Geology Reference
In-Depth Information
Group constrained by a single age (890
35 Ma,
1988). Disappearance of fl at-clast breccias and
fl at-laminated microbial carbonate interbeds in
stratigraphically higher siliciclastic intervals and
their replacement by small (<2 m thick) discontinu-
ous bioherms of irregularly branching columnar
stromatolites (
Tungussia confusa
; Bertrand-
Sarfati & Moussine-Pouchkine, 1999), as well as
rare intervals of 'molar-tooth' carbonate (Furniss
et al.
, 1998; James
et al.
, 1998; Pollock
et al.
,
2006) suggest shallow-marine deposition and
overall greater accommodation space across the
platform.
Carbonate-dominated intervals in the Atar
Formation form three distinct stromatolite bio-
stromes (R1-R3, 9-39 m thick; Fig. 3) that are the
focus of this study. Biostromal intervals typically
preserve a thin (<1 m thick) and discontinuous
transgressive horizon at their base, composed of
fl at-clast breccia and low relief, divergent branch-
ing stromatolites (
Tilemsina divergens
; Bertrand-
Sarfati & Moussine-Pouchkine, 1999). The main
biostromes are composed of conical to branch-
ing conical stromatolites (
Conophyton ressoti
,
Conophyton jacqueti
and
Jacutophyton sahari-
ensis
; Bertrand-Sarfati & Moussine-Pouchkine,
1999) that are interpreted to have developed
as deep-water reef tracts (80-100 m; Bertrand-
Sarfati & Moussine-Pouchkine, 1985) during high-
stand conditions. Termination of each biostrome
appears to have resulted from loss of accom-
modation space. Termination of the R1 biostrome
is marked by regional toppling of
Conophyton
(Fig. 4a), suggesting an abrupt decrease in accom-
modation space and impingement of storm wave
base on the biostrome. Similarly, termination of
the R2 biostrome is marked by a regional pave-
ment of broken
Conophyton
and stromatolitic
breccia (Fig. 4b), suggesting an abrupt decrease
in accommodation space and prolonged expo-
sure of the biostrome. By contrast, termination of
the R3 biostrome is marked by a transition from
dominantly conical stromatolites to a 10 m thick
interval of irregularly branching forms (
Baicalia
safi a
and
Baicalia mauritanica
; Bertrand-Sarfati &
Moussine-Pouchkine, 1999) that are overlain
by black shale and 'molar-tooth' carbonate of
the lower Oued Tarioufet Formation (unit I-6).
Gradational contacts between stromatolite mor-
phologies in the R3 biostrome suggest a gradual
decrease in accommodation space driven by
aggradational growth of the biostrome complex.
Combined, lithological and stratigraphic
features indicate that the Atar Formation bio-
strome complexes developed during a series of
Atar Formation, unit I-5; 874
22 Ma, Oued
Tarioufet Formation, unit I-6; 866
67 Ma, Oued
Terrarit Formation, unit I-8; 775
52 Ma,
Aouleigate Formation, unit I-10).
Although these Rb-Sr ages clearly represent
diagenetic mineralization, the consistent decrease
in ages through the stratigraphic column has been
used to argue for early diagenetic stabilization of
clay minerals and Neoproterozoic depositional
ages for these strata. By contrast, recent C-isotope
data from the Atar Group (Fairchild
et al.
, 1990;
Teal & Kah, 2005) reveal moderately positive
13
C
values near +2‰ with several distinct negative
excursions to nearly -2.5‰. The range of C-isotope
values preserved in the Atar Group is inconsistent
with the strongly positive values (
13
C >+5‰)
recorded in well-constrained isotopic compila-
tions for the post-850 Ma Neoproterozoic (Kaufman
& Knoll, 1995; Halverson
et al.
, 2005). Atar Group
chemostratigraphy is also distinctly different from
that of strata probably deposited in the earliest
Neoproterozoic (Knoll
et al.
, 1995; Bartley
et al.
,
2001) and more closely refl ects isotopic patterns
preserved globally in mid to late Mesoproterozoic
strata (Knoll
et al.
, 1995; Kah
et al.
, 1999a; Bartley
et al.,
2001, 2007; Frank
et al.,
2003; ), suggesting
that the Atar Group may be as old as ~1200 Ma.
STROMATOLITE BIOSTROMES OF THE
ATAR FORMATION
Lithology and sequence stratigraphic framework
Like the rest of the Atar Group, the ~125 m thick
Atar Formation (unit I-5) consists of alternating
fi ne-grained siliciclastic strata and stromatolite-
bearing carbonate (Fig. 3). Siliciclastic inter-
vals are composed primarily of dark coloured
shale containing laterally discontinuous inter-
beds of fi ne-grained sandstone and a variable
carbonate component. Basal siliciclastