Comments on the EXXON cycle chart for the Cretaceous system
Jake M. HANCOCK *
* Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, ¿1K.
ABSTRACT
Correlation between the various columns of chronostratigraphy on the EXXON chart has often been assumed without being tested. A considerable number of the tie-lines can be shown to be wrong. As a result one cannot know what many of their ‘3rd order cycles’ refer to in the stratigraphical column. In most examples where such cycles of sea-level change can be dated, somebody else has aíready recognised them. Most of their ‘2nd order cycles’ are suspect. Paradoxically, current research shows that changes of sea-level through intervals of a few million years may have been simultaneous in Europe and the USA to an accuracy of ±100,000years. Key-Words: EXXON chart, Brd order cycles, 2nd order cycles, Cretaceous, custasy, sea-level changes, accuracy in correlation.
RESUMEN
En términos generales se asume la validez de las correlaciones entre las distintas columnas cronoestratigráficas incluidas en la escala de la EXXON. Sin embargo, es posible demostrar que un número considerable de éstas no
0 17, 57-78. Cuadernos de Geología Ibérica, n. Editorial Complutense, Madrid, 1993 58 lake M. Hancock son correctas. Como consecuencia de esto es difícil saber cuáles de los ciclos de tercer orden existen realmente en el registro estratigráfico. Por otro lado, en la mayor parte de los casos en los que ha sido posible datar los ciclos eustáticos, éstos han sido reconocidos por otros autores. La mayoría de los ciclos de segundo orden son dudosos. Paradójicamente, las investigaciones más recientes parecen indicar que pa- ra intervalos de tiempo de unos pocos millones de años, los cambios eustáti- cos podrían ser simultáneos en Europa y Estados Unidos con una precisión de ±100.000años. Palabras clave: Curva EXXON, ciclos de 32 orden, ciclos de 2.0 orden, Cretácico, eustatismo, cambios del nivel del mar, correlación detallada.
INTRODUCTION
There is a long tradition in religions and mythology of a world-wide flooding of the land (Dean, 1985). It is now more than 100 years since Eduard Suess put this on a scientific basis. He noticed that there were many regions of the world where Upper Cretaceous sediments spread on to much older rocks. He noted that the widest extent of this transgression of sea on to land was represented by ‘Senonian’ sediments, but because the transgression seemed to have started during the Cenomanian, he wrote of ‘the Cenomanian transgression’ (1875, Pp. 104-117; 1906, Pp. 289-292). Fewer people noticed that Suess also discovered other times in the history of the Earth when sea-levels were exceptionally high, e.g. Middle Devonian, or exceptionally low, e.g. near the end of tbe Jurassic period. He distinguished these major world-wide changes as ‘eustatic move- ments’, but he also wrote, ‘A close examination of the stratified series often leads us to suspect the existence of numerous smaller oscillations which are hard to reconcile with eustatic processes’ (1906, Pp. 544-545). For many years after Suess’ work, ideas on sea-level changes were developed from other systems: for a lucid and learned summary, see the essay by Dott (1992). Local curves for parts of the Cretaceous were produced, cg. Kahrs (1927) for the south-west margin of the Múnster basin in Germany. Under the influence of Stille (1924) sea-levels were always related to tectonics, cg. Kirkaldy (1939), Arkell (1947), Owen (1971); a eustatic element was not even considered. As recently as 1976, Hughes wrote, ‘no world-wide pattern of transgressions is discernible; they are really better regarded as epeirogenic ‘immersions’ of parts of continental edges and therefore of regional significance or less’ (1976, p.66). Comments on the EXXON cycle chart... 59
The first broader survey was by Matsumoto (1967). This remarkable but obscure papa has still not been superseded. Matsumoto showed that the data demanded that both eustasy and broad tectonic warping must be allowed for if sea-level changes are considered on a world-wide scale. Happily, Matsumoto published a summary in English in 1977. In te 1970’s a team of geologists, led by Peter Vail, working for EXXON in Houston, developed a theory of relationships between cycles of coastal onlap and offlap of sediments which they related to eustatic changes of sea-level. Their cycles or ‘sequences’ were divided by unconformities — rather like Suess considered that his eustatic lows corresponded to boundaries between systems. From an interpretation of the facies in each ‘sequence’ and te lateral relation- ships of these facies, it would be possible to make allowances for a tectonic interference or overprint on the sequence of facies. Their concepts are now usually called ‘sequence-stratigraphy’. The principies of sequence-stratigraphy have been described in English many times, from the original first fulí account by Vail et al. (1977) to a readable summary by Hallam (1992). No attempt is being made here to repeat these discussions of the theory. The valuable review by Cross & Lessenger (1988) sometimes shows more insight into the ideas than are easily extracted from the papers of the EXXON team. Jervey (1988) has provided a fulí theoretical account of the subject, with beautiful coloured diagrams. Obections to the theory are also numerous. There are the alí- embracing views of Mómer (1976, 1981, 1989) who argues that shelf-unconformities cannot be explained in terms of changes in global sea-level because these are affected by deformations of the geoid and differential rotation. Burton e>’ al. (1987) and Kendall & Lerche (1988) show that the quantitative relations between sediment accumulation, eustasy and tectonic subsidence can never be calculated exactly. Criticisms of the sedimentological theory have been made by Mialí (1991) and of the seismic theory by Neidelí (1979), or the resolution of the seismie stratigraphy (Cartwrightetal., 1993). The biggestgeneral problem is that the data and origin of te EXXON chart has never been documented, as Mialí has pointed out repeatedly (1986, 1991, 1992), and Mialí has also argued that the resolution of biostratigraphic correlation will be inadequate totest the eustatic origin of the smaller cycles in the EXXON curves. We are alí expected to take te accuracy and reliability of their chad on trust. Altogether there has been a plethora of discussions of the theory. In this paper 1 try to see how far the EXXON chart works, or does not work, in practice. For if it works, theoretical objections must collapse. If it does not work at alí, we should abandon it as a dangerous doctrine. If it works in some parts and not in others, we ought to ask why. 60 Jake M. l-lancock
Ideally, a distinction should be made between: (i) whether there have been synchronous changes of sea-level or not, i.e. is eustasy true or false, and (u) if there havebeen eustatic changes of sea-level, does sequence-stratigraphy provide a reliable technique to detect, date and measure the changes? To complicate matters further, there are some geologists who recognise simultaneous sub- mersions of widely separated cratons, but still argue that these áre controlled by tectonic forces’, e.g. Sloss (1991), and before him, Stille (1924).
TIff COLUMNS OF THE EXXON CHART
The EXXON chart contains 27 columns, most of which are lumped into five groups. a) TIME IN MILLtONS OF YEARS
Since many of their changes of coastal onlap only lasted a few hundred thousand years, the accuracy of this scale is important, particularly because the authors name their sequence-boundaries by these radiometric dates. Their scale for the Jurassic-Cretaceous is taken from dates for stage-boundaries by Harland e>’ al. (1982 edition, not the 1990 edition). 1-faq e>’ al. (1988) themselves estimate that the average limits of uncertainty are ±3.0 m.y. in the Early Cretaceous, and + 1.75 m.y. in the Late Cretaceous. Ihis is bad enough, but some parts of the Cretaceous are even more uncertain than tRis. Thus dates for te Iurassic- Cretaceous boundary range from 145.6 Ma. with a possible error of 9 m.y. (Harland et al., 1990) to 130±3 Ma (Kennedy & 0dm, 1982), a difference of
more than 15 m.y. — equivalent to about three stages of the Lower Cretaceous! Note that this inaccuracy is independent of different biostratigraphic definitions of the boundary between the two systems. Although Late Cretaceous radiometric dates are much better constrained tan Early Cretaceous dates (particularly compared with the pre-Aptian), an un- certainy of 1.75 m.y. is nearly as longas the Coniacian age (2.1 m.y. according to Obradovich (in press); 1 m.y. on the EXXON chart). The EXXON team had to plot their data on some stated scale, and a published radiometric scale is as sensible as any, but one must not conclude that it is as accurate as the implied resolution of the chart. If one uses the dates to name the coastal onlap changes, ah the names will have to be changed as the radiometric scale is revised. An example of the resultant oddities is tliat the reference date for the ‘latest Valanginian’ at 119.0±2.0Ma (Haq e>’ al., 1988, Appendix B) would falí half Comments on Me EXXON cycle chart... 61 way through the Hauterivian (121 tu 116.5 Ma) un the chan. This results from using ‘best-fit data’. A trenchant criticism of such laxity has beenmade by 0dm (1986). The Coniacian stage may be half as long or three times longer than the 1.5 m.y. average of Obradovich (in press) and the EXXON chan, i.e. 0.75to 3.5 m.y. ‘This corresponds tu a factor of variation of six tu be added to the authurs’ estimate for the mean zone duratiuns in this stage’ (0dm, 1986, pl97).
(b) ‘STANDARD
The classification of the Cretaceous system under erathems, systems, series and stages is in accurd with the recummendations of the International Sub- commission un Cretaceous stratigraphy (Birkelund e>’ al., 1984). Two altemative stages for the base of the Cretaceuus, Berriasian and Ryazanian, are shown. The meanings uf the stages are discussed, where necessary, in tlie next section.
(c) ‘BJOCI-IRONOSTRATIORAPHY’
Five columns are used here: ‘planktonic foram biochronozones’, ‘nannufossil biuchronuzones’, ‘macrufussil biochronozones Great Britain’, ‘ammonuid bio- chrunuzunes (tethyan region)’ and ‘dinoflagellate biohorizuns’. Three problems exist here. (1) Are the zonal divisions widely recugnisable; in particular, can they be used in regions where the sequence-stratigraphy is guing tu be worked out? (2) Do the zones in any une culumn currelate currectly with the zones in other culumns? (3) Are the duratiuns uf the zones correct? For alí criteria, the EXXON team had set themselves an almost impossible task, and there are mistakes that are quantitatively serious in alí three.
