01_1098 A new geochemical--sequence stratigraphic model for the Mahakam Delta and Makassar slope,

A new geochemical–sequence stratigraphic model for the Mahakam Delta and Makassar slope,Kalimantan, Indonesia:DiscussionTobias H.D.Payenberg*and Andrew D.Miall** I N T R O D U C T I O NThe article by Peters et al.(2000)presents a new geochemical–sequence stratigraphic model for the middle Miocene and younger rocks in the Kutei basin, east Kalimantan,Indonesia.Peters et al.’s(2000)re-interpretation of the lowstand systems tract containing transported terrigenous source rocks helps to explain the recent offshore discoveries in the Makassar Strait. The work is primarily based on the geochemicalfinger-prints of oil condensatesfitted into a sequence strati-graphic model modified from a previous version by Snedden et al.(1996).It is this sequence stratigraphic framework with which we take issue in this article.The model lacks supportive data,it uses an outdated global cycle chart,and it implies eustatic origin of the middle Miocene and younger sequences in the Kutei basin.We realize that the sequence stratigraphic model is not the primary objective of Peters et al.(2000);however,the stratigraphic framework is an integral part of the geo-chemical–sequence stratigraphic model,and the model can only be as good as its stratigraphic framework.Copyright᭧2001.The American Association of Petroleum Geologists.All rights reserved.*Department of Geology,University of Toronto,22Russell Street,Toronto,Ontario, M5S3B1Canada;tobi@geology.utoronto.ca.**Department of Geology,University of Toronto,22Russell Street,Toronto,Ontario, M5S3B1,Canada;miall@quartz.geology.utoronto.ca.This article benefited from suggestions by AAPG associate editor John Lorenz.We acknowledge ongoing research support through an NSERC grant to Andrew Miall and a University of Toronto Fellowship to Tobi Payenberg.Manuscript received February22,2000;revised manuscript received November27, 2000;final acceptance December19,2000.L A C K O F D A T ATo explain the origin of the sequence stratigraphicmodel,Peters et al.(2000)state:“Standard sequence stratigraphic analysis allowed identification of se-quence bounding unconformities and maximumflood-ing surfaces.Seismic facies within sequences were cal-ibrated with available well(lithology)control.The sequences were dated using paleontological picks inwell ties and compared to available coastal onlap charts(e.g.,Haq et al.,1987)”(Peters et al.,2000,p.18).Although seismic interpretation is now a standard toolin stratigraphic analysis,the assignment of absoluteages as described in this quotation is not,or should notbe.The reader is left in the dark regarding what kindof paleontological data was used in the study.Also,noindication is given of the precision of the paleontolog-ical data set.The common practice of employing the global cy-cle chart of Haq et al.(1987)as a template for cor-relation and as a means for the identification of eustatically controlled sequence boundaries has beencriticized elsewhere(Miall,1992,1994)on the basis(1)that the data justifying the construction of the origi-nal chart have never been published and its validity istherefore unproven,and(2)that the accuracy and pre-cision of chronostratigraphic techniques do not permitrigorous dating and global correlation of sequence boundaries(see also Ricken,1991;Johnson,1992). Therefore,the proposition of the eustatic dominanceof global stratigraphy remains unproven.In theirfigure2,Peters et al.(2000)highlight sev-eral sequence boundaries they claim to have identifiedin their data set(Figure1).None of these surfaces cor-respond to a biozone boundary or to a log top.UnlessMobil has refined the paleontological data set,and sub-divided each biozone,a correlation of the sequence boundaries with the global cycle chart is highly errorprone.For example,in the case of the N16biozone,which spans about2.5m.y.,this error could be up to2.5m.y.,depending on the position of the sequence boundary.W H I C H C Y C L E O N W H I C H C Y C L E C H A R T?To date sequence boundaries,Peters et al.(2000)com-pare their data to the global cycle chart of Haq et al. (1987).Snedden et al.