Geology Reference
In-Depth Information
can be followed over tens of kilometres and that
they are independent of facies is best explained
by sea-level changes that affected the entire plat-
form. Depending on the position on the platform
and on the type of environment (Fig. 2), a sea-level
cycle produced a deepening and shallowing or
an aggradational pattern. Thicknesses of the
small-scale sequences vary between 2 and 8 m
(Fig. 6). If facies-dependent decompaction is
applied (procedure described in Strasser
et al
.,
2004) this would correspond to about 3 to 15 m.
The amplitude of a sea-level change that created a
specifi c sequence must have been at least as high
as this decompacted sequence if it ends in tidal
facies, or somewhat higher if the sequence stayed
subtidal throughout.
In many elementary sequences it is diffi cult to
discern deepening and shallowing facies evolu-
tions. However, bed limits may display features
(e.g. hardgrounds, bioturbation, channelling) that
indicate changes in the depositional environment
(Fig. 9). Bed limits are caused by thin marly lay-
ers. The input of clays can be related to sea-level
changes (erosion of soils during sea-level low-
stands, ponding in deeper environments during
maximum fl ooding), to more rainfall in the hin-
terland increasing fl uvial transport, and/or to cur-
rents that redistribute the clays independently of
sea level or climate (Strasser & Hillgärtner, 1998).
In ooid shoals, bedding limits form through bed-
form migration and channelling. These limits are
fi rst of all controlled by tidal currents, and only
the intensity of the latter may be related to sea
level. Consequently, a bed of ooid grainstone seen
in outcrop does not necessarily represent a cycle
of sea-level change.
The hierarchical stacking of elementary, small-
scale and medium-scale sequences and the bio-
chronostratigraphic time control suggest that the
formation of these sequences was in tune with the
orbital cycles of precession, short eccentricity and
long eccentricity, respectively (see above). The
quasi-periodic perturbations of the Earth's orbit
produce changes in the insolation received at the
top of the atmosphere, which then translate into
climate changes (Schwarzacher, 1993; De Boer &
Smith, 1994; Matthews & Perlmutter, 1994). In the
Late Jurassic, ice in high latitudes probably was
present, but land-bound ice-volumes were too
small to induce important glacio-eustatic fl uc-
tuations (Frakes
et al
., 1992; Eyles, 1993; Valdes
et al
., 1995). However, volume changes of alpine
glaciers could have made a small contribution
(Fairbridge, 1976; Valdes
et al
., 1995). An important
component in low-amplitude sea-level changes
is thermal expansion and contraction of the
uppermost layer of ocean water (Gornitz
et al
.,
1982; Church & Gregory, 2001). Contributions
may also have been made by thermally induced
volume changes in deep-water circulation (Schulz
& Schäfer-Neth, 1998), and/or by water reten-
tion and release in lakes and aquifers (Jacobs &
Sahagian, 1993). Consequently, it is assumed that
Oxfordian high-frequency sea-level changes were
coupled to climate changes, which themselves
were linked to the Earth's orbital parameters. The
above-mentioned mechanisms could easily pro-
duce the few metres of amplitude necessary for
the accommodation of the observed depositional
sequences.
On a shallow platform, even very low-ampli-
tude sea-level changes can have dramatic effects.
For example, a sea-level fall of a few tens of centi-
metres can lead to emersion and erosion of a reef
that has previously built up to sea level, and ooid
shoals can be forced to migrate laterally or will
emerge to form islands. On the other hand, a small
sea-level rise can fl ood a barrier that has hitherto
protected a lagoon and produce a rapid change in
water energy, salinity and ecology. Facies changes
and depositional sequences can also be induced
independently of sea level changes by lateral
migration of sediment bodies controlled by cur-
rents (Pratt & James, 1986) or by progradation
(Ginsburg, 1971). Such 'autocyclic' processes may
lead to stacked sequences but will never be able to
create a hierarchical stacking pattern as observed
in the sections studied (Strasser, 1991). In the
defi nition of individual elementary sequences,
however, such processes must be considered.
Differential subsidence
The Swiss Jura in Late Jurassic times was part of
the passive northern margin of the Tethys Ocean
(Fig. 5). The tectonic regime was extensional,
and a pattern of structural highs and depressions
formed (Allenbach, 2001). Average subsidence
rates are estimated at 20-40 m per million years
(Wildi
et al
., 1989). The thickness variations
of the sections studied (Fig. 6) can therefore be
attributed to differential subsidence of individual
tectonic blocks. The general facies distribution
in the sections studied suggests that the blocks
were slightly tilted: coral reefs and ooid shoals
developed on the higher margins towards the
