout the ISSC paper.”
If you want to delineate a given stratigraphic unit, say a TST in a given sequence, then one has to correlate the boundaries of that unit over the study area. If this cannot be done, such a unit has little to no utility for subsequent facies analysis and reconstructing depositional history. Of course it is recognized that at some localities the data may not be sufficient for the exact placement of a given boundary but this can only be determined if one knows what data are needed to recognize the boundary in the first place. In most cases the placement of the empirical surfaces of sequence stratigraphy can be reliably done at most localities at which strata are well exposed and the work of MacNeil and Jones clearly demonstrates this. Their MFSs and MRSs are reliably delineated in most sections. The same cannot be said for the BSFRs and CC.
“Systems tracts are delineated by sequence stratigraphic surfaces, but they are defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.”
This is a common error in understanding. The systems tracts are defined by their boundaries. Their boundaries are “defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.” It is important to understand the difference between how surfaces are defined and how systems tracts are defined. Data sets yield surfaces and surfaces yield systems tracts. Data sets do not directly yield systems tracts. One needs to go though the surface identification step first.
“So yes, if one chooses to limit themselves to sequence stratigraphic surfaces, they may find themselves largely limited to defining T-R sequences. If one chooses, however, to actually include study of the sedimentology and stratigraphy of the deposits between the surfaces, they can conclude with much more advanced interpretations – and therein lies the powerful nature of sequence stratigraphic concepts and methods with significant implications for how we understand strata and advance the science”.
We are getting close here. If we stick with surfaces, we can reliably identify an R-T sequence and a depositional sequence as defined in the ISSC report. So Alex is almost right with his first sentence.
In his second sentence Alex suggests we can make some more advanced interpretations by examining the strata within the empirical systems tracts, the TST and the RST. There is no argument with this. Thus in the RST we can sometimes say these specific strata were deposited under conditions of rising base level and these specific strata were deposited under conditions of falling base level (e.g. they have a RSME at the base and a SU at the top). The critical point that must be stressed is that we CANNOT draw the boundary between rising base level deposits and falling base level deposits with any semblance of objectivity and thus cannot have separate stratigraphic units based on this concept. There will always be a large interval of uncertainty, and any attempt to draw a line between rising and falling base level deposits superimposes a very speculative and unsupportable interpretation on the data. This is the last thing we want to do.
“I can not imagine that its guidelines, in the current form, will be accepted by the greater research community. In the very least, they need to correct their assertions that the BSFR is purely theoretical and has never been demonstrated in core or outcrop, as this is incorrect.”
I like to think most sedimentary geologists are rational, critical thinkers. To me, when one applies critical thinking to Catuneanu (2006) and the IWGSS draft report, the fruitlessness of trying to delineate abstract time surfaces and to use them for unit boundaries becomes readily apparent. It just seems few have ever thought about the problems of such an approach. I hope Alex will do such critical thinking and always ask himself “Can this sedimentological change I want to label as a BSFR be instantaneous over the basin and coincide with the moment of base level fall?”.
Based on current data, including the MacNeil and Jones paper, we stand by our “assertions that the BSFR is purely theore