Geology Reference
In-Depth Information
Fig. 4.6
Composite Fischer plot,
sequence
stratigraphy and
Pr´at
2012
). The shallowing upward lithofacies succession is the fol-
lowing: lf1, shales; lf2, psammites; lf3, sandstones and quartzites or
conglomerates (Based on a total of 133 elementary parasequences [fifth
order]). Abbreviations:
TST
Transgressive Systems Tract,
HST
Highstand Systems Tract,
TS
Transgressive Surface,
MFS
Marine
Flood Surface
13
C
carb
curves in the Mbuji-Mayi Supergroup of the northern (and
central) Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Elementary
parasequences in BII carbonates are delimited by the microfacies
standard sequence (MF1-MF11) and by the lithofacies standard
sequence (lf1-lf3) in the mixed clastic-BI group (Delpomdor and
δ
4.6.3 Paleodepth
(Bertrand-Sarfati
1972a
) could indicate that a new transgres-
sive systems tract will replace the BIIe HST.
Finally a comparison of clastic and mixed clastic/carbon-
ate sedimentation in part of the northern (and central) SMLL
Basin is revealing. The Fischer plot of the Kafuku 15
drillcore displays a regular succession of 30 plurimetric
shallowing-upward clastic/carbonate parasequences
We estimate that the clastic and carbonate sequences of
the Mbuji-Mayi Supergroup have an average thickness of
17 m for subtidal cycles.
Using approximate depositional depths of microfacies (or
lithofacies lf1 to lf3 in siliciclastics and mixed siliciclastics/
carbonates of the S70 Tshinyama and Kafuku 15 drillcores),
we thus suggest that
4 m for peritidal cycles, and
(on
average
10 m-thick
parasequences). The curve is stabilized in the BId Subgroup
supporting a HST. Parasequences thicken progressively,
with overlying lithostratigraphic units starting with the BIE
subgroup pointing to a TST in an outer ramp setting. Com-
paring the S70 Tshinyama (northern basin) and Kafuku 15
drillcores (central basin) separated by a distance of 190 km,
it appears that Fischer curves are different with a positive
slope (Kafuku) and a negative one (Tshinyama). In this case,
mixed clastic/carbonate sedimentation is evidently dia-
chronic and related to a slow transgression across the craton.
In addition, the BIE Subgroup is missing in the S70
Tshinyama drillcore, suggesting that differential subsidence
characterized different parts of the basin.
4.1 m-thick
excluding
four
>
within the shelf (or
ramp with MF6-MF10/11 succession) records relative sea
level fluctuations of a maximum of 4 m, and that most
fluctuations probably ranged between 1 to 4 m. Precise
relative sea level fluctuations are more difficult to determine
in the
peritidal
cyclicity
'
'
'
subtidal cycles, because facies could have
been deposited in a wider range of water depths (example of
shales and stromatolites), rarely reaching shallow waters
(see MF1-MF5 sequence). It is probable, therefore, that
fluctuations with a magnitude of 10-20 m occurred on the
ramp, but whether the depositional environments were able
to record these fluctuations is not easy to determine. Both
cycle-types (thin and thick) are well expressed on the
'
deeper
'
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