Discussions about the Jurassic-Cretaceous boundary (Part I)
Let me try to make a few things clear to you by elucidating my opinion on the matter. I am at one with you about the stratigraphic position of the Jurassic-Cretaceous boundary at the base of the Neocomian (= boundary between the ammonite zones of 'Berriasella' jacobi and 'Tirnovella' subalpina), because this boundary practically correlates with the top Portlandian, the top Volgian and the original base of the Berriasian. If we want to speed up the procedure of the Working Group on the the Jurassic-Cretaceous Boundary, we should not waste time on finding a level for the GSSP at the base of the Berriasella jacobi Zone, but start as quick as possible to find an appropriate level at or close to the base of the Neocomian. (...)
First: the stratigraphic position of the J/K boundary should be as close as possible to the original boundary. Priority should be respected, and disrespected only when absolutely necessary. Slight deviations are tolerated on account of modern achievements in science.
Second: The GSSP should be defined in the Tethyan realm, not in the boreal realm. This is because of the continuous and easy accessibility of Tethyan GSSP locations.
Third: The GSSP should be located in open marine sediments deposited in not too shallow and not too deep environment.
Fourth: The objects or features used to determine and correlate the boundary should preferably be visible objects or features, i.e. of lithostratigraphical (sedimentological) or biostratigraphical (palaeontological) nature. Chemostratigraphical or magnetostratigraphical features in the rock are not visible and require long and expensive field and laboratory work to detect them; they can, however, be used to controle and support the correlation by visible features.
Argumentation, history, priority
1. The stratigraphic position of the J-K boundary has not been established yet. There is only a recommendation of the former 'IUGS Working group for the Jurassic-Cretaceous Boundary' (Meeting in Sümeg, Hungary, 18-21 September, 1984), that this boundary should, for the time being, be in accordance with the outcome of the vote among the participants of the 'Colloque sur la limite Jurassique-Crétacé, 1975.' This outcome endorses the proposal of Raymond Enay and his group that the stratigraphic position of the J-K boundary should be at the base of the Berriasella jacobi Zone. This species belongs to the generic group Hegaratella Nikolov, 1977, type species Berriasella paramacilenta Mazenot, 1939.
2. The Berriasian Stage defines the Cretaceous System, and the Berriasian Stage is defined by its base, which is by definition also the base of the Cretaceous System. According to the recommendation worded above, the base of the Berriasian is at this moment defined by the base of the H. jacobi Zone, which is on its turn defined by the first appearance of H. jacobi. This means that the Cretaceous is recommended to be defined by the first appearance of H. jacobi.
3. I reject this recommended definition, because it is unacceptably far away from the original stratigraphic position being the base of the Neocomian in the Neuchâtel region (Desor & Gressly, 1859; Oppel 1865), i.e. between the Goldberg Formation and the 'Unité oolitique inférieur' of the Marbre Bâtard. (This boundary is the original one, and is, according to the ammonites found and translated into current ammonite zones, situated between the H. jacobi and S. occitanica zones (Clavel et al., 1986; Détraz & Mojon, 1989; Hoedemaeker, 1987, 1991). This boundary has been defined in sediments deposited in a very shallow sea. The Maximum Flooding Interval in the upper part of the S. subalpina Subzone presumably correlates with the basal part of the Pierre Châtel Formation. This stratigraphic position lasted about 30 years, from 1865 to1895 and with a small shift up to 1973, thus more than 100 years.
4. 4. The base of the H. jacobi Zone as the Jurassic-Cretaceous Boundary is also unacceptably far from the base of the Berriasian Stage as it was originally defined. about two (Gradstein et al., 2004) to three (about 150 precessions of the equinoxes earlier) million years earlier. According to the original definition of Coquand (1869, 1870, 1871, 1875) the Berriasian comprises the 'Calcaires marneux de Berrias' containing the ammonite fauna described by Pictet (1867), which embraces, translated into current ammonite zones, the S. occitanica and S. boissieri zones. The important thing is that this original base of the Berriasian unintendedly correlates with the original base of the Cretaceous, i.e. the base of the Neocomian.
5. Finally Hoedemaeker (1987, 2002, 2003) showed by interrealmal correlation that the boundary between the B. jacobi and S. occitanica zones practically correlates with the modern top of the Portlandian Stage (since Cope et al., 1980) between the Subcraspedites lamplughi and Runctonia runctoni zones) and the top of the Volgian Stage in Siberia (between the Chetaites chetae and Praesurites sibericus zones) and European Russia (between the Subcraspedites nodiger Zone and the base of the Ryazanian Stage). So, the tops of the Boreal Portlandian and Volgian stages correlate with the original base of the Cretaceous System and with the original base of the Berriasian stage. Also the American Parodontoceras-Substeueroceras-Proniceras beds and the Argentine Substeueroceras koeneni Zone remain in the Tithonian. Only non-typical 'Parodontoceras' reedi and 'Substeueroceras' disputabile occur in younger beds. Hoedemaeker's correlations were confirmed by magnetostratigraphy (Housa et al., 2007); actually he traced major sequence boundary Be3 all over the world. I cannot find any legitimate and compelling reason to deviate from this boundary. What Enay and his group did is unacceptable and solely based on fossil taxonomy; he disturbed the already established good correlation. I cannot find any legitimate and compelling reason to deviate from this boundary. What Enay and his group did is unacceptable and solely based on fossil taxonomy; he disturbed the already established good correlation.
6. The first small deviation from the original J-K boundary was done by Kilian (1895), who respected the original definition of the Berriasian all his life. He found directly below the Berriasian sensu stricto of Berrias a few beds with a mixed Berriasian-Tithonian ammonite fauna, which he regarded as a transitional fauna between the Tithonian and Berriasian. Translated into current ammonite zones these beds, which he called Niveau de Jansiac (or Gensiac) and can be studied in the Montagne de Lure, approximately represent a part of the former 'Pseudosubplanites grandis Zone.' He surprisingly united these transitional beds and the Berriasian sensu stricto' into what he called the 'Infravalanginien.' He considered the base of the Infravalanginian the base of the Cretaceous, but he keeps the base of the Berriasian where it always has been (Coquand 1871), i.e. a few beds higher. This would be an unacceptable situation nowadays. The base of the Infravalanginian (only a few beds below the base of the original Berriasian) became known as the 'classical boundary' of the Cretaceous System, which lasted 80 years: from 1895 to 1975.
7. Mazenot (1939) corrected this unacceptable condition by considering the 'Infravalanginian' equivalent to the Berriasian and the base of the Berriasian equivalent to the base of the Cretaceous System. The 'Pseudosubplanites grandis Zone' became the basal ammonite zone of an extended Berriasian and therefore also of the Cretaceous. This boundary was respected by Le Hégarat (1973) as the J-K boundary; he introduced the H. jacobi Zone as the uppermost Tithonian ammonite zone. This zone covers the greater part of the "Calcaire blanc d'Ardèche," which represents the Ardescian Substage (Toucas, 1890), the uppermost substage of the Tithonian Stage (Cecca et al., 1989). In the Berriasian stratotype the 'Calcaires blancs de l'Ardèche' include even the P. grandis Subzone. The minor lower part of the 'Calcaire Blanc' is occupied by the M. microcanthum and Durangites zones.
8. It appears difficult to distinguish the P. grandis zone form the H. jacobi zone, because nearly all ammonite, belemnite, echinoid and calpionellid species of P. grandis Zone occur also in the H. jacobi Zone. The former zone is hardly recognizable without the presence of the latter and it is not considered workable to define the base of the Cretaceous at the base of the P. grandis Zone. During the Colloque sur la limite Jurassique-Crétacé (1975) both zones were, therefore, united into the 'jacobi-grandis zone'. In 1990 the IGCP (projects 262 and 362) Lower Cretaceous Cephalopod Team (= the predecessor of the IUGS Lower Cretaceous Ammonite Working Group, the "Kilian Group,"of the Subcommission on Cretaceous Stratigraphy) regarded the two zones as subzones of an extended H. jacobi Zone (Hoedemaeker & Bulot, 1990). In 1993 the Lower Cretaceous Ammonite Team abandoned the P. grandis Subzone as unworkable, and only kept the extended H. jacobi Zone (Hoedemaeker & Company, 1993).
9. It therefore became desirable to shift the Jurassic-Cretaceous boundary and the base of the Berriasian Stage to the top of the extended H. jacobi Subzone again, i.e. to their original position. However, Enay and his students studied the uppermost Jurassic succession in Andalusia (Spain) where the top of the P. grandis Subzone (=) cannot be well defined; it is probably a level within the Retowskiceras andrussowi Zone of J. M. Tavera, 1985, the totality of which he correlated with the S. occitanica Zone, but which probably correlates only with the top part of the former 'P. grandis Zone' and the S. subalpina Subzone, because it represents the total range of Hegaratella paramacilenta. Enay & Geyssant (1975) proposed to place the Jurassic-Cretaceous boundary either at the base of the H. jacobi Zone where the genus Berriasella suddenly becomes dominant. This level correlates with the base of Calpionellid zone B and with major sequence boundary Be1. According to Enay & Geyssant the only alternative would be a level within or at the top of the Berriasian Stage. This procedure would make the way free for every stratigrapher to indicate at random the events that he likes to be a boundary between stages or systems This cannot be. Every indication of a boundary should take account of already existing boundaries and priority should be violated only when absolutely necessary.
10. After an inquiry among the attendants of the Colloque sur la Limite Jurassique-Crétacé, the base of the H. jacobi zone was accepted by the majority of the attendants, presumably because of the perfect and convincing presentation of Enay and Geyssant, and because Enay and Geyssant did not give an acceptable alternative. There were only two dissidents mentioned: Jost Wiedmann and Raymond Casey, who are actually working around the J/K boundary. In 1984 during the meeting of the IUGS Working Group on the Berriasian Stage in Sümeg (Hungary) the members decided to respect for the time being the outcome of the iquiry until a final definition in the future (see point 1).
11. The base of the H. jacobi Zone is too far from the original Jurassic-Cretaceous boundary of Oppel (1865). It cannot be indicated in the stratotype of the Berriasian near Berrias, where only the uppermost bed of the former P. grandis Zone yielded a few ammonites. Also a large part of the original Ardescian Substage of the Tithonian was severely truncated and reduced to an insignificant part of the upper Tithonian. Nevertheless the base of the H. jacobi Zone has all qualification to be an important stratigraphic boundary: major sequence boundary attended by a clear change in the calpionellid fauna and a clear change in ammonite fauna (Tavera, 1985 found only three ammonite species (= 15%) crossing the boundary and occurring in the basalmost part of the H. jacobi Zone, viz. Corongoceras köllikeri, Protacanthodiscus heterocosmus and Durangites sutneroides), which makes its recognition and identification easy.
12. From 1981 onward I proposed with ample argumentation to put the J-K boundary at the base of the S. occitanica Zone, which is defined by the base of the S. subalpina Subzone, and which can be recognized by the first appearance of at least Strambergella subalpina, Berriasella privasensis, Neocosmoceras sayni, Negreliceras paranegreli, 'Delphinella' boisseti, Subthurmannia clareti, Mazenoticeras malbosiforme, Mazenoticeras curelense, Mazenoticeras broussei, Malbosiceras pouyannei (M. pictetiforme), Malbosiceras malbosi, Retowskiceras andrussowi, 'D.' ellenica, 'D.' sevenieri, 'Tirnovella' occitanica, and Spiticeras praegratianopolitense. None of these ammonite species that characterize the Berriasian S. occitanica and S. boissieri zones, does occur in the extended H. jacobi Zone, whereas only eight of the 62 species (13%) that occur in the H. jacobi zone, viz. Hegaratella paramacilenta, Hegaratella subcallisto, Strambergella shipkovensis, Strambergella floquinenesis, Subalpinites aristides, Dalmasiceras gigas and Dalmasiceras djanelidzei, and remarkably Schaireria longaeva, occur in the S. subalpina subzone. This means that the change in the ammonite fauna is very great and the measure of extinction surpasses the normal background extinction to a large extent, which greatly enhances the biostratigraphic recognizability of the boundary. This is interpreted to be due to the extra deep fall of the sea level connected with major sequence boundary Be3.
13. The most preferable locality for the GSSP (Global stratotype section and point) should of course be sought for in the stratotype of the Berriasian Stage near Berrias, because of the thorough pluridisciplinar investigations performed in this stratotype section (e.g. Jan du Chêne et al.,1993). According to Le Hégarat (1973) the S. subalpina Subzone begins at the base of bed 147 (= base of bed 147/12 in Jan du Chêne et al., 1993). However, the only S. subalpina mentioned from the Berriasian stratotype occurs in bed 148 (= bed 148/16 in Jan du Chêne et al., 1993), and from beds 146 and 147 no ammonites were reported. The first and last ammonites of the extended B. jacobi Zone occur in bed 145.
14. Halfway up bed 147 there is a major sequence boundary (Be3) at the erosive base of bed 147/14A. This sequence boundary coincides with the base of magnetostrigraphic chron M17r. Also the base of the 'Calcaires marneux de Berrias' virtually coincides with this sequence boundary. It is the base of the Berriasian defined by Coquand (1869, 1870, 1871, and 1875). The presence of about 20 % kaoliniet begins at the base of the first non reworked sediments above this sequence boundary. The expression of this major sequence boundary is lithostratigraphically as well as biostratigraphically discernable and recognizable in most successions in the Tethyan and Boreal realms. It is biostratigraphically well recognizable by ammonites, belemnites and calpionellids. The faunas of all these groups show a marked change-over at this sequence boundary. The GSSP may be placed at the major sequence boundary Be3. The only problem is the hiatus along this sequence boundary. The Berriasian near Angles is very incomplete around the B. jacobi-S. subalpina zonal boundary (very large hiatus between the B. jacobi and S. occitanica zones) and the magnetostratigraphy has not been studied.