(i) Recognition of zones For the Middle Albian and pan of the Upper Albian the ammonite zones fur the tethyan realm are named after species of Huplitidae, an exclusively oíd world boreal family. One must suppuse that the coastal unlap curve is actually based entirely un boreal evidence at this time. Four ammonite unes have been fitted, with sume difficulty, into the tethyan Cuniacian. These zones are from the wurk uf Kennedy (1984) un the Coníacían ammonites of France. Most of this research was done in nurthern Aquitaine and the luwest uf the fuur zones has not been found in France outside Aquitaine; whilst the top zune is nut clearly demarcated in Aquitaine. Three uf the four genera uf the index species are also known in the western interior of the United 62 Jake M. Hancock
States, but it has been fuund necessary tu use different index species there (Kennedy & Cobban, 1991). Such fine local divisiuns un the EXXON chart give a spurious appearance uf precision. The ammunite índex used fur their Upper Maastrichtian is Pachydiscus neubergicus. There is one specimen of P. neubergicus (von Hauer) known frum the base uf the Upper Maastrichtian (un the belemnite seale) in Denmark; and in Australia there are chrunulugical subspecies in the Upper Maastrichtian (Hendersun & McNamara, 1984), but P. neubergicus is typically a Luwer Maas- trichtian species, probably passing into 1’. gollevillensis (d’Orbigny) in the Upper Maastrichtian (Kennedy, 1986). Does this mean that the Upper Maastrichtian parts of the EXXON curves are actually Luwer Maastrichtian?
(u) Correlation between columns It is seldum realised by geulugists at large how insecure most zonal sehemes are, and how few are the regiuns in which any une scheme has been successfully tested. Papers with titíes such as ‘Standard of Cretaceuus System’ (Muller & Schenck, 1943) do uur science a disservice. A few years ago a small gruup uf biostratigraphers was asked tu produce dic standard scale of the Cretaceuus system un Earth, along the lines of the EXXON chart, for the use uf sedimentolugists, tectunicians and similar geolugists. That such a standard did not exist; that pussible scales needed disclaimers of applicatiun and accuracy; that almust no reliable currelatiuns existed between different scales; alí these limitatiuns were met with amazement. An example uf the current state of knuwledge is shuwn by comparisun of expert opinion un different gruups of fossils frum une unit uf a few metres at Tercis in south-west France which, prior tu discussiun between the ‘experts’, ranged frum Lower Santonian tu Luwer Maastrichtian (Hancuck e>’ al., 1993)! Hence, unly a few examples uf the problems can be taken from the EXXON chart because must of the tie-lines between these culumns have yet tu be pruved. Every wide-ranging stratigrapher wurking un the Mesozoic meets the difficulty uf correlatiuns between boreal and tethyan realms. One uf the times uf particular difficulty lies aruund the Jurassic-Cretaceuus buundary, however that is defined. The EXXON chan shows the tethyan Berriasian extending duwn into the Jurassic, aud starts the Cretaceuus with the base of the Ryazanian stage, i.e. gives the boreal definitiun priurity uver a tethyan definitiun. Whilst it is probably correct tuplace the tethyan jacobi-grandis Zone well below tlie boreal Runetonia runctoni Zone, it must largely be a matter uf faith (see Hoedemaeker, 1987). The current fluidity uf understanding and knowledge uf Luwer Cretaceuus ammonite zonatiuns can be seen by cumparing three schemes which havebeen published in Comments on the EXXON cycle char>’... 63 the last few years (Huedemaeker & Bulut, 1990; Hancuck, 1991; Hoedemaeker e>’ al., 1993). It is nut a criticism uf Peter Vail and his colleagues but an emphasis un the difficulties with which they are faced. Thus fur the tethyan Aptian stage there are 11 ammunuid zunes un the EXXONchart. In Huedemaeker e>’ al. (1993) there are eight zones; only twu index species are commun tu the twu charts! Whereas sume of the ‘tethyan’ ammonite zones are actually boreal, the planktic furaminiferal zunes are tethyan, most of them ‘tropical’, rather than
temperate’ — where there might be an interfingering currelatiun with a boreal area (see Caron, 1985, fig. 5). Thus the extinctiun level of Globotruncanita calcarato, here placed at the summit of the Campanian, is known tu be several milliun years ulder than the start of the Maastrichtian as defined un te chart by the base uf the Zune of Belemnella lanceolata (‘macrufussil zunes Great Britain’) and the base uf the Zune uf Acanthoscaphi>’es tridens (ammunuid zones, tethyan regiun, althuugh A. i~ridens has nut yet been recurded frum the tethys) (Salaj & Wiedmann, 1989; Schñnfeld & Burnett, 1991; Burnett et al., 1992>. Interestingly, this has recently beenconfirmed by eustatic changes uf sea-level (Hancock e>’ al., 1992)!
(iii) Duration of he zones It is cummun pra9tice, in the absence uf uther criteria, tu assume that zunes were of equal duration, particularly if they are based un the same group of urganisms (e.g. Hallam e>’ al., 1986). The EXXON chart fulluws this principle within each stage, having chusen the isotopic dates fur each stage boundary.
