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2 Volumina Jurassica

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2 Morton N.

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GSSPs approved by ICS and IUGS since Mondello (2002)

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Morton N.

tom Vol. 4, no. 1

195-196

The priority activity at present for the International Commission on Stratigraphy and its Systemrelated Subcommissions, including the Jurassic Subcommission, is completion of the programme of selection and ratification of Global Stratotype Sections and Points for all the Stages of the International Chronostratigraphic Scale. The deadline for completion set by IUGS is the next International Geological Congress in 2008. The current state of progress for the Subcommissions is highly variable – from having completed this process some years ago (and now having to consider revisions!) to being far away from completion. Hopefully there will be a “flood” of proposals over the next two years. The situation for the various Subcommissions will be summarised with illustration and comment on some examples of GSSP proposals that have been approved since I reviewed this subject in the last Jurassic Symposium in Mondello (2002). The situation for completion of the Stage GSSPs in the Jurassic is reasonably promising. Four of the eleven have been ratified, though only one since Mondello. Several others should be decided during the Kraków Congress, leading to postal/email votes within the Working Groups and submission of proposals to the Jurassic Subcommission. The 23 Voting Members of the Subcommission must study a proposal, make comments if appropriate and vote (usually by email) YES/NO/ABSTAIN. If a proposal is approved by a significant majority, the Working Groups have the opportunity to modify the proposal in the light of comments received before it is submitted to ICS. The same procedure follows before the proposal is submitted by ICS to IUGS for ratification. For the Jurassic the greatest problems concern the lower and upper boundaries of the Jurassic. Great progress has been made with the Triassic/Jurassic boundary and a GSSP proposal may be possible in the near future. However, there has been little progress with the Jurassic/Cretaceous boundary, technically the responsibility of the Cretaceous Subcommission.

Evolution of Graphoceratidae and the Aalenian/Bajocian boundary at Bearreraig, Isle of Skye, NW Scotland

100%

Morton N.

tom Vol. 4, no. 1

196-196

The GSSP for the base of the Bajocian Stage, and therefore the Aalenian/Bajocian boundary, has been defined at Cabo Mondego, western Portugal and ratified by IUGS. It was the first of the Jurassic Stages to be so defined. At the same time an Auxiliary Stratotype Point (ASP) at Bearreraig, Isle of Skye, NW Scotland was ratified by IUGS. The key marker event for the Aalenian/Bajocian boundary is evolution within the ammonite family Graphoceratidae. The original intention was to use as marker the first appearance of the genus Hyperlioceras (s.l. to include Toxolioceras and other mainly microconch “genera”) which evolved from the genus Graphoceras. However, in the light of detailed study of successions in various areas of western Europe and North Africa, this was modified. The earliest horizon of Hyperlioceras (H. incisum) proved to be too limited in distribution to be useful for correlation, so that the second Hyperlioceras horizon (H. mundum) was selected as the key marker for definition and correlation of the base of the Bajocian. The details of the evolution of Hyperlioceras from Graphoceras are best preserved and documented at Bearreraig, which is why this section was accepted as ASP. The succession is relatively thick (c. 24 m for the topmost Aalenian and lowermost Bajocian) and the ammonites are preserved mainly in a sequence of nodules. Each nodule contains an assemblage, dominated by juveniles, which approximates to a biological population. The dimorphic Graphoceras limitatum/carbatinum species remains virtually unchanged morphologically through some 15 m of strata (evolutionary equilibrium?) and overlap with the first two Hyperlioceras species. The main morphological change into the first Hyperlioceras species (dimorphic H. incisum/rotabilis) is in the shape of the venter (a punctuation event?). This is followed over 13 m of strata by gradational increases in size and involution through H. mundum/aspera and continuing into H. walkeri/contorta.