Geology Reference
In-Depth Information
SB
sea
Time
SB
Sedimentary
record
TS
MF
SB
TS
SB
TS
MF
SB
SB
SB/TS
SB/TS
TS
TS
MF
MF
hidden SB
MF
MF maximum flooding
TS transgressive surface
SB sequence boundary
bioturbation
birdseyes
lithoclasts
SB/TS
MF
SB/TS
TS
Fig. 7.
Superimposed frequencies of sea-level fl uctuations lead to a complex sedimentary record. Note that sequence
boundaries may be hidden if sea-level fall is not suffi cient to create a facies contrast. When accommodation is low due to a
long-term sea-level fall, elementary sequences become very thin and have eroded tops, or they are not deposited at all. For
more discussion refer to text. Modifi ed from Strasser
et al
. (2004).
Pertuis
Savagnières
Gorges de Court
Hautes-Roches
3
Hardground
(medium-scale
maximum-flooding
surface)
Palaeosol
?
2
m M W P G
m M W P G
m M W P G F/R
m M W P G
Fig. 8.
Correlation of small-scale sequence 2 (compare with Fig. 6). While at Pertuis there is a well developed maximum-
fl ooding surface separating the transgressive ooid shoals from the muddy lagoonal highstand, the transgressive deposits
overlying a palaeosol at Hautes-Roches are rich in siliciclastics and the maximum fl ooding of the small-scale sequence is
diffi cult to place. The maximum fl ooding of the medium-scale sequence, however, is expressed by a hardground within
small-scale sequence 3. Symbols are as in Fig. 6.
facies contrast, it will not be expressed in the
record. When accommodation is low, elementary
sequences may not be deposited or eroded ('missed
beats' of Goldhammer
et al
., 1990). Consequently,
the best chance of recording elementary sequences
is during long-term transgression when accommo-
dation space is created while carbonate production
keeps up and the depositional environments stay
shallow. For a detailed analysis we have there-
fore chosen small-scale sequence 9 (Figs 6 and 9),
which formed at the beginning of a medium-scale
(400-kyr) transgression (Hug, 2003).
Figure 9 presents a correlation between the fi ve
sections studied. Small-scale sequence 9 is com-
posed of individual beds, which are separated by
thin layers of marls. The number of beds varies
from one section to the other. The identifi cation of
elementary sequences is most straightforward at
Vorbourg, where small channels indicate shallow-
est water and thus sequence boundaries. The max-
imum-fl ooding surface of small-scale sequence 9
is implied by the change from high-energy to low-
energy facies and by bioturbation. In the second
and third elementary sequence, their maximum
fl ooding can be identifi ed by thin marly layers
and subtle facies changes. The Hautes-Roches
section displays coral carpets at the base. The top
of a conspicuous ooid sand wave is interpreted
as the maximum-fl ooding surface of small-scale
sequence 9. In the following ooid dunes it is
