Report by Bill Wimbledon (Chair of the Berriasian WG)
Participants: Daniela Rehakova, Otilia Lintnerova, Martin Kostak, Petr Pruner, Petr Schnabl, Kristyna Cizkovak, Andrea Svobodova, Jozef Michalik, Tiiu Elbra, Camille Frau, Jacek Grabowski, Li Gang, Li Jianguo, István Főzy, Kristalina Stoykova & Bill Wimbledon.
A formal WG meeting was held on 16th May 2018 under the auspices of Institute of Geology of the Czech Academy of Sciences. The meeting to discuss the compilation of a shortlist of sites for consideration as potential GSSPs. This was founded on a short presentation by WAPW (largely concerning earlier decisions by the WG on a primary and secondary markers, the details of calpionellid boundaries and evolving nannofossil FADs: as well as the need to choose sites with both vertical and lateral continuity) and correlation charts circulated to all members prior to the Kroměříž meeting.
WAPW emphasised the enormous recent advances made in Mexico and the Andes on the Tithonian-Berriasian interval by our colleagues in the WG, discussing the latest finds of calpionellids in Mexico (by Lopez-Martinez) and the need for complementary magnetostratigraphic and nannofossils data. The calpionellid advances in the Andes at Las Loicas (Lopez et al.) and magnetostratigraphic results at Arroyo Lonconche (Inglesia Llanos, Kietzmann et al.) are key advances. However, the disagreement of the ammonite zonations between these two localities remains a stumbling block which Argentine colleagues are addressing (Riccardi). More substantial nannofossil results from a wider range of localities can be anticipated in due course. We were reminded (Fozy) that numerous good sections exist in Argentina, notably those documented by Parent.
After consideration of the ICS guidelines for selection of chronostratigraphic units, and mention of the already agreed parameters for a preferred Berriasian GSSP (notably a well- developed Alpina Subzone, and the inclusion of complete M19r, M19n and 18r), the group moved to a discussion of localities showing a good magnetostratigraphic record and the primary marker (small orbicular) C. alpina with supporting nannofossil and ammonite datums. Our debate focussed, particularly, on the viability of the original type locality for the Berriasian, the locality first examined by the WG, and the many sites that have since been studied and documented by the group (see Appendix) through the Tithonian/Berriasian interval. Key localities were demonstrated in correlation charts for the T/B interval. A number of these localities were also the subject of individual presentations during the Kroměříž workshop.
The discussion, in particular, included mention of Berrias, and the more useful localities of Puerto Escano, Brodno, Fiume Bosso, the Drome (Vocontian trough) plexus of sites (Le Chouet, Font de St Bertrand, Haute Beaume, Charens & Tre Maroua), Torre de’ Busi, Rio Argos, and Kurovice. The pros and cons of these localities were discussed, and are (briefly) as follows.
Berrias was earlier ruled out as a contender: the outcrop ends before the base of the Alpina Subzone and the stage are not seen. Though the profile is still being studied by the group for the upper Lower, Middle and Upper Berriasian. The adverse facies at Puerto Escano were noted, as were earlier comments (Cristina Casellato) about the condensation of the FADs of nannofossil marker species, supported in the discussion (Stoykova), though the very good palaeontological record was emphasised (Kostak). Brodno is well documented and has been understood since our earliest WG meetings to lack much of M18r, and to have a weaker record of nannofossils in upper M19n. Fiume Bosso remains a strong contender: there calpionellids have been re-collected and revised in the Chitinoidella-Ferasini interval that was described by Housa et al.: and a nannofossil revision is still in progress. The top and base of the Alpina Subzone show slight changes in position from those published previously. Torre de’ Busi appeared to be a good prospect in 2010, but its calpionellid zonation shows some aberrant features, notably the very high placing of the base of the Alpina Subzone. The Vocontian trough plexus of sites are geographically closely spaced (in the Dept. of Drome) and show a detailed composite record, with good calpionellid preservation, essentially showing the same T/B facies succession and overlapping stratigraphic intervals within the pre-Chitinoidella to Oblonga biozonal range so far documented. The need for description of the local sedimentary context was stressed (Grabowski), and the fact that Haute Beaume (near to Le Chouet) lacks breccias, as does most of the more distant Tre Maroua section was noted (Frau).
Two sites where great hopes were entertained in earlier times have proved to be a considerable disappointment: Theodosia (recently published) because, though it has ammonites, nannofossils and magnetostratigraphy, no coherent calpionellid zonation can be constructed; whereas Rio Argos yields ammonites, nannofossils and calpionellids, but is entirely remagnetised. Kurovice (recently published) shows a good fossil record, though it has three tectonised intervals which closely bracket the Alpina zonal base, and it lacks ammonites (Kostak); calpionellid preservation is poorer than, for instance, France (Rehakova).
Velikiy Kamenets, currently being documented, was also mentioned (Grabowski): though some uncertainty over the position of the Alpina/Ferasini subzonal boundary (In M17 or M18?) means that presently it does not qualify as a contender site.
Therefore, in summary, Berrias, Theodosia, Torre de’ Busi, Brodno and Rio Argos were considered less suitable; Fiume Bosso and the Vocontian Trough sites were judged to be better candidates; and question marks remain over the localities of Puerto Escano and Kurovice.
This is tabulated below:
|Theodosia||nannos, ammonites, magnetostrat. (M19n.1r, M18r)||-no calpionellid zonation
-base of Alpina Sbz. and M19n not detected
|Rio Argos||calpionellids, nannos, ammonites||-no magnetostrat. remagnetised|
|Torre de’ Busi||nannos, calpionellids, magnetostrat. (M19r, M19n.1r and M18r)||-Alpina Sbz. base in M19n.1n is anomalous
|Brodno||calpionellids, nannos, magnetostrat. (M19r, M19n.1r)||-incomplete M18R
-limited nannofossil record
|Puerto Escaño||ammonites, nannos, calpionellids, magnetostrat. (M19r, M19n.1r and M18r)||-Rosso Ammonitico facies
-compressed nannofossil FADs
|Kurovice||calpionellids, nannos, magnetostrat. (M19r, M19n.1r and M18r)||-tectonics in Alpina Sbz.
|Fiume Bosso||calpionellids, nannos, magnetostrat. (M19r, M19n.1r and M18r)||-no ammonites
-nannofossil revision in progress
|Drôme composite||calpionellids, nannos, ammonites, magnetostrat. (M19r, M19n.1r and M19n.1r is M18r)||-M19r absent at Tre Maroua, but M19n.1r is present|
The meeting was concluded with an agreement that WAPW would circulate a summary of the meeting discussion for the agreement of the attendees, and then the shortlist is to be circulated to the entire WG for their opinions and votes.
29 May 2018
Subsequent to our group discussion Jacek Grabowski wrote suggesting we should include mention of a site not proposed and discussed at the Kroměříž business meeting – Lokut, and Petr Pruner suggested another – Nutzhof. So I append these in the same format.
|Lokut||calpionellids, nannos, magnetostrat. (M19r, M19n.1r)||-top of Alpina sbz. not detected
-late FADs for key nannofossil markers
|Nutzhof||calpionellids, nannos, magnetostrat. (M19r, M19.1r, M18r)||-abnormally thin M19n.2n
-abnormal level of base of Elliptica Sbz. in M18r
-late FADs for key nannofossil markers.
|Italy||Torre de’ Busi
Fonte de Giordano
(pending Col Santino)
(pending Cortese quarry)
Mintaq Salt Dome
San Jose de Iturbide
St Mateus, Zacatecas
San Pedro de Gallo
Isle of Portland quarries