Unfortunately, this assumptiun — which 1 have used myself — can be wildly false: fur example, the number uf rhythms in chalk-marl facies in the Cenumanian indicate that the Zune uf Acan>’hoceras rho>’omagense was sume three times as longas the uverlying Zune uf A.jukesbrownei (Gale, 1990). In the Turonian te Zune of Collignoniceras woollgari is given four times the amuunt uf time alluwed tu the Zune uf Subprionocyclus neptuni, apparently tu fit in the fuur subzunal divisiuns recognised fur the woollgari Zune in Tuuraine. Fur the Campanian equal time is given tu the Zunes uf Offaster pilula, Gonioteu>’his quadrata aud Belemnitella mucronata, although it has been known for sume time that the luwer twu cumbined lasted unly abuut 3 m.y. cumpared with 8 m.y. fur te mucronata Zone (Ernst e>’ al., 1979), a disparity now increased (Hancuck, 1991; Kennedy a al., 1992). The highest macrofossil zunes listed for the Campanian are actually well below the top uf the stage in the sense used. Thus, aboye the Zune uf Belemnitella mucronata s.s. there are the Zunes uf B. minor aud B. langel, although it is true that these younger zunes are nut always recugnised. But since the wurk uf Blaszkiewicz (1980) it has been knuwn that 64 Jake M. Hancock there are une, pussibly two, ammunite zones aboye that uf Bostrychoceras polyplocum and beluw that of A. tridens. This is the gap uf about 2 m.y. between the extinctiun level uf Globo¿’runcanita calcarata aud the appearance of Belem- nella lanceolata referred tu earlier. AIí these present weaknesses in the. biu-chrunostratigraphy are upen tu future ímpruvement, but it dues mean that, fur the time being, in thuse parts of the cuastal unlap curve, where these inaccuracies exist, une cannut knuw what changes in sea-level are being postulated. There are sufficient uf these prublems tu make it very difficult, and sometimes impussible, tu recuncile their ubservatiuns with thuse uf uther geolugists. An example uf such prublems is illustrated in fig. 1.
Hg. l —Sea-level changes during the Campanian-Maastrichtian. The continuous line re- presents my own view of the times of the principal peás and troughs of sea-level during the latest Cretaceous. For the basis and construction of this graph, see l-lancock (1990). The dotted line represents the EXXON ‘curve’, taking only Iheir peaks and troughs; the horizontal axis is from EXXON’s radiurnetric scale; the vertical axis assumes that the height of tbeir peak aL 72.5 Ma on their scale equaís my langei Zone peak. and that the depth uf the late Turonian trough is the saíne as mine. The dashed une represents the EXXON ‘curve’, taking their cephalopod biostratigraphic scale. Since they have no represenlation for the belemnite Zone of Belemnitella langei or the ammonite Zones of Didymoc.eras donezianum and Nostoceras kvatti, there is a gap In the curve corresponding tu Ibis pali of the biostratigraphic scale. ‘There is no simple right or wrong answer tu each poinr on these graphs. They have been plotted not just tu show that diere are disagreements, but the impossibility of comparing results unless one is working on identical stratigraphic scales. Fig. 1.—Cambios en el nivel del mar durante el Campaniense-Maastrichiense. La línea continua representa mi interpretación del momento en el que tienen lugar los principales máximos y mínimos del nivel del mar durante el Cretácico tenninal. Para una descripción más detallada del método utilizado en su construcción, ver l-lancock (1990). La línea de puntos representa la “curva” de la EXXON, y en la que únicamente se representan los máximos y mínimos. El eje horizontal corresponde a la escalaradiométrica de la EXXON. El eje vertical se representa asumiendo que el valor del máximo que se observa a los 72,5 Ma en su escala, es equivalente al máximo que aparece en mi Zona langel, y que el valor del mínimo que se observa en el Turoniense superior es el mismo que el que aparece en el mío. La línea discontinua representa la “curva” de la EXXON, dibujada con base en su escala biostratigráfica de cefalópodos. El tramo sin datos que se observa en esta curva se debe a que estos autores no han representado en su escalabioe.stratigráfica la Zuna de belerunite Belemnitíella langei ni las zonas de ammunites Didyrrwcera~ donezianum y Nostoceras hyatti. Es difícil determinar cuáles son los errores y los aciertos de cada una de las curvas que se representan en estos gráficos. Estas se han representado no sólo para demostrar la existencia de discrepancias notorias, sino también para poner de manifiesto la imposibilidad de comparar los resultados, a no ser que se trabaje con escalas estratigráficas equivalentes. Comments on Me EXXON cycle char>’... 65
12 11 10 9 8 CD
5-- Belomnitella .5~4 euj- CO rninOT 5-- 0 z e.- e.- -J CD e.- Belemnitella o, mucronata e.- o CD CD Gonioteuthis C’J quad CD o, Odaster CD pi CD
LO CD CO CD e.- a, a, CD high— Relative sea-level —~ Iow 66 lake M. JJancock
(d) ‘SEQUENCE CHRONO5TRATIORAPHY’ AND ‘EU5TATIC CURVES
The EXXON chart agrees with sume long established trends and widely agreed changes in sea-level, e.g. Suess nuted that there were extreme luws very early in the Cretaceuus. That there was a pruminent peak sumetime cluse tu the start of the Turunian was already recugnised in suuthern England by Hume (1894) and in the south-west part uf the Paris basin by Cayeux (1897); inthe USA this was shuwn by Kauffman (1969) and has been maintained by him since (Kauffman & Caldwell, in press). A very strung and relatively brief falí in sea- level early in the Late Turunian has been known since the l9th century (Héberí, 1876), and shuwn un many later charts. Since their first comprehensive paper un global cycles of sea-level chariges (Vail e>’ al., 1977), the íearn have claimed that their charts shuw cycles of three urders of magnitude. It is cunvenient tu discuss the chart in these terms, even though their classification may be an illusion: it is notable that neither Kauffman & Caldwell (in press) in North America nur myself in Europe have used any but the shurtest cycles.
