Geology Reference
In-Depth Information
the shelf during marine transgression (Kah
et al.,
2006), thick encrustations of herringbone carbon-
ate within coarse-grained platy breccias (Fig. 9d)
suggest that stabilization of the sedimentary sub-
strate occurred over multiple parasequences.
This scenario is supported by the presence of
microsparitic detrital carbonate within herring-
bone-lined voids, which probably represents the
winnowing of fi ne-grained detritus through a
highly permeable substrate. If substrate stabiliza-
tion is protracted through several sea-level cycles,
it is possible that some parasequences within the
Atar Formation reefs may have experienced sig-
nifi cant post-depositional modifi cation.
stromatolite growth, which could potentially bring
the biostrome top above wave base.
Jacutophyton
,
which requires a signifi cant fall in eustatic sea
level and exposure of entire cones to wave energy,
would not be expected to form.
By contrast, over much of the cratonal platform,
intermediate water depths and low depositional
relief should result in environments in which
parasequence-scale changes in sea level would
exert a greater infl uence over stromatolite form,
resulting in greater variability of stromatolite
morphology within biostromes. In these regions,
total accommodation space available for stroma-
tolite growth relative to the magnitude of parase-
quence-scale sea-level changes is critical in the
distribution of different stromatolite morpho-
logies. First, larger magnitude sea-level rises
would bring more of the cratonal platform beneath
wave base, resulting in widespread development
of conical stromatolites. Similarly, smaller magni-
tude sea-level changes would result in restricted
spatial development of conical stromatolites.
Second, during eustatic falls in sea level, deeper-
water portions of the platform would experience
less wave energy (perhaps episodic wave energy
if between fair weather and storm wave-base),
resulting in biostromes dominated by
Conophyton
and
Jacutophyton
. Similarly, shallower-water
portions of the shelf would experience wave
energy suffi cient to produce quantities of detrital
material. In these regions, the biostrome would
be dominated by
Jacutophyton
and the branching
form
Tilemsina
, with sparse
Conophyton
refl ect-
ing conical forms that were well-lithifi ed at the
time of sea-level fall. Throughout these platfor-
mal regions, unusual juxtaposition of stromatolite
forms results not from penecontemporaneous
growth, as previously inferred (Bertrand-Sarfati &
Moussine-Pouchkine 1985, 1999), but from lat-
erally adjacent growth during distinct phases of
sea-level cycles.
Shallowest regions of the craton should also
contain a third, distinct stromatolite tract. In
these regions, low overall accommodation space
will result in deposition controlled mainly by
parasequence-scale eustatic sea-level changes.
Biostrome development should be absent except
during brief periods of cratonal inundation and
should be dominated by low-relief, irregularly
branching stromatolite forms. In fact, high water
energy might fully preclude development of
Conophyton
, with coniform growth represented
only by the low-relief conical laminae of
Baicalia
safi a
. Low synoptic relief of stromatolites and
DISCUSSION
Idealized model for reef growth
Taken together, the development of Atar
Formation biostromes on a low-relief, cratonal
platform (Bertrand-Sarfati & Moussine-Pouchkine,
1988), the laterally variable thicknesses of parase-
quences and the possible post-depositional
modifi cation of parasequences make deciphering
the precise architecture of the Atar reefs diffi cult.
Our understanding of parasequence development,
however, allows construction of an idealized
model for biostrome development that is a func-
tion of (1) regional platform geometry, which con-
trols the position of the substrate relative to wave
base and the total accommodation space available
for stromatolite growth, and (2) the magnitude of
parasequence scale (fourth or fi fth order) sea-level
changes, which act to modify total accommoda-
tion space and the position of wave base.
At distal craton margins, the steeper gradient of
the substrate and potentially greater rates of sub-
sidence should result in available accommodation
space that is large relative to parasequence-scale
sea-level changes. In this scenario, upward growth
of conical stromatolites would remain uninter-
rupted by small-scale changes in sea level and low
water energy would prohibit growth of branching
stromatolites and the production and deposition
of interstromatolitic detritus, resulting in a stro-
matolite tract that is dominated by high relief
Conophyton
(cf. Dismal Lakes reef, Donaldson,
1976; Kah
et al.,
2006). Branching stromatolites
such as
Baicalia safi a
and
Baicalia mauritanica
or the incipient branching of
Conophyton jac-
queti
would form only with large magnitude falls
in sea level or with the continued aggradation of