15. The perfect boundary would be at a locality where the hiatus produced by sequence boundary Be3 does not exist, and the beds are conformable. The Lower Cretaceous succession along the Río Argos west of Caravaca (Province of Murcia, SE Spain), for instance, is such a locality. The sediments were deposited at a depth of about 200 to 300 m (upper slope) (Hoedemaeker & Leereveld, 1995; Hoedemaeker, 1998). In this locality the correlative conformity of sequence boundary Be3 separates the last bed with Pseudosubplanites of the extended H. jacobi Zone, bed Z204, from the first bed with Strambergella subalpina of the S. subalpina Zone, bed Z206. The first Calpionella elliptica appears in bed Z200, which is120.000 years (6 precessions of the equinonoxes) before the top of the last bed of the H. jacobi Zone. According to Allemann in Allemann et al. (1975) the base of the C. elliptica Zone characterizes "one of the most easily definable breaks in the calpionellid faunas" (viz. the appearance of many well developed Calpionella elliptica; Tintinnopsella carpathica becomes large and frequent and develops subspecies/varieties; within the C. elliptica Zone the T. carpathica-T. longa transition sets in, and the first T. longa appears; the appearance of the forerunner of Calpionellopsis simplex. Crassicolaria parvula and Calpionella alpina disappear within the C. elliptica Zone; C. alpina is still prent and develops subspecies/varieties). Unfortunately magnetostratigraphy is not possible in the Río Argos succession because of remagnetization during the orogenic movements in Miocene times (Hoedemaeker et al. 1998), and pollen and dinoflagellate cysts are absent around this boundary, because they are all carbonized by geothermal heat. However the magnetic susceptibility has been measured and can be used in magnotostratigraphic correlation.
16. A section for the GSSP of the base of the Cretaceous System that is perfect in all aspects, has not been described yet for the Mediterranean Faunal Province.
J/K-markers of the top of the extended H. jacobi Zone
There are several levels that are suitable to function as boundary planes for the base of the Berriasian/Cretaceous:
1. The entry of Calpionella elliptica, i.e. the base of the C. elliptica Zone, which according to Allemann in Allemann et al. (1975) characterizes "one of the most easily definable breaks in the calpionellid faunas." Why? Because it heralds a new association which begins with the presence of C. elliptica s.s., the increase in the size and abundance of Tintinnopsella carpathica, the appearance of Tintinnopsella longa preceded by transitional forms between T. carpathica and T. longa, the entry of the forerunner of Calpionellopsis simplex; Crassicolaria parvula and Calpionella alpina disappear within the C. elliptica Zone. (Tropical Realm)
2. The first appearence of either Subthurmannia (Strambergella) subalpina, or Berriasella privasensis, or Neocosmoceras sayni, or Negreliceras paranegreli, or or Subthurmannia clareti, or Mazenoticeras malbosiforme, or Mz. curelense, or Mz. broussei, or Malbosiceras pouyannei, or M. pictetiforme, or M. malbosi, or Retowskiceras andrussowi, or 'Delphinella' boisseti, or 'D.' ellenica, or 'D.' sevenieri, or 'Tirnovella' occitanica, or Spiticeras praegratianopolitense, which start their ranges in the S. subalpina Subzone. (Mediterranean Faunal Province)
3. The base of magnetostratigraphic zone M17r. In the type section of the Berriasian the base of M17r coincides with the sequence boundary Be3. Magnetostratigraphic boundaries are not visible in the field and therefore not as apparent as the sedimentological and palaeontological characteristics of the sediments. They can only be detected by expensive and time-consuming laboratorium work. Many rocks are not suitable to do magnetostratigraphic studies. (Mondial)
4. Sequence boundary Be3, which is a major (type 1) sequence boundary, which means that the sea level falls below the platform edge, and erosion of the platform is severe. This causes a severe turn-over in the ammonite fauna. 87% of the ammonite fauna of the subjacent H. jacobi Zone becomes extinct, whereas the overlying S. subalpina Subzone is characterized by a great number of new ammonite species, none of which occur in the H. jacobi Zone. 13% of the ammonite fauna of the H. jacobi Zone becomes extinct in the S. subalpina Zone. (Mondial)
5. The entry of kaolinite in France, England and Germany at correlatable levels. The appearance of kaolinite at the base of the S. subalpina Zone is interpreted as a change in climat, viz. from semiarid to semihumid. (Europe)
All these markers of the Jurassic-Cretaceous boundary have the same importance, but I prefer the base of the S. subalpina Subzone or sequence boundary Be3.