Geology Reference
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shoals (MF9) under restricted conditions. Association of
microbially laminated or stratiform carbonates, desiccation
cracks and muddy intraclasts are frequently attributed to
peritidal origins in ancient successions (Grotzinger
1986
;
Pratt et al.
1992
). Along the inner-ramp, ephemeral
supratidal lacustrine ponds, seasonally flooded by marine
water and followed by precipitation of evaporitic minerals,
characterize organic-rich carbonate mud (MF10a) during
dry season. Buildups of cyanobacterial mats and stratiform
stromatolites (MF10b) occurred preferentially in the
shallow-margin of the lacustrine ponds (Freytet and Plaziat
1972
), and finally during wet season coated pisoids along a
lacustrine shore resulted from aggregation of stromatoclasts
and reworked ooids (MF10c). Vadose or meteoric cements,
suggesting subaerial exposures and temporarily humid
periods affected these microfacies (Delpomdor and Pr´at
2012
). Increasing concentration of evaporation may result
in deposition of subtidal aragonitic muds, intertidal micro-
bial mats and precipitation of gypsum, halite in the sabkha
domains (MF11).
replaced clastic and mixed clastic/carbonate facies after
depth shallowed. This evolution could record the change
from an early highstand system succeeding one (HST). The
available accommodation space generated in the upper part
of the BIe subgroup declined, producing a slight
long term
'
'
negative slope in the Fischer curve.
The two thicker parasequences (48 and 49) create to a
short positive slope in the curve that could record an aggra-
dational step within the general progradational trend,
highlighted by shallowing parasequences with intertidal
and supratidal caps and thin calcrete levels. Oolitic shoals
disappeared during this unstable phase and are only
associated with stabilized portion of the Fischer curve. The
two thicker parasequences at the beginning of HST record
partly underfilled accomodation spaces due to a short deep-
ening event related to sea-level fluctuations or temporary
tectonic activation. This HST probably overlies a transgres-
sive systems tract, consistent with stromatolitic buildups in
the non-studied BIIa subgroup. Stromatolites are well devel-
oped in the lower part of next subgroup (BIIc).
A major process or facies and parasequence thickness
change occurs in parasequences 68-72. Here, stromatolite
buildups and shaly dolomudstones record a major shift from
previous inner ramp setting to an outer/middle ramp system.
The change is recorded in very thick parasequences (22.4 m-
thick on average) typical of deepening-upward sediments
(shales) overlain by slight shallowing-upward facies of stro-
matolitic boundstones. This suggests a transgressive systems
tract (TST) with deposition in deeper water at a depth in
which the former carbonate
4.6.2 Sea Level Fluctuations in the SMLL Basin
The
general
thickening-upward
trends
of
both
parasequences and parasequences sets (from
20 m to
40 m) in the BIb and BIc-d subgroups point to a retrogra-
dational stacking pattern, progressively dominated by
subtidal deposition with interbedded shales. Parasequence
sets I, II and III (including elementary parasequences 1-9,
Fig.
4.6
and Delpomdor and Pr
´
at
2012
), containing shaly
levels, are characterized by a deepening-upwards trend
highlighting a transgressive systems tract (TST). However,
deposition of the BIc-d Subgroup is characterized by a
relatively thick, single shallowing-upward parasequence as
resurgent carbonate production was able to outpace relative
sea level change succeeding the previous deepening phase.
The long-term sea level is relative constant and
parasequences are thinner in the upper part of the BIc-d
subgroup than in its lower part. Facies are similar along the
unit and point to a constant water depth during deposition.
Carbonate supratidal facies (BIIb subgroup) progressively
was no longer able to
return to state of optimum carbonate production. The deep-
ening, or
factory
'
'
, was probably not very significant as
carbonate deposition never ceased. Overlying parasequences
have thicknesses more-or-less near the average BIIc
parasequence thickness with an evolution toward thinner
thicknesses along this stratigraphic interval. The general
evolution then is the one of a highstand systems tract
(HST), locally interrupted by aggradational thicker
parasequences (95-97), as previously recorded in the BIIc
Subgroup. Distribution of different parasequences or cycle-
types (aggradational vs. progradational) along the BIIc Sub-
group therefore provides evidence of the variation of relative
drowning
'
'
Fig. 4.5
(continued) replaced gypsum by silica in a medium-grained
dolomicritic matrix. MF11a, BIId Subgroup, sample ULB549, depth:
55.15 m, core 41, Bena Tshovu drillcore; (
c
) bedded and laminated
replaced pale anhydrite by dolomitic crystals in brown/grey and pale
laminated dolomite matrix. MF11b, BIIb Subgroup, sample ULB41,
depth:
435.80 m, B13 Kanshi drillcore; (
d
) bedded light nodules of
replaced anhydrite by dolomitic cements in a dolomitic matrix. MF11c,
BIIb subgroup, sample ULB434, depth:
417.40 m, B13 Kanshi
drillcore; (
e
) packed aggregates of tiny lath-shaped crystals
(20-50
m
m long and 5-10
m
m wide) of replaced anhydrite forming a
massive structure. MF11d, BIId Subgroup, sample ULB577, depth:
43.15 m, Bena Tshovu drillcore; (
f
) grain-supported breccia with
angular dolomitic clasts and replaced anhydrite (bladed crystals).
MF11e, BIId Subgroup, sample ULB576, depth:
43.00 m, Bena
Tshovu drillcore; (
g
) silicified pyramidal hopper structures in a
silicified massive evaporite. MF11f, BIIc subgroup, sample ULB561,
depth:
47.20 m, Bena Tshovu drillcore; (
h
) filled mimetic molds of
halite crystals in loosely packed aggregates of tiny lath-shaped
silicified crystals of anhydrite. MF11f, BIId subgroup,
sample
ULB561, depth:
47.20 m, Bena Tshovu drillcore
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