(1996)previously noted,how-ever,that the9.0Ma sequence boundary“has no coun-terpart on the global chart,suggesting local variationsContentsSearch ResultsSearch Index1098AAPG Bulletin,v.85,no.6(June2001),pp.1098–1101Pay enberg andMiall1099Figure 1.Sequence boundaries and ages used by Peters et al.(2000)and original global cycle chart data from Haq et al.(1987)and Vanderberghe and Hardenbol (1998).Although Peters et al.(2000)claim to be using the Haq et al.(1987)chart,one sequence boundary was added (9.0Ma),one omitted (4.2Ma),and one age was changed from 2.9Ma to 3.0Ma.The latest global cycle chart from Vanderberghe and Hardenbol (1998)shows 16sequence boundaries that have mostly different ages compared to the 12from Haq et al.(1987).If Peters et al.(2000)used the newest cycle chart,their timing of the sequence boundaries would change substantially.in relative sea level may be responsible”(Snedden et al.,1996,p.285).Peters et al.(2000)do not mention that the 4.2Ma sequence boundary from Haq et al.(1987)is not present in their data set,and they also have changed Haq et al.’s.(1987)2.9Ma boundary to 3.0Ma (Figure 1).If eustasy were responsible for the formation of the sequences in the Kutei basin,then all observed sequence boundaries would be synchronous globally and present in the geological record.This is not the case presented by Peters et al.(2000).Also unclear is why,if Peters et al.(2000)attribute sequence boundaries to eustatic control,they used the Haq et al.(1987)global cycle chart instead of Van-derberghe and Hardenbol’s (1998)new cycle chart de-rived from several European basins (de Graciansky et al.,1998).Figure 1shows the new cycle chart along-side the Haq et al.(1987)and Peters et al.(2000)charts.In addition to more sequence boundaries on the new chart,most of the dates have also been changed (Figure 1).Because Vanderberghe and Hardenbol(1998)propose a glacio-eustatic mechanism for the Neogene sequences,the same number of sequence boundaries having exactly the same dates should show in the data presented by Peters et al.(2000).E U S T A T I C O R I G I NDating of sequence boundaries using the global cycle chart(Haq et al.,1987)necessarily implies a eustatic origin of the stratigraphic sequences.Despite their brief tectonic review,Peters et al.(2000)fail to rec-ognize the importance of tectonism in the Kutei basin. The Kutei basin is a tectonically very active basin that has extraordinary subsidence rates.It accumulated14 km of sediments in some parts of the basin during the Tertiary(Chambers and Daley,1995).Since the mid-dle Miocene alone,it has accumulated more than4km offluvial,deltaic,and shelf deposits in deltaic cycles (Magnier et al.,1975).Paleocurrent analysis of middle Miocene rocks in the Mutiara area by Payenberg (1998)showed distributary channels diverging around growing anticlinal structures.Stratigraphic thinning also occurred across these anticlines,indicating the im-portance of tectonism during this time of deposition (seefigure1in Peters et al.[2000]for location).Peters et al.(2000)state that the offshore Ma-hakam Delta comprises two different tectonic phases: middle Miocene rocks experienced compressional fold-ing and thrusting,whereas upper Miocene and Plio-cene rocks are deposited in an extensional regime(Al-len and Chambers,1998).The two distinct tectonic regimes are separated by a sequence boundary,which “is a significant angular unconformity”(Peters et al., 2000).Regional compressional stress during the mid-dle Miocene increased from west to east and inverted earlier extensional faults(Allen and Chambers,1998), leading to shallower,less tight anticlines and thrusted anticlines toward the east.During this compressional time,the Mahakam River was incising into the growing anticlinal structures belonging to the Samarinda anti-clinorium.As the compressional stress moved from west to east,the Mahakam River also locked into the anticlines progressively from the west to the east(Allen and Chambers,1998;Payenberg,1998).At the end of the middle Miocene,the sedimentation in the present-day onshore part of the Mahakam Delta ceased,ex-posing the rocks to the north and south of the Ma-hakam River(Payenberg,1998).Only to the east of the Sanga-Sanga anticline has sedimentation continued intermittently until today.The prominent angular un-conformity dated as the10.5Ma unconformity by Pe-ters et al.