(i) .3rd order cycles The finest cycles uf cuastal onlap un the chart, i.e. the finest resulutiun used in the cunstructiun uf the chart, lasted b m.y. These are called ‘3rd urder cycles’. Since Cretaceous ammonite zunes (ur subzones) ranged frum aruund 0.4 m,y. tu perhaps 2 m.y. (Kennedy & Cobban, 1976; Hancuck, 1991), the 3rd order cycles wíll have averaged very appruximately une tu twu ammunite zones. Therefure, where ammunite correlatiuns can be applied, it will be pussible tu test the reality uf 3rd urder cycles. Valanginian.—-At the base of the Valanginian there is a eustatic truugh recugnised by buth the EXXON team and by Huedemaeker (1987), but his next low is at the base of the Zune uf Himantoceras trinodosum where EXXON has a eustatic peak. Sumehuw, Huedemaeker, with his well nigh encyclupedic knuw- ledge uf the bottom fuur stages uf the Cretaceous in south-west Europe, has nut noticed the must marked cuastal offlap un the EXXON curve in the Zune of Thurmanniceras per>’ransiens. This may be because they are actually getting their data frum different faunal realms. Aphan.—It so happens that the sequence-stratigraphy of the Aptian and must of the Albian uf Kent in south-east Englanó, where there is a precise bio- stratigraphic control, has been studied in detail by Hesselbo e>’ al. (1990). Their diagram (fig. 8> cumparing their sequence-buundaries with those of Haq e>’ al. Comments on the EXXON cycle chart... 67
(1988) shows guod agreement. However, when this succession is interpreted in terms uf sea-level changes, difficulties appear. Their boundary LG2 currespunds tu sequence-buundary 109.5 Ma uf Haq e>’ al. It is shown at outcrup by the Sandgate Beds resting uncunformably un the Hythe Beds. Accurding tu the EXXON eustatic curve, sea-level was falling at the sequence-buundary; it is shuwn as reaching a eustatic luw in the Zune of Parahoplites nutJieldíensís. Sediments uf this zone are transgressive acruss the stable London platfurm, juining the sea of suuthern England with that of the Nurtli Sea regiun. It seems difficult tu reconcile this with a falí in sea-level. Cenomaman.—Although most uf the EXXON chau is not backed by recurded observations, fur the Cenumanian stage we have nuw been given a sequence- stratigraphic interpretatiun of an actual area, namely for Sarthe un the south-west flank of the Paris basin (Haq e>’ al., 1988, fig. 12). The síage is shown as cuntaining three sequence-buundaries, that is strong ufflap shurtly after a eustatic peak, at: 95.5 Ma (middle of the Lower Cenumanian, in the upper part of Amédro’s Zune of Maníelliceras can tianum); 94 Ma (in the Middle Cenumanian at the junctiun of the Zunes of Acanthoceras rho>’omagense and A. jukes- brownei); and 93 Ma high in the Upper Cenomanian, a little below the top uf the Zune uf Me>’oicoceras geslinianum) (radiometrie dates by EXXON). Each sequence-buundary is said tu be immediately preceded by highstand depusits with eustatic highs a little earlier than the aboye dates. Immediately fulluwing each date there is believed tu be a rapid and distinct falí in sea-level, of which that at the base of thejukesbrownei Zone is said tu be the strungest. The person with the must knuwledge of the Cenumanian uf the Sarthe is Pierre Juignet(1974, 1977) and, as it happens, Juignethas published his uwn analysis of the transgressive-regressive liistury (1980). He recognises ‘deux épisudes transgressifs atteignant leur phase paruxysmale au milieu de Cénomanien inférieur et it la fin du Cénumanien muyen’. The first uf these, correspunding tu the Mames de Hallon, agrees with Haq e>’ al.; the secund in the jukesbrowne¡ Zune, near the top uf the Craie de Théligny, is where Haq e>’ al. have their strungest ufflap. He dues nut separate a third ‘phase’ in the Late Cenomanian, but like Haq e>’ al., recognises ‘une tendance régressive’ at the end of ihe Cenumanian. Juignet’s regressive phases are late in the Early Cenumanian and in the Late Cenumanian. The second probably currespunds with te eustatic falí immediately after the 93 Ma sequence-boundary uf Haq e>’ al.; the earlier regressive phase is placed by Juignet thruugh the Sables et Orés de Lamnay, which embrace the top part uf the ‘can>’ianum Zone’ but largely belong tu the Zune of Mantelliceras dixoni. This is a lunger regressive phase than that 68 Ja/ce M. Hancock immediately following the 95.5 Ma sequence-boundary uf Haq et al., but tlie two began at abuut the same time. Ihere is sume agreement here between te twu, but also a majur disagreement abuut what was happening late in the Middle Cenumanian. Further cumparison is difficult because the twu authurs use a different number uf phases or cycles. In fact, Juignet recognises considerably more oscillations but did not try tu map them uut in the 1980 papen lfthis basin-marginal successiun is traced northwards from Sarihe intu the less marginal facies of the Pays de Caux in Nurmandy and un the nortli French cuast, as Juignethimself has done (1980, figs. 3,4 & 5), une cangive mure precise dates fur the main changes. Thus the biggest regressive