Geology Reference
In-Depth Information
reef consists primarily of
Conophyton
, whose
steep-sided walls, synoptic relief and large height-
to-width ratio indicate deposition in a low-
energy environment with little or no sedimentary
infl ux. Such environments are typical of marine
highstands, when accommodation space is at its
greatest and backstepping of the shoreline inhibits
sediment infl ux into the basin. Finally, each reef
interval concludes with either a regional pave-
ment of broken
Conophyton
(R1, R2) or the appear-
ance of irregularly branching elements, each of
which can be interpreted as refl ecting a loss in
accommodation space associated with either sea-
level fall or aggradational stromatolitic growth
(Bertrand-Sarfati & Moussine-Pouchkine, 1988).
Within each third-order package, however,
unusual vertical and lateral juxtapositions of
stromatolite forms suggest a much more complex
developmental history. Detailed superpositional
relationships between and among reef elements,
however, suggest that the complex juxtaposition
of stromatolite forms can be interpreted in terms
of parasequence-scale (fourth to fi fth order)
changes in relative sea level. The most common
and conspicuous elements of parasequences in the
Atar Formation reef are
Conophyton
. These stro-
matolites represent dominantly subtidal (beneath
storm wave-base; Donaldson, 1976; Bertrand-
Sarfati & Moussine-Pouchkine, 1985; Kah
et al.,
2006) stromatolite nucleation during marine trans-
gression and upward growth throughout trans-
gression and early highstand (Fig. 10a, V1-V6).
Rare variation in
Conophyton
morphology, such
as the presence of
Conophyton
with elliptical
cross sections, irregular margins and occasional
branch development (Fig. 6b;
Conophyton jac-
queti
; Bertrand-Sarfati & Moussine-Pouchkine,
1999) may represent the presence of low-energy
currents during
Conophyton
growth. The trajec-
tory of parasequence formation, however, and
the resultant complexity of individual parase-
quence (Fig. 10; see below) depends critically
upon the position of
Conophyton
with respect to
wave base, the magnitude of parasequence-scale
sea-level changes, and the lithifi cation history of
the
Conophyton
.
If
Conophyton
tops are suffi ciently beneath
wave base, aggradational growth may continue
even during falls in relative sea level, resulting
in
Conophyton
with the greatest synoptic relief.
Alternatively, upward growth may simply termin-
ate and become a nucleus for later Conophyton
development (Figs 10a-c, V1 and 6a). Parasequence
complexity increases dramatically if, during
falls in relative sea level,
Conophyton
tops are
subjected to subaerial exposure or increased wave
energy. High-energy storm events or prolonged
subaerial exposure may topple cones, forming a
substrate for subsequent microbial growth or ter-
minating parasequence development (Figs 10b,
V2 and 4a and b). Wave energy may also deeply
erode fully lithifi ed cones (Fig. 6d) or delaminate
poorly lithifi ed outer margins of the cones
(Fig. 6c). In the former case, upon subsequent rise
in sea level, stromatolite growth may resume, with
stromatolite morphologies (conical vs. branching)
refl ecting the hydrodynamic conditions within
the environment (Fig. 10b, V2). In the latter case,
stromatolitic debris may be transported, hydro-
dynamically sorted and deposited in interstro-
matolitic regions (Fig. 10b, V3), with lowstand
nucleation and growth of stromatolites such as
Tilemsina
in high-energy environments between
adjacent
Conophyton
(Fig. 10b, V4). During sub-
sequent rises in sea level, nucleation of new
Conophyton
may occur on either exposed cone
tops or on interstromatolitic detritus.
Additional complexity of parasequences
occurs when falls in relative sea level expose
Conophyton
to wave energy, but interstroma-
tolitic detritus is generally absent. In this case,
disruption of the outermost laminae of living
Conophyton
(i.e. those with unlithifi ed outer
margins) by wave energy would result in
Conophyton
encircled by partially delaminated
microbial elements that, in turn, would form ini-
tial substrate for the petaloid branching elements
characteristic of
Jacutophyton
(Fig. 10b, V5).
Laterally adjacent non-living or more heavily
lithifi ed
Conophyton
may experience either no
delamination, or more extreme erosion (Fig. 6d)
during lowstand. The absence of platy breccia
between petaloid branches suggests that wave
energy may not be suffi cient to create or trans-
port signifi cant breccia, although some brec-
cia may be deposited during latest lowstand. In
the Atar Formation biostromes,
Jacutophyton
parasequences show the most variability upon
subsequent sea-level rise, with new
Conophyton
nucleating adjacent to
Jacutophyton
, initiating
atop the central cone of
Jacutophyton
(Figs 10c, V5a
and 11a), or developing directly on
Jacutophyton
branches (Figs 10c, V5b and 11b). In rare cases,
subsequent growth of branching stromatolites
(Fig. 10c, V5c) suggests sea-level rise insuffi cient
to place microbial growth beneath wave-base.
Finally, if falls in relative sea level expose only
the tip of
Conophyton
to wave energy, either
because of the position of wave-base relative to
stromatolites or the presence of abundant detrital