(2000)is thus tectonic in origin and cannot be associated with the eustatic sea level drop inferred from the cycle chart,let alone dated using it.C O N C L U S I O N SThe new geochemical–sequence stratigraphic model uses a precision of the stratigraphic model that is not supported by data and thus cannot be verified.The biozone boundaries and log tops presented do not correspond to any of the sequence boundaries,cast-ing doubt on the precision of those sequence bound-aries.An outdated cycle chart was used,and the new chart shows more sequence boundaries of different ages.Most important,active,syndepositional tecton-ism was downplayed in the stratigraphic framework and eustasy inferred for the origin of the stratigraphic sequences observed in the Neogene of the Kutei basin.The assignment of absolute ages to sequence boundaries without supportive data is a practice that should be abandoned.Extrapolation of a stratigraphic synthesis based on this approach to other areas of the basin that have more precise data can lead to mis-interpretations.Because the cycle chart implies solely eustatic origins for sequence boundaries,its use in tectonically active basins should be abandoned.In the Kutei basin,more evidence exists for a tectonic origin of the sequences than for a eustatic origin,despite the Neogene being a time of active glacio-eustasy.R E F E R E N C E S C I T E DAllen,G.P.,and J.L.C.Chambers,1998,Sedimentation in the modern and Miocene Mahakam Delta:Jakarta,Indonesian Pe-troleum Association,236p.Chambers,J.L.C.,and T.E.Daley,1995,A tectonic model for the onshore Kutei basin,east Kalimantan,based upon an in-tegrated geological and geophysical interpretation:Jakarta, Proceedings of the Indonesian Petroleum Association,v.24, p.115–130.de Graciansky,P.-C.,J.Hardenbol,T.Jacquin,and P.R.Vail,eds., 1998,Mesozoic and Cenozoic sequence stratigraphy of Euro-pean basins:SEPM Special Publication60,786p.Haq,B.U.,J.Hardenbol,and P.R.Vail,1987,Chronology offluc-tuating sea levels since the Triassic(250million years ago to present):Science,v.235,p.1156–1167.Johnson,J.G.,1992,Belief and reality in biostratigraphic zonation: Newsletters in Stratigraphy,v.26,p.41–48.Magnier,P.,T.Oki,and L.Kartaadiputra,1975,The Mahakam Delta,Kalimantan,Indonesia:Proceedings of the9th World Petroleum Congress,v.2,p.239–250.1100Discussions and RepliesMiall,A.D.,1992,The Exxon global cycle chart:an event for every occasion?:Geology,v.20,p.787–790.Miall,A.D.,1994,Sequence stratigraphy and chronostratigraphy: problems of definition and precision in correlation,and their implications for global eustasy:Geoscience Canada,v.21, p.1–26.Payenberg,T.H.D.,1998,Paleocurrents and reservoir architecture of the middle Miocene channel deposits in Mutiarafield,Kutei basin,east Kalimantan,Indonesia:M.Sc.thesis,Queensland University of Technology,Brisbane,Australia,235p. Peters,K.E.,J.W.Snedden,A.Sulaeman,J.F.Sarg,and R.J.Enrico,2000,A new geochemical–sequence stratigraphic model for the Mahakam Delta and Makassar slope,Kaliman-tan,Indonesia:AAPG Bulletin v.84,p.12–44.Ricken,W.,1991,Time span assessment—an overview,in G.Ein-sele,W.Ricken,and A.Seilacher,eds.,Cycles and events in stratigraphy:Berlin,Springer-Verlag,p.773–794. Snedden,J.W.,J.F.Sarg,M.J.Clutson,M.Maas,T.E.Okon, M.H.Carter,B.S.Smith,T.H.Kolich,and M.Y.Mansor, 1996,Using sequence stratigraphic methods in high-sediment supply deltas:examples from the ancient Mahakam and Ra-jang-Lupar deltas:Jakarta,Proceedings of the Indonesian Pe-troleum Association,v.25,no.1,p.281–296. Vanderberghe,N..and J.Hardenbol,1998,Introduction to the Neogene,in P.-C.de Graciansky,J.Hardenbol,T.Jacquin, and P.R.Vail,eds.,Mesozoic and Cenozoic sequence stratig-raphy of European basins:SEPM Special Publication60, p.83–85.Pay enberg andMiall1101A new geochemical–sequence stratigraphic model for the Mahakam Delta and Makassar slope,Kalimantan, Indonesia:ReplyJohn W.Snedden*,J.F.