Geology Reference
In-Depth Information
packages) associated with changes in sea level
and accommodation space. They also preserve
a peculiar juxtaposition of conical and branch-
ing stromatolite forms, which typically occur in
distinct depositional environments.
Careful assessment of superpositional relation-
ACKNOWLEDGEMENTS
Field work was supported by grants from the
National Geographic Society and the National
Science Foundation (EAR-0106089). We thank
Janine Sarfati and Alexis Moussine-Pouchkine
for introducing us to the Atar Group during an
IGCP Project 380 fi eld trip in 1998 and gratefully
acknowledge them for their steadfast support and
aid in expediting our 2003 fi eld season. We also
acknowledge all participants of the 1998 IGCP
fi eld trip, and especially Bob Ginsburg, for enthu-
siastic discussion of stromatolite morphologies.
Finally, many thanks go to Mohammed Lemin,
Malik, Mohammed and Maloud for their assis-
tance in the fi eld. Thoughtful reviews by Hans
Hofmann and Miriam Andres helped to improve
the clarity of the manuscript.
2
ships among stromatolitic elements indicates
that the diversity of stromatolite morphologies
in the Atar Formation can readily be explained
via parasequence-scale sea-level changes and
the incomplete and laterally discontinuous
fi lling of accommodation space. Each parase-
quence is represented by subtidal nucleation
and growth of conical stromatolites during
initial transgression and sea-level highstands,
followed by a variety of complex interactions
between production and deposition of interstro-
matolitic debris and stromatolite growth during
sea-level lowstands. The character of lowstand
deposition is controlled by the amount of avail-
able accommodation space and wave energy,
and the unusual juxtaposition of stromatolite
forms refl ects lateral infi lling of accommoda-
tion space at different times under very
different environmental conditions. In this
scenario, the enigmatic stromatolite form
Jacutophyton
spp
.
represents a single depos-
itional parasequence, wherein transgression
and sea level highstand are marked by growth
of the central cone, and sea level lowstand is
marked by growth of superimposed branching
columns and deposition of interstromatolitic
debris.
Interpretation of the parasequential growth of
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3
Atar stromatolites allows construction of an ide-
alized model for biostrome development that is
a function of regional platform geometry, which
controls the position of the substrate relative to
wave base and the total accommodation space
available for reef growth, and the magnitude
of parasequence-scale (fourth or fi fth order)
sea-level changes, which act to modify total
accommodation space and the position of wave
base. In light of this model, the parasequence
structure of the Atar Formation suggests that it
is unlikely that stromatolite buildups formed a
regional barrier to wave or current energy. This
interpretation of Atar Formation biostromes as
stacked, low-relief depositional parasequences
differs markedly from previous interpretation
of these biostromes as constructional, high-re-
lief reefs that played an active role in develop-
ment of inter-reef basins.
