in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
66
Progress in Defining Jurassic Stages – A Learning Process
Nicol Morton
Former Chair of ISJS, Le Chardon, Quartier Brugière, 07200 Vogüé, France (E-mail: nicol.morton@orange.fr)
In Jurassic stratigraphy we have a long and
influential heritage. By the end of the 18 th Century
Jurassic successions in several countries had been
divided into small named units that we would now call
lithostratigraphy. These were shown on early geolo-
gical maps such as that of William Smith. It was also
known that stratigraphical units could be recognised by
the fossils they contained.
By the middle of the 19 th Century the fossil con-
tent was used to recognise the same unit in different
areas, despite changes in lithology, implying cor-
relation in time–what we now call chronostratigraphy.
The pioneers in the Jurassic, Alcide D’Orbigny (1850)
and Albert Oppel (1856-58), established, respectively,
the concepts of stages and zones much as these are used
today in Jurassic stratigraphy.
During the latter half of the 19 th Century and into
the 20 th Century many new stage names were intro-
duced into Jurassic stratigraphy, to the extent that W.J.
Arkell (1930) listed 120; others have been added since!
Clearly, there were too many, with too many partial
overlaps and synonyms, to be useful for international
correlation and communication. Fortunately we had in
Jurassic stratigraphy the widely respected authority of
W.J. Arkell (1956) who was able to convince most to
use a simplified scheme of only eleven stages as the
basis for a global time-scale. This has stood the test of
time, with only two modifications–international agree-
ment at Luxembourg in 1962 and 1967 to reintroduce
the Aalenian Stage in the Middle Jurassic and by ISJS
vote in 1984 to use Tithonian as the international
standard top Jurassic stage in place of Portlandian and
Purbeckian or Volgian.
The Stages were defined in terms of the ammonite
zones they contained, but it became evident in the
1960s that more precise and unambiguous definitions
were required. During the 1962 1 st International
Jurassic Colloquium in Luxembourg organised by P.L.
Maubeuge, it became clear that the traditional strato-
types were of limited value. To establish a sequence of
stages that did not overlap or leave gaps only the base of
each stage should be defined. At the 1967 2 nd
International Jurassic Colloquium also organised in
Luxembourg by P.L. Maubeuge, specific levels (at
zone, subzone or horizon) were proposed with
suggestions as to where a stratotype section and point
could be selected to define the basal boundary of each
stage. There was even one stage (Bathonian) for which
a specific bed and section were proposed–10 years
before the first formal GSSP definition (base Devonian)!
Unfortunately the then Jurassic Subcommission be-
came inactive.
With the re-establishment of a Jurassic Sub-
commission (first Chair Arnold Zeiss, Secretary Olaf
Michelsen) a new series of International Jurassic
Symposia/Colloquia was initiated. During the first at
Erlangen in 1984, Working Groups with a Convenor
and a flexible number of members, were established for
each of the standard Jurassic stages. Their primary
responsibility was to research alternatives and propose
the level for recognizing the base of the stage and a
specific point and section (the Global Stratotype
Section and Point, GSSP) to define the boundary (see
Morton, 2008). After approval by majority vote (at least
60%) within the Working Group membership, the
proposal should be submitted to the Executive (Chair,
Vice-Chair, Secretary) and Voting Members (20) of the
International Subcommission on Jurassic Stratigraphy
(ISJS) for comment and approval (again 60% YES
votes required). After revision, if required in the light of
comments received, the proposal should then be
submitted by ISJS to the International Commission on
Stratigraphy (ICS) for comment and approval, and
finally by ICS to the Executive Committee of IUGS for
ratification.
To date six of the eleven Jurassic stages have been
ratified (see Morton, 2008 for details). These will be
discussed briefly, in the order in which they were
ratified by IUGS, with my comments based on personal
experience with most as to what could be learned from
each.
Bajocian: at base of the Hyperlioceras mundum
Horizon, Discites Zone, with GSSP at the base of bed
AB11, Murtinheira, Cabo Mondego, Portugal and
Auxiliary Stratotype Point (ASP) at the base of bed U10,
Bearreraig, Isle of Skye, Scotland. Ratified by IUGS
1996 (Pavia & Enay, 1997).
Aalenian (also Middle Jurassic): at base of the
Leioceras opalinum Horizon, Opalinum Zone, at the
base of bed FZ197, Fuentelsaz, Iberian Chain, Spain.
Ratified by IUGS January 2000 (Cresta et al., 2001).
Sinemurian: at the base of the Vermiceras
quantoxense Horizon, Bucklandi Zone, 0.9m above the
base of bed 145, East Quantoxhead section, Kilve,
Somerset, England. Ratified by IUGS August 2000
(Bloos & Page, 2002).
Pliensbachian: at base of Bifericeras donovani
Horizon, Jamesoni Zone, base of bed 73b, Wine Haven
section, Robin Hood’s Bay, Yorkshire, England.
Ratified by IUGS March 2005 (Meister et al., 2006).
Bathonian: at base of Bigotites diniensis Horizon,
Convergens Zone, base of bed RB071, Ravin du Bès