Geology Reference
In-Depth Information
were most probably the Bohemian and/or Central
massifs, which implies long transport distances
(Fig. 5). Consequently, a time lag must be assumed
between the increased erosion of the hinterland
and the fi nal deposition of the material in the
study area.
At the scale of this study (sections spaced a few
kilometres, time lines spaced a few ten-thousand
years), facies mosaics are not random but linked to
sea-level changes, tectonics, climate and ecology.
On smaller scales, however, random processes
defi ning the exact position and composition
of each facies may become important (Rankey,
2002; Wright & Burgess, 2005). In a road-cut
close to Roderis (18 km ENE of Vorbourg), in the
Steinebach Member, Samankassou
et al
. (2003)
have described important facies changes occur-
ring over a few metres only: a coral reef is laterally
replaced by coral rubble then by ooids over a dis-
tance of 2.5 m, another reef passes laterally into a
tidal fl at over 14 m. Thus, reef growth determined
the depositional environments in the immediate
neighbourhood. Such relationships demonstrate
that ancient facies mosaics were as complex as
modern ones.
before it fi nally accumulates and is preserved, and/
or by bioturbation that may vertically mix tens of
centimetres of sediment. Hardgrounds or emer-
sion surfaces that are interpreted as time lines on
the cyclostratigraphical (10-kyr) scale may record
hundreds of years of non-deposition, physical ero-
sion and/or dissolution and cannot be considered
isochronous if a higher time resolution is sought.
Different carbonate environments have differ-
ent sediment production rates, which is clearly
demonstrated for the Holocene (Enos, 1991;
Bosence, 1995; Gischler, 2003). Therefore, a com-
parison between recent and ancient platforms is
only reasonable if ancient sedimentation rates
are calculated separately for each facies, and if
non-deposition, reworking and erosion are also
considered (Strasser & Samankassou, 2003). In
the protected lagoonal setting of Pertuis, elemen-
tary sequences 3 and 4 do not show any signs of
reworking or erosion. If a decompaction factor of
2 is applied for these homogeneous wackestones
(Strasser & Samankassou, 2003), the original thick-
nesses amount to 245 and 220 cm, respectively.
They have been deposited over a period of 20 kyr
each, which results in average sedimentation rates
of about 0.11-0.12 mm yr
. The maximum-fl ood-
ing surfaces have not been placed in the middle of
these sequences (Fig. 9): either the chosen bedding
surfaces do not correspond to maximum fl ood-
ing, or the sea-level cycle was asymmetric, or the
sedimentation rates varied through the sequences.
Tropical-subtropical Holocene lagoons have sedi-
mentation rates ranging widely from 0.1 to 4 mm
(Enos, 1991). The coral reef at Gorges de Court
probably formed mostly during the transgressive
phase of a 20-kyr sea-level cycle. The highstand is
very thin and composed of a tidal fl at (Fig. 9). The
reef is 226 cm thick; compaction of this bound-
stone is negligible. Consequently, reef growth
rate can be estimated at 0.23 mm yr
on aver-
age. This is low when compared with Holocene
reefs, which have vertical accretion rates of up
to 14 mm yr
(Enos, 1991). The low growth rate
of the Oxfordian patch reefs is explained by the
at times unfavourable ecological conditions (see
above). The ooid shoal in the highstand of ele-
mentary sequence 2 (Fig. 9) is preserved with a
thickness of 120 cm. If decompacted, this would
represent about 144 cm (assuming a decom-
paction factor for grainstones of 1.2; Strasser &
Samankassou, 2003). The time span is estimated
at 10 kyr (half a precession cycle), which results
in a rate of 0.144 mm yr
. This number of course
does not have any sedimentological meaning,
Sedimentation rates
When estimating sedimentation rates, it is para-
mount to specify the time interval over which
they are calculated (Sadler, 1981; Schlager, 1999).
When simply dividing the thickness of a strati-
graphic column by the number of million years
it represents, only average rates of sediment pres-
ervation are obtained. Original carbonate produc-
tion rates certainly were much higher, but part
of the sediment was exported, and time was con-
sumed in hiatuses. Sediment compaction through
time also has to be considered. To approximate
the sediment production of ancient carbonate
systems, the time interval over which the rates
are calculated has to be as short as possible, in
order to avoid inclusion of hiatuses. The time
resolution obtained by cyclostratigraphy is 10 kyr
at best, if a maximum-fl ooding surface can be
recognized within an elementary sequence shown
to correspond to the 20-kyr precession cycle
(assuming that the sea-level cycle was symmetri-
cal). On the other hand, it is improbable to fi nd
perfectly isochronous layers or surfaces in ancient
shallow-water carbonates. Time-averaging is
caused by reworking of older particles that are
incorporated in younger sediment, by sediment
being washed back and forth for hundreds of years
