Geology Reference
In-Depth Information
sediment infi lling (i.e. wall structure, Hofmann,
1969; Semikhatov
et al.
, 1979) and the infl uence
of wave and/or current action on mat growth
(i.e. stromatolite elongation, Hoffman, 1967;
Logan
et al.
, 1974). Not only do laminae closely
record the morphology of a microbial mat on the
seafl oor, but the geometry of successive laminae
also produces a record of microbial mat growth
over time that is refl ected in stromatolite mor-
phology. More recently, process-oriented models
(Grotzinger & Rothman, 1996; Grotzinger & Knoll,
1999; Dupraz
et al.
, 2006) have emphasized the
importance of laminae geometry by using variable
interface parameters including surface roughness
(mat growth and degradation), upward growth
(mineral precipitation) and surface dampen-
ing (draping sedimentation) to simulate lamina
accretion and stromatolite morphogenesis.
The relationship between stromatolite morpho-
logy and depositional setting has been examined in a
number of key studies (Hoffman, 1974; Donaldson,
1976; Grey & Thorne, 1985; Bertrand-Sarfati &
Moussine-Pouchkine, 1985). These studies, sum-
marized by Hoffman (1976) and Grotzinger (1989),
indicate that specifi c stromatolite morphologies
represent distinct depositional settings and are well
correlated with inferred palaeowater depth, water
energy and sediment supply. Upper intertidal to
supratidal environments are characterized by stro-
matolites with low synoptic relief, frequently asso-
ciated with intraclastic debris. Intertidal to shallow
subtidal (above fair weather wave-base) environ-
ments are characterized by relatively low-relief
domal and irregular columnar stromatolites asso-
ciated with intraclastic and/or fi ne-grained detrital
material. Deeper subtidal (below fair weather wave-
base) environments are characterized by high-relief
columnar to conical stromatolite forms that lack
evidence of interstromatolitic sediment deposition.
Empirically, relationships between stromatolite
morphology and inferred depositional parameters
appear relatively robust, yet similar relationships
observed in modern environments highlight poten-
tial variability resulting from local and regional
variation in accommodation space, hydrodynamic
regime and patterns of sediment deposition (Andres
& Reid, 2006).
The Proterozoic Atar Group, exposed along the
northern edge of the Taoudeni Basin in Mauritania,
Mali and Algeria, preserves a spectacular diversity
of stromatolite morphologies, including three dis-
tinct biostromes composed of the conical stroma-
tolite form
Conophyton,
the enigmatic branching
conical stromatolite
Jacutophyton
and a variety of
irregularly branching forms, including
Tilemsina
and
Baicalia
(Bertrand-Sarfati, 1972; Bertrand-
Sarfati & Moussine-Pouchkine, 1999). These
bioherms, herein referred to as Atar Formation
biostromal units R1-R3, show striking strati-
graphic similarity in both western (Mauritania) and
eastern (Mali-Algeria) sections of the Taoudeni
Basin, and have been interpreted as regional reef
tracts that played a critical role in basin develop-
ment (Bertrand-Sarfati & Moussine-Pouchkine,
1992, 1999; Moussine-Pouchkine & Bertrand-
Sarfati, 1997).
Two aspects of the Atar Formation biostromes,
however, pose a conundrum for an environmental
interpretation of stromatolite morphology. First,
biostromal intervals do not exhibit the large-scale
changes in geometry that are commonly observed
in stromatolite reefs and interpreted as result-
ing from changes in sea level or accommodation
space (Grotzinger, 1989; Narbonne & James, 1996;
Turner
et al.
, 1997; Lemon, 2000). Second, pecu-
liar juxtaposition of conical and branching stro-
matolite forms (Fig. 1), such as laterally adjacent
Conophyton
and
Jacutophyton
or
Conophyton
and
Tilemsina
, which typically occur in distinct
depositional environments, has been used to con-
clude that microbial community structure, rather
than depositional environment, may exert the
primary control over stromatolite morphology
(Bertrand-Sarfati & Moussine-Pouchkine, 1985).
Recent work in the Mesoproterozoic Dismal
Lakes Group provides a framework for interpreting
the enigmatic relationships observed in the Atar
Group biostromes (Kah
et al.
, 2006). The Dismal
Lakes reef consists of a thin (<40 m), laterally exten-
sive (>2400 km
2
) biostromal complex that contains
both exceptionally high-relief (up to 15 m) conical
stromatolites and lower relief domal and columnar
forms. In the Dismal Lakes Group, relationships
among stromatolites are best explained in terms
of the sea-level history of the platform, wherein
high-frequency (i.e. parasequence-scale) sea-level
changes produce a complex internal reef architec-
ture, which results from lateral juxtaposition of
facies across non-horizontal timelines (Kah
et al.,
2006). High-relief conical stromatolites initiated
during major transgression and refl ect transgressive
and highstand stromatolite growth. Subsequent
sea-level fall exposed the tops of these conical stro-
matolites, creating a sequence boundary that traced
the rugged seafl oor topography delineated by the
decametre-scale synoptic relief of the stroma-
tolites. During the sea-level cycles that followed,
lower-relief domal and columnar stromatolites