(Rick)Sarg**,and Kenneth E.Peters**I N T R O D U C T I O NWe appreciate the opportunity to address the sequence stratigraphic issues raised in the discussion by Payen-berg and Miall.Although stratigraphy was a small part of the new geochemical model presented in Peters et al.(2000),we agree that it is an important framework element in study of source rocks,hydrocarbon kitch-ens,and basin history of the Kutei petroleum province.Payenberg and Miall identify three points of dis-agreement:supportive data,outdated global cycle chart,and eustatic origin of middle Miocene and younger sequences.In fact,their argument concerning supportive data pertains to the precision of biostrati-graphic age assignments and global cycle assignments discussed in the second point.For consistency,how-ever,we will address their concerns in the same order.S U P P O R T I V E D A T APayenberg and Miall argue that Peters et al.(2000) lacks supportive data for the stratigraphic framework of the Mahakam Delta.The concerns extend to a pre-vious article by Snedden et al.(1996).Copyright᭧2001.The American Association of Petroleum Geologists.All rights reserved.*ExxonMobil Exploration Company,P.O.Box4778,Houston,Texas,77210; john.w.snedden@.**ExxonMobil Exploration Company,P.O.Box4778,Houston,Texas,77210; sarg@;ken_peters@.Manuscript received December19,2000;final acceptance January11,2001.The focus of Peters et al.(2000)is organic geo-chemistry and the new model that reasonably explains a series of recent discoveries on the outer Mahakam shelf and slope.For editorial reasons,only limited space was available to discuss the stratigraphic meth-odology and background on the stratigraphic analysis that itself spanned two years of investigation,involving 500ft of core,4000km of two-dimensional(2-D)seis-mic data,logs and cuttings from40-plus wells,and pa-leontological and isotopic dating.Proprietary data in a two-volume company report could not be entirely shared with the readers of the Peters et al.(2000)or Snedden et al.(1996)articles because of continuing exploration efforts by Mobil and now ExxonMobil and its partners in the Mahakam slope and adjacent Ma-kassar Straits.More information has been approved for release,however,beginning with recent AAPG presen-tations and expanded abstracts(Snedden and Sarg, 1998a,b;2000).We intend to publish further docu-mentation of our framework as proprietary controls are relaxed.Our framework is a clear improvement on previ-ous publications that stressed lithostratigraphic picks (e.g.,Sujatmiko and Irawan,1984)or had limited ties to the Mahakam Delta slope/Makassar Straits region (e.g.,Duval et al.,1992).Miscorrelation of the pre–middle Miocene section as shown in Burrus et al. (1992)had significant impact on the calculated source maturation trends.Our stratigraphic correlations,il-lustrated by a seismic section in Peters et al.(2000), continue to be confirmed by new drilling.Payenberg and Miall point out that many of the sequence boundaries do not coincide with biozone boundaries and thus have uncertain ages.The biozones shown for reference are planktonic foram biochrono-zones;however,a more refined subdivision was made using a combination of nannofossils,dinoflagellates, and strontium-isotopic dating.Thus,a stratigraphic as-signment of a particular sequence boundary can be made with precision within the biozones shown for reference.The argument made by Payenberg and Miall con-cerning imprecision of biostratigraphic methods and the uncertainty of global synchronicity dates back to Miall(1991).Much progress has been made in the last ten years toward improving age resolution using a va-riety of biostratigraphic and isotopic dating techniques, especially in the Neogene(de Graciansky et al.,1998). In the Gulf of Mexico Basin,for example,age resolu-tion approaching100–200k.y.has been achieved (Wornardt et al.,1998).1102AAPG Bulletin,v.85,no.6(June2001),pp.1102–1105Snedden et al.1103W H I C H C Y C L E O N W H I C H C H A R TPayenberg and Miall point out the inconsistency be-tween the Mahakam stratigraphy of Peters et al.(2000)and the Haq et al.(1987)chart.Specifically,they list recognition of a possible new sequence boundary at 9.0Ma and lack of a sequence boundary at 4.2Ma in Sned-den et al.