truugh is marked by hard- gruund Ruuen No. 1, at the base of the Craie de Ruuen, at the bottom of the Zune of Acan>’hoceras rho>’omagense; this correspunds tu the basal phosphatic fauna of the Craie de Théligny in Sarthe and the sub-Tutternhoe Stone erusiun in suuthem England (Hancock, 1990). In the coastal sectiun at Cap d’Antifer there are no hardgruunds aboye Rouen Nos. 1 and 2 until Antifer No. 1 at the base of the Zone of Metoicoceras geslinianum (Juignet, 1974, fig. 27), thus cunfirming Juignet’s eustatic high near the top of the jukesbrownei Zune (also knuwn in England). Further cunfusiun has resulted frum a mis-dating of the top Cenomanian luwesl Turunian furmatiuns in Fig. 12 of Haq et al. Very reasunably, they have placed a eustatic luw immediately aboye the base uf the Sables de Bousse, but this is at the base of the M. geslinianum Zone, nut the base of the Zone uf Neocardioceras juddii as they have shown it. Their 94 Ma sequence-buundary lies at Ihe base uf the Plenus Marís in suuthern Englaud. These inaccuracies by Haq et al. (assuming that Juignet and 1 are correct do nut dispruve EXXON theory, but they are certainly worrying. If each pan of their general curve is based un several arcas, there should be an autumatic check frum a cumparison between them tu prevent mistakes. It also cunfirms that it is better tu use relatively upen sea successions rather than basin-marginal success¡ons, where vagaries uf topugraphy or tectonics, sumetimes difficult tu alluw fur, have a more marked effect. Campanian.—Anyune familiar with the general geology of the Campanian in the nurthern hemisphere wuuld probably be puzzled by the EXXON sea-level changes fur this stage. It shows five eustatic peaks, wliich un their biozone scale fur Great Britain are: (i) just aboye the base of Ihe stage; (ji) at the junction of the Zunes of Offaster pilula and Gonio>’euthis quadrata; (iii) in the middle uf the Zune uf G. quadrata; (iv) just below the top of the Zone of G. quadrata; (y) a little beluw the middle uf the Zune of Belemnitella mucronata. Their two highest sea-level peaks are nos. iii and iv, buth in the quadrata Zone. Commen>’s on dic EXXON cycle char>’... 69
Whatever number uf peaks une believes can be recugnised in Campanian successions, there is une high in the Campanian which stands uut in buth nurth- west Europe and the U.S.A. (Hancuck, 1993). This spread te Chalk uver Ireland and ~5 10w in the Zone of Belemnitella langei as this zone has been used in the U.K. (Hancock, 1990). Irunically, it is this great eustatic peak which has enabled critical currelations tu be made between the USA and Europe (Hancuck & Kauffman, 1989; Hancock e>’al., 1992). Within the limits uf experimental error it agrees with buth ammunite and struntium isutupe intercontinental currelatiun of the Campanian-Maastrichtian buundary (McArthur e>’ al., 1992). Where is this great eustatic peak un the EXXON curve? Even their fifth peak is much tuu early; whilst their highest peak, No. iii, does nut seem tu correspond tu a peak un anyune’s curves fur nurthern Eurupe, with the just pussible exceptiun uf the western slupe uf the Urals in Arctic Russia (Naidin e>’ al., 1980). Where un Earth did the EXXON team cullect their data? Conclusions on 3rd order cycles.—-1 find myseff in the udd positiun uf believing in eustasy but finding many mistakes in the EXXON curves wherever biustratigraphy alluws une tu test them. Sume of the sources uf mistakes and errors have been indicated earlier in this paper; still others un the accuracy uf te results have been discussed by MiaU (1991, Pp. 503-504). It wurries me that many uf their successes are where peaks anó troughs have previuusly been recurded, but that many uf those that are purely their own are difficult tu substantiate. TUs suunds unfair, su can their detailed techniques be used hy uther geulugists? Luts uf geulogists think they can, but alí tou often they seem tu bejust titting their own recurds into the EXXON chart; the putential errurs in te Late Cretaceuus, and even mure in the Early Cretaceuus, make this easy. An example is shown by the wurk of Ernst and Kúchler (1992, fig. 1) where they have ignured their own, appruximately currect, radiometric dates and used EXXON’s false apportionment between Early and Late Campanian time! In suuth-east France, Ferry (1990) has interpreted the meaning of limestunes in limestune-clay alternatiuns in exactly the uppusite sense uf tlie EXXON teaín, and ammunite stratigraphy is nut adequate tu see whicli is correct. Howeyer, there is une criticism by Mialí (1992) which is partly misplaced. He has puinted uut that synthetic sectiuns cunstructed frum tables uf randum numbers alluw a minimum of 77% successful currelatiun with the EXXON chart. This experiment makes no alluwances fur relative heights uf individual peaks and truughs: they are not alí the same. 1-le partly recugnises this by referring tu the possibility uf a second order eustatic changes as shuwn by writers such as Hallam (1984) and Weimer (1986). 70 Ja/ce M. Hancock
(u) Cycles longer >‘han 4Q m.y.
