Geology Reference
In-Depth Information
sections studied are fi rst correlated on the level of
small-scale (100-kyr) and medium-scale (400-kyr)
sequences (Fig. 6). Sequence boundaries are placed
where facies indicate relatively shallow water or
intertidal conditions with birdseyes and micro-
bial mats, where very thin elementary sequences
suggest that accommodation was low, and/or
where channels and overlying lithoclasts imply
erosion. Maximum-fl ooding surfaces or intervals
are indicated by relatively deep facies, by shifts
from carbonate-dominated deposits to marls that
imply highstand progradation, and/or by strong
bioturbation due to low sedimentation rates.
In one section alone it is often diffi cult to place
these boundaries unequivocally and to estimate
their magnitude (i.e. if they belong to elementary,
small-scale, or medium-scale sequences). Through
correlation with other sections, however, a best-fi t
solution can be found (Fig. 6).
The model in Fig. 7 illustrates how superim-
posed high-frequency sea-level changes lead to
the formation of complexly stacked depositional
sequences in one spot on the shallow platform.
Adding to this platform morphology and the fact
that different depositional environments react dif-
ferently to the same sea-level change, it is to be
expected that sequence defi nition and correlation
is often not straightforward. For example, a reef
may create a shallowing-up succession if it catches
up with rising sea level (Kendall & Schlager,
1981), whereas during the same time a lagoon will
produce a deepening-up succession. Also, if sea-
level fall is minor, it will lead to emersion and a
well-developed sequence boundary in previously
very shallow environments, whereas in a deeper
lagoon the same sea-level fall will not be recorded
at all, or only indirectly through input of material
reworked from the emergent areas ('hidden SB' in
Fig. 7).
Overlying the coral reefs, ooid shoals and
oncoid lagoons shown in the lowermost parts
of the sections in Fig. 6, there is rapid change
towards low-energy, plant-bearing strata (at 4 m in
Gorges de Court and at 6.3 m in Hautes-Roches)
and local erosion surfaces (at 5 m in Pertuis and
at 6.3 m in Hautes-Roches). This is interpreted as
representing a sequence boundary on the 400-kyr
scale. In Savagnières, this boundary has been
placed according to the stacking pattern of the
elementary sequences at 4.8 m: a thick elementary
sequence following several thin ones implies a
rapid increase in accommodation, which is attrib-
uted to transgression after the sequence boundary.
The maximum fl ooding of the fi rst medium-scale
sequence coincides with the maximum-fl ooding
surface of small-scale sequence 3, which is
developed as a strongly bioturbated surface in
Pertuis, Savagnières and Court, and as a hard-
ground at Hautes-Roches (Figs 6 and 8). According
to the biostratigraphic position, the following
medium-scale sequence boundary corresponds
to Ox6 of Hardenbol
et al
. (1998). It displays
plant fragments at Pertuis and tidal-fl at facies at
Gorges de Court but is not very conspicuous in
Savagnières or Hautes-Roches.
The overlying medium-scale sequence begins
with locally well-developed ooid shoals in small-
scale sequence 5. Maximum-fl ooding conditions
are implied by a rapid change to a marl-domin-
ated succession in Pertuis and Hautes-Roches,
and by a change from high-energy shoals to low-
energy lagoonal deposits in Savagnières. However,
there is no feature in Gorges de Court that would
allow placing this maximum fl ooding. The fol-
lowing medium-scale sequence boundary marks
the end of the marly facies and the beginning of
the massive beds of the Hauptmumienbank and
Steinebach members. Small-scale sequence 9 gen-
erally is very thick and thus suggests rapid gain
in accommodation. This sequence will be the
subject of a high-resolution analysis in the next
chapter. Maximum fl ooding of the corresponding
medium-scale sequence is inferred by the appear-
ance of open-marine fauna in all sections and by
a change from ooid dunes and coral reefs to marly
and low-energy facies in Court, Hautes-Roches
and Vorbourg (Fig. 6).
When comparing the facies of time-equivalent
small-scale sequences between the sections it
becomes evident that there are signifi cant lateral
facies changes: for example, ooid shoals are later-
ally replaced by oncoid lagoons, or tidal fl ats are
juxtaposed to marls and palaeosols (Figs 6 and 8).
Ooid shoals do not always occur at the same time
in the sections studied, which suggests that they
migrated across the platform through time or were
stranded in one position and later reinitiated in
another one. Futhermore, the distribution of sili-
ciclastics is heterogeneous, which may be due to
preferential deposition in troughs. This and the
varying thicknesses of the sequences point to a
highly structured platform.
Elementary sequences
An elementary sequence is the smallest recogniz-
able unit of the sedimentary record and is defi ned
as having formed through one cycle of environ-
mental change (Strasser
et al
., 1999). However,
if this cycle is not strong enough to create a
