Geology Reference
In-Depth Information
high rates of deposition of interstromatolitic
debris will also result in parasequences that pre-
serve much less lateral variability in thickness.
When cratonal inundation is insuffi cient for
stromatolite development, biostromes should be
replaced by thin parasequences characterized
by fi ne-grained detrital carbonate and/or fl at-
laminated microbialites, with or without evaporite
lithologies.
of central and eastern Mauritania strongly suggest
a proximal cratonal setting, where development
of stromatolitic build-ups is restricted to brief
periods of cratonal inundation.
The parasequence structure throughout the Atar
Formation stromatolite buildups suggests that it
is unlikely that these buildups formed a regional
barrier to wave or current energy, although stro-
matolites may have served as local baffl es to water
energy and sediment transport. The interpreta-
tion that Atar Formation biostromes did not form
regional hydrodynamic barriers differs markedly
from previous interpretation of these biostromes
as basin-forming, constructional high-relief reefs
(Bertrand-Sarfati & Moussine-Pouchkine, 1992,
1999; Moussine-Pouchkine & Bertrand-Sarfati,
1997). In the constructional reef model, based in
large part on data collected in easternmost expo-
sures of the Atar Group (Hank Group in Algeria),
near the tectonic edge of the West African cra-
ton, stromatolite biostromes are interpreted to
have stood 20-25 m above the seafl oor, in water
depths >50 m (Bertrand-Sarfati & Moussine-
Pouchkine, 1988). Reef margins dropped steeply
at their edges, producing protected depressions
devoid of sedimentary input during reef growth.
The resultant platform geometry was one of
stromatolitic platforms separated by rimmed
depressions 15-20 m deeper than the platforms
(Bertrand-Sarfati & Moussine-Pouchkine, 1992).
During sea-level fall, reef tops became exposed
subaerially, shedding debris into intrashelf
basins. Over the course of three sea-level cycles,
intrashelf basins were fi lled with shale, marls
and reef debris (Bertrand-Sarfati & Moussine-
Pouchkine, 1992).
Implications for basin geometry and
development of Atar Formation reefs
In light of the idealized model described above,
several generalizations can be made regarding
environments of deposition of the Atar Formation
biostromes. The stratigraphically lower R1 and
R2 biostromes record a complex juxtaposition of
Conophyton
,
Jacutophyton
and
Tilemsina
, and
parasequences show lateral variability in thick-
ness from a few centimetres to <2 m (i.e. the total
synoptic relief of
Conophyton
present in the
interval), with an average parasequence thickness
of 60-70 cm. These reef intervals are interpreted
as having formed in intermediate water depths
along a broad expanse of the stable, West African
craton. By contrast, the stratigraphically higher
R3 interval preserves a very different stromatolite
structure, which is dominated by large, high-relief
Conophyton
and typically lacks
Jacutophyton
and
Tilemsina
. Branching forms are rare except for
several horizons containing the incipient branch-
ing
Conophyton jacqueti
and an upper interval
dominated by
Baicalia safi a
and
Baicalia mau-
ritanica
. The abundance of large
Conophyton
and relative lack of
Jacutophyton
suggests that
accommodation space during deposition of the
R3 interval was suffi ciently great that parase-
quence-scale changes in sea level were unable to
expose stromatolite bases to wave energy. Gradual,
rather than abrupt transitions to branching forms
suggest, instead, that stromatolite aggradation
was the main factor involved in decreasing
accommodation space. Finally, recent fi eld
exploration in central and eastern Mauritania has
demonstrated that Atar Formation bioherms are
laterally replaced by discontinuous thin (typically
<5 m thick) stromatolitic intervals of low-relief,
irregularly coniform to columnar stromatolites
with possible affi nities to
Baicalia safi a
and
Baicalia mauritanica
and by thin (15-35 cm thick)
laterally continuous parasequences of fi ne-grained
detrital carbonate (Tourist Formation). Lithologies
SUMMARY AND CONCLUSIONS
1
The Atar Formation (Mesoproterozoic Atar
Group, Taoudeni Basin, Mauritania) preserves
a series of three stromatolite biostromal com-
plexes. Biostromes initiated during successive
third-order rises in sea level and terminated
with the fi lling of available accommodation
space, either via an abrupt fall in eustatic sea
level (R1, R2) or the continued aggradational
growth of stromatolitic facies (R3). In contrast
to other stromatolitic complexes, the Atar
Formation biostromes lack unambiguous, large-
scale changes in geometry (i.e. vertical stacking
of expanding, contracting, or backstepping reef