In their original work (Vail e>’ al., 1977, fig. 2) their ‘secund urder cycles’ ranged frum 10-80 m.y. In the current chart their secund order cycles are subdivided into ‘supercycles’ and ‘supercycle sets’. The luwer and upper buundaries uf the Cretaceous system do nut coincide with ‘supercycle’ boundaries, buí there are approximately seven supercycles (4 ~/2 tu 12 m.y.) and nearly twu supercycle sets (31 tu 39 ~/2m.y.) in the Cretaceous system. These cycles do not seem real tu me. The resultant sea-level curves do nut fit witb well knuwn published evidence. The nomenclature is derived frum the classificatiun uf ‘sequences’ un Ihe North American Craton by Sloss (1988); fur anyune uutside Nurtli America, it smacks of mumbo-jumbo; paradoxically, Sluss himself believes íhat the sequences reflect a tectunic history rather than eustatic changes uf sea-level (Sluss, 1991). There may or may not be long cycles uf sea-level change but the EXXON curve alluws tuu little change uver a long time: there are tuu many retums tu the same level. Such simple oscillations make it easier tu see cycles in the pattem. Take their ‘supercycle set Upper Zuní A’, which rau from 107.4 Ma, ihe middle uf the Zune uf Hypacanthopli>’es milleticides in the Lower Albian, tu 67.8 Ma, somewhere in the Zune uf Pachydiscus neubergicus, placed in the Upper Maastrichtian but actually Luwer Maastrichtian. This supercycle set cuntains fuur supercycles which cari be tabulated thus:
somcwhcrc within Upper Maastrichtian ______
Farlier half of bread Low in tong ter, curve: minimo, sea-level, height 215 m, maximo, UZA 4 245 m. In short te~ curve ¡be minimo, it about líO m at top boundary of supercycle. maxin,un, 245 m in the ntiddle of Ihe Zone of G. quadrala.
______within the lower half of the Zone of CL quadrata
Complete bigh-low-b¡gh in Long ter, curve: minioso, sea-tevel height 225 m, maxima at ¡ UZA 3 240 and c.25O m. In short tcrm curve the lowest minimun, it cl 25 m (Low in the Upper Turonian); a ¡ester mininsun, of 180 mis placed o thc middte ofibe Upper Santonian.
______little below top of Zone o>’ 6. woai/gar¿ N 235 m at start of cycLe. to sison tem, curve and 1) Breadlong ten~highcurvejo longibe¡cmimaximumcurve;itminimon,260,. lowc. u e 7.oue of fiL aoci/gori; this it Ihe highcst ~ UZA2 sea-tevel in the whole of the EXXON chau tbr the Mesozoic md Ccnozoic. The lowest minimo, o tbc sborl ter, curve it 180 m near the top of she Middle Cenomanian.
