Geology Reference
In-Depth Information
(b)
(a)
10 cm
Fig. 4.
Termination of stromatolite reef deposition. (a) Termination of the R1 reef interval is marked by regional toppling
of
Conophyton
; oblique bedding plane view; Jacob staff marked in 10 cm intervals. (b) Termination of the R2 reef interval
is marked by a regional pavement of broken
Conophyton
and stromatolitic breccia; bedding plane view.
thin (30-50 m thick) third-order stratigraphic
sequences (Emery & Meyers, 1996; Miall, 1997).
The base of each biostromal complex is marked by
a transgressive surface and initiation of deep-water
stromatolite growth. Stromatolite growth contin-
ued during transgressive and highstand phases
and was terminated by a loss of accommodation
space, either by a fall in sea level or continued
aggradation of the biostrome complex. Finally,
biostromes are overlain by late highstand to
lowstand, shale-dominated deposits. Relatively
thin sequences and abrupt transitions between
siliciclastic and carbonate facies refl ect a cra-
tonal sedimentation regime (Bertrand-Sarfati &
Moussine-Pouchkine, 1988), wherein minimal
subsidence and the extremely low relief of the
sedimentary substrate resulted in dramatic changes
in depositional environment with even small
changes in accommodation space.
of the seafl oor at the time of lamina formation
(Hofmann, 1969). Inheritance of antecedent
topography in these stromatolites is high, and a
single columnar structure often represents several
distinct phases of stromatolite growth, with each
cone growing directly atop the previous structure
(Fig. 6a). Thus, a column may be up to 4 m tall;
however, no single lamina exceeds 2 m in height,
indicating that individual stromatolites stood no
more than 2 m above the seafl oor at any one time.
Despite a generally uniform shape, differences
in morphology are apparent. Cross-sectional
shapes range from circular (
C. ressoti
) to elliptical
(
C. jacqueti
; Fig. 6b; see discussion in Bertrand-
Sarfati & Moussine-Pouchkine, 1999), and indi-
vidual cones may display thickening of outer
laminae, forming superfi cial protrusions along
the outer surface of the cone (Fig. 6b). Cones also
show variable modifi cation after growth, includ-
ing delamination (Fig. 6c) and erosional incision
(Fig. 6d) of cone margins.
Individual conical stromatolites range in diam-
eter from 10 to 50 cm and show interstromatolitic
spacing of 5-70 cm. Interstromatolitic regions
contain both platy breccia and fi ne-grained detri-
tal carbonate, as well as several generations of
precipitated carbonate cement (cf. Fig. 9; see
descriptions below). The high synoptic relief of
stromatolitic laminae suggests that conical stro-
matolites formed largely below wave base, in
quiet-water environments, and that lithifi cation
occurred largely through
in situ
carbonate precipi-
tation (cf. Bertrand-Sarfati & Moussine-Pouchkine,
1985; Kah
et al.
, 2006). Although high synoptic
relief may have inhibited incorporation of sedi-
ment from the water column into the
Conophyton
,
Reef elements
Conical stromatolites
Conical stromatolites (
Conophyton
spp.) are the
most common element of the Atar Formation
biostromes (Fig. 5a), and a number of form-taxa
have been recognized, including
Conophyton
ressoti
and
Conophyton jacqueti
(Bertrand-
Sarfati & Moussine-Pouchkine, 1985, 1999).
Conical stromatolites have steeply dipping
(75°-90°) wall-parallel laminae, narrow and
nearly vertical axial zones and high synoptic
relief. Synoptic relief of a stromatolite above
the seafl oor is delineated by the height of a sin-
gle lamina, which represents the morphology