(1996).Most practitioners of sequence strati-graphic techniques understand that the Haq et al.(1987)chart was a composite of many different basins and that any single basin will not display all 119se-quence boundaries (213in the Triassic to Holocene chronostratigraphy of de Graciansky et al.[1998]).Experienced seismic interpreters also know that conventional 2-D seismic data has resolution limits.Where a sequence falls below one-quarter to one-half wavelength of the dominant seismic frequency,it is generally not observed (Sheriff,1977).This may be one explanation for the “missing”4.2Ma sequence in the Mahakam stratigraphy.The new 9.0sequence may actually be a fourth-order or high-frequency sequence,following the definition of Mitchum and Van Wagoner (1991).The fact that our framework differs somewhat from Haq et al.(1987)indicates that we are not force-fitting the Mahakam stratigraphy to the Haq et al.(1987)coastal onlap chart.Payenberg and Miall are unclear as to why we did not use the new European basins chronostratigraphy (de Graciansky et al.,1998).Unfortunately,this new set of chronostratigraphic charts was not available to us at the time of the submission of our article in 1998.Although the publication date of the European basins system is 1998,the actual SEPM volume was not in wide release until 1999,following submission of our manuscript.As Payenberg and Miall illustrate in their figure 1,use of the new chronostratigraphic charts changes tim-ing of the sequence boundaries emphasized in the stratigraphic framework of Peters et al.(2000).This is due,however,to a recalibration of all biostratigraphic,isotope stratigraphic,and sequence stratigraphic en-tries to the new Gradstein et al.(1995)and Berggren et al.(1995)time scales in the European basins chrono-stratigraphy (de Graciansky et al.,1998).The sequence boundaries of Peters et al.(2000)have not changed their biozone position.Thus,relative timing and strati-graphic position of the sequences and sequence bound-aries remain unchanged.In fact,one of the key products from the new European basins chronostratigraphy is the elimination of absolute age dates for sequences and sequence-boundary names.For example,instead of the 10.5Ma sequence boundary,one should refer to the Ser-ravallian 4(Ser 4)sequence boundary (Figure 1).Payenberg and Miall unduly focus on absolute ages for sequence boundaries in their figure 1,which are prone to change with evolution of the Mesozoic and Cenozoic time e of the newly calibrated European basins chronostratigraphy should eliminate this problem.The sequence boundaries are keyed to biozone position and thus to the chronologic scale,not directly to the time scale as implied byPayenbergFigure 1.Translation chart for sequences and sequenceboundaries used in Peters et al.(2000)and new European ba-sins chronostratigraphy of de Graciansky et al.(1998).Changes in the ages of the se-quence boundaries are primar-ily due to a change in the geo-chronologic scale,not biozone e of European ba-sins nomenclature (e.g.,Ser 1)for sequences and sequence boundary names is an obvious improvement over absolute ages,which are prone to change with evolution of the time scales.and Miall.Figure1shows our translation of the ear-lier published Mahakam stratigraphic cycles to this new chronostratigraphy.E U S T A T I C O R I G I N OF S T R A T IG R A PHI CS E Q U E N C E SPayenberg and Miall point out that Peters et al. (2000)ignore tectonics as a factor in formation of the stratigraphic sequences in the Kutei basin.Ap-parently Payenberg and Miall did fully appreciate our explanation of how tectonic processes forced deltaic depocenters and shelf margin positions to shift within and between sequences.Our interpretation of the tectonic styles in our study area is based on a large set of regional seismic lines and extends much farther seaward of the area discussed by Chambers and Daley(1995).Furthermore,we have presented arti-cles elsewhere suggesting that the Mahakam strati-graphic sequences are strongly influenced by synse-dimentary tectonics(Snedden and Sarg,1998a, 2000).Payenberg and Miall also suggest that the Mio-cene