little bcLow ¡be top of Zone of M. inflatu.n
second ha]>’ of bread low in tung ten, curve; minirnun, sea-Level beight c.160 m at stan of cyelc, maximum of 245 m at the eod of the eycte. On the short ter, curve ibera are tive lIZA 1 minima md five maxima, ¡be tast two in the Upper Albian being al 230,, low in ¡he Zone of Euhopliírs bulos: aud 245 m. higb ¡o the Zone of M infla lower pan of ihe zone of L. lardcforcnlo Comments cm the EXXON cycle chart... 71 The maximum heights in these ‘supercycles’ should give a pattern of the relative areal extents of sedimentatiun at these times. Of course, une must investigate the evidence with sume care because foriner sediments may have been removed by erosiun, thus destroying the evidence, althuugh experience suggests that it is difficult tu remuve alí evidence of a great spread of sediment. A guod example of this problem is illustrated by the eustatic high which lies 10w in the C. woollgari Zune, where 1 happen tu agree with the EXXON team. There are few areas in northern Europe where une can see uverlapping woollgari Zune sediments because they were usually eruded away, alung with carlier zunes, in basin-marginal districts immediately after, during the subsequent S. neptuni Zone eustatic luw. But onlap by the luwer woollgari Zune can be seen un the south- west flank uf the Massif Central tu tlie east uf Périgueux in France, where it rests directly un Calluvian-Oxfurdian limestunes (Platel, 1979). However, in a broader sense the relative heights during the Middle and Late Albian cannut have been of the same urder as thuse given fur te Campanian. This wuuld give an equal extent for the Zone uf Morloniceras in.tla>’um and C,onioteuth¡s quadrata. Where is such a pattern uf distribution seen? The slipper- cLay Gault of England and Germany would have extended uver Ireland and Sweden. Accurding tu Haq e>’ al. (1988, Pp. 107-108) must uf their Cretaceous evidence came from France, plus Belgium and the Netherlands fur the Campanian-Maastrichtian; in the U.S.A. they used Colorado, Utah and central Texas. These regions do not show an equal spread uf Upper Albian and Luwer Campanian, althuugh yuu might conclude this in central Texas ur the Pays d’Auge in France if yuu ignured the facies invulved (Ménillet & Monciardini, 1991). Against this EXXON view is the far greater spread and/ur deeper water facies uf Upper Campanian sediments acruss Europe frum Ireland tu Bulgaria; alung the Atlantic seabuard of the U.S.A., thruugh the Gulf Cuast frum Alabama, íhrough Mississippi, Arkansas and into north-east Texas; in te westem interior uf the U.S.A. and Canada in spite of late Cretaceuus uplift in the regiun; in suuthern Argentina; prubably in Western Australia. In Nigeria the maximum assuciated transgressiun peaked slightly later in the Early Maastrichtian (Reyment, 1980). The first graphs that 1 pruduced uf Cretaceuus sea-levels (Hancock, 1975) show an uveralí rise thruugh the Albian and Late Cretaceuus. Such graphs have nuw been refined but the trend has nut changed: te levels during the Albian were never as high as thuse during the Late Campanian. What seems tu be an ubviuus mistake makes une suspiciuus uf uther ‘2nd order cycles’. The EXXON team are guod geulogists but since we knuw unly tlie general basis uf their method and not the details (except fur the Cenumanian of te 72 Ja/ce M. l-Iancoc/c Sarihe, see aboye), it is necessary tu make sume guesses, hopefully inspired. There are lucalities which shuw very high Cretaceous sediments resting directly un mid-Cretaceous, resting directly un pre-Cretaceous rucks. In the Cotentin peninsula in Normandy there are Upper Maastrichtian tuffeau facies resting directly un Cenumanian or Albian, which in tum is underlain by pre-Mesuzoic rucks. Note that the raw evidence dues not accurd with either EXXON ur myself, but it is certainly cluser tu the EXXON view. The real puint is that in the investigation of eustasy by the study uf marginal facies it is necessary tu consider a very wide budy uf evidence. This is the majur reasun why ErIe Kauffman and 1 have argued in favuur uf using basin-centre ur upen sea successiuns for the evtdence, and nut alluw yuurself tu depend un the vagaries of local shurelines (Hancuck & Kauffman, 1979). CONCLUSIONS It is perhaps inevitable that discussion uf the EXXON chart should seem tu be a catalugue uf cumplaints. It did nut need this chan tu persuade many geulugists tu accept eustasy as a major control un Cretaceuus facies. Indeed, it is pussible that by assuming tou fine a resulutiun of successions from seismic stratigraphy, they have damaged their own theury. The main virtue of the work by Peter Vail and his culleagues, whu are mure mudest abuut their results than many uf the fulluwers, is that it has furced us tu examine uur results in much greater detail: compare the curves published in Cretacecus Research, volume 1, with the EXXON chad. Are we yet in a position tu recugnise simultaneous changes uf sea-level in different cuntinents al the finest biustratigraphic resulutiun? The sort uf simultaneous changes that Hancuck & Kauffman (1979) indicated, were unly tu sub-stage resulution at best. New wurk with Bilí Cubban uf the US. Geulugical Survey suggests that sume peaks and truughs uf sea-level were cuincidení between England and the western interior of the USA duwn tu a zune ur fraction uf a zone: a resolutiun uf the order uf less than 100,000 years. These are changes uver time intervais which wuuld be called 3rd urder cycles by the EXXON team. As their biustratigraphy is impruved, it shuuld be possible, in spite of Miall’s wurries, tu test uther 3rd urder cycles. Sequence-síratigraphy ur similar techniques, can date relatively fine changes of sea-level, but Ihe best dates are still given by facies- successiuns in the upen sea, far from a shure-line. At time-scales lunger than abuut 12 m.y. plate muvements are commonly strung enuugh tu give a false picture, i.e. alí EXXON’s secund urder cycles are Comments on tite EXXON cycle char>’... .73 suspect. Eustatic changes uver this time interval are not always strung enough ur fast enuugh tu prevent majur plate movements duminating the regional picture. ACKNOWLEDGEMENTS Rick Sarg, Peter Vail, Bernie Vining and particularly Jan Hardenbul of ihe EX- XON team have patiently answered questiuns un Iheir theories and results. 1 am tn- debted tu the Natural Environment Research Cuuncil fur financial suppurt fur must uf Ihe research behind this paper. REFERENCES ARKELL, W.J., (1947). 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