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knowing that there is an erosion surface at the top
of the bed, that ooid shoals prograde and migrate
laterally, and that much of the sediment may
have been exported from the site of produc-
tion. Ooid shoals today accumulate with rates of
0.5-2 mm yr
(Schlager, 1981; Enos, 1991).
through time. Especially the sea-level falls related
to the 100-kyr cyclicity appear to have punctuated
the sedimentation history and forced the plat-
form to reorganize itself. Thus, even if the general
facies distribution was controlled by the tectoni-
cally induced platform morphology, the establish-
ment of complex facies mosaics probably also had
a random component.
High time resolution also allows for the estim-
ation of sedimentation rates separately for each
small-scale or elementary sequence and, if a max-
imum-fl ooding surface can be identifi ed, separately
for transgressive and highstand deposits. The
sedimentation rates derived from this Oxfordian
study are more or less comparable to the ones
estimated for Holocene tropical-subtropical car-
bonate environments. Nevertheless, ancient reefal
and lagoonal ecosystems must be interpreted with
care because of preservational bias. Also, the eco-
logical requirements and carbonate production
rates may have differed considerably from today's
reefs and lagoons, and high-frequency changes of
water temperature, water chemistry and nutrient
levels can only be guessed at. On the other hand,
processes such as transport of ooid sands by tidal
currents, mud accumulation in lagoons and on
tidal fl ats, or burrowing by animals mixing the
sediment certainly occurred in similar conditions
as they happen today.
Late Jurassic climatic and sea-level changes
had much lower amplitudes than those in the
Quaternary, plate-tectonic confi guration and oce-
anic circulation were fundamentally different,
and there were many evolutionary steps between
the Late Jurassic and today's fauna and fl ora, yet
there are still many common features. As such,
looking at modern shallow-water carbonate sys-
tems therefore remains a good means to improve
one's understanding of ancient carbonate plat-
forms, and there is no reason to abandon the con-
cept of 'Comparative Sedimentology' put forward
by Robert N. Ginsburg in the early 1970s.
CONCLUSIONS
A study in the Oxfordian of the Swiss Jura has
shown that ancient shallow-water carbonate sys-
tems are as complex as modern ones. In order to
obtain a short time-frame within which the lat-
eral and vertical facies distribution in ancient
strata can be monitored, a cyclostratigraphic
analysis is necessary. In the case of the interval
studied, bio- and sequence-stratigraphic calibra-
tion as well as the hierarchical stacking pattern of
depositional sequences allows a time resolution of
100 to 20 kyr to be obtained, which corresponds to
the orbital short eccentricity and precession cycles,
respectively. Small-scale sequences related to the
100-kyr orbital cycles are generally well devel-
oped, while elementary (20-kyr) sequences often
are diffi cult to identify because facies contrasts
are weak, or because local factors create facies
changes that are independent of an orbital con-
trol. Correlation of elementary sequences between
sections therefore is not always possible.
On the 100-kyr as well as on the 20-kyr scales,
signifi cant lateral facies heterogeneity is observed
in the interval studied. While differential sub-
sidence created a relief that pre-determined the
positions of coral reefs, ooid shoals and protected
lagoons, high-frequency sea-level changes in tune
with the orbital insolation cycles gave the beat for
the formation of depositional sequences. Lateral
migration of sediment bodies may have been
forced by sea-level fall but could also have hap-
pened under the infl uence of currents, indepen-
dently of sea level. Climate fl uctuated between
humid and dry, which at least partly controlled
the input of siliciclastics and nutrients from the
hinterland. This terrigenous input had a negative
impact on the coral patch-reefs, which suffered
from eutrophication.
Although the distances between the sections
studied are of several kilometres and the recon-
structed time lines have a resolution of 10 kyr at
the most (from sequence boundary to maximum
fl ooding of a 20-kyr precession cycle), it can be
demonstrated that different depositional envir-
onments were juxtaposed and shifted position
ACKNOWLEDGEMENTS
The fi nancial support of the Swiss National
Science Foundation (Projects Nos. 20-67736.02
and 109214.05) is gratefully acknowledged. We
thank Elias Samankassou for his constructive
remarks on a fi rst version of this paper, and David
Osleger and Christophe Dupraz for their careful
reviews. We also thank Peter Swart and Judith
McKenzie for the editing.