and younger stratigraphy cannot be related to or is unlikely to be influenced by eustatic processes. This opinion ignores the well-established fact that the Miocene and younger time frame is clearly an ice-house period in earth rge ice sheets existed in Antarctica as far back as the Oligocene(Abreu and Anderson,1998).L arge,globally correlative glacio-eustaticfluctuations clearly occurred during the Mio-cene period in discussion(e.g.,Miller et al.,1996). These relatively high-frequency eustatic changes worked in combination with tectonic forces related to plate collision and formation of the central Borneo Highlands(early Miocene)and later uplift of the Meratus Mountains closer to the Kutei basin(middle Miocene).The observed results are pronounced an-gular truncations seen in Sarawak(Langhian2[Lan 2]sequence boundary)and offshore Mahakam(Ser 4sequence boundary).Our comparison of the two basins(discussed in Snedden and Sarg[1998a, 2000])demonstrates the utility of sequence strati-graphic techniques in high sedimentation,tectoni-cally active basins such as Sarawak and Kutei.Finally,we should point out that rigorous anal-ysis,drilling,and sequence dating continues to con-firm the causal link between coastal plain and slope sequence boundaries formed during global sea level falls(e.g.,Miller et al.,1996;Eberli,2000).C O N C L U S I O N SThe inquiry by Payenberg and Miall has allowed us to further elaborate the stratigraphy of the offshore Kutei basin,beyond what was discussed in the geochemically focused article by Peters et al.(2000).Updating the previous stratigraphy using the new global chronostra-tigraphy is also another benefit of this discussion and reply(Figure1).Of course,we do not agree with their rather bleak assessment of the supportive data,cycle chart issues,and implied eustatic origin of the strati-graphic sequences in our study area.Many improve-ments in stratigraphic resolution,global chronostratig-raphy,and the sequence stratigraphic approach itself have been made since concerns were raised by Miall (1991).The validity of the global stratigraphic frame-work continues to be supported by rigorous analysis of outcrops and deep-sea cores(Miller et al.,1996; Thiede and Myhre,1996;Franseen et al.,1998;Eberli, 2000).We expect,and look forward to,further discus-sions of global chronostratigraphy and sequence stra-tigraphy with these authors in the future.R E F E R E N C E S C I T E DAbreu,V.S.,and J.B.Anderson,1998,Glacial eustasy during the Cenozoic:sequence stratigraphic implications:AAPG,v.82, p.1385–1400.Berggren,W.A.,D.V.Kent,C.C.Swisher,and M.-P.Aubry,1995,A revised Cenozoic geochronology and chronostratigraphy,inW.A.Berggren,D.V.Kent,and J.Hardenbol,eds.,Geochro-nology,time scales,and global stratigraphic correlations:SEPM Special Publication54,p.129–212.Burrus,J.E.,E.Brosse,G.C.de Janvry,Y.Grosjean,and J.L.Oudin, 1992,Basin modeling in the Mahakam Delta based upon the integrated2D Temispack:Proceedings of the Indonesian Petro-leum Association Twenty-First Annual Convention,v.1,p.23–43.Chambers,J.L.C.,and T.E.Daley,1995,A tectonic model for the onshore Kutei basin,east Kalimantan,based upon an integrated geological and geophysical interpretation:Proceedings of the Indonesian Petroleum Association Twenty-Fourth Annual Con-vention,v.1,p.111–117.de Graciansky,P.-C.,J.Hardenbol,T.Jacquin,and P.R.Vail,1998, Mesozoic and Cenozoic sequence stratigraphy of European ba-sins:SEPM Special Publication60,786p.Duval,B.C.,C.de Janvry,and B.L oiret,1992,Detailed geoscience reinterpretation of Indonesia’s Mahakam Delta scores:Oil& Gas Journal,v.90,no.22,p.67–72.Eberli,G.P.,2000,The record of Neogene sea-level changes in the prograding carbonates along the Bahamas Transect—Leg166 synthesis,in P.K.Swart,G.P.Eberli,M J.Malone,and J.F.Sarg,eds.,Proceedings of the Ocean Drilling Program,scientific results:v.166,p.167–176.Franseen,E.K.,R.H.Goldstein,and M.R.Farr,1998,Quantitative controls on location and architecture of carbonate depositional1104Discussions and Repliessequences:upper Miocene,Cabo De Gata region,SE Spain: Journal of Sedimentary Research,v.68,p.283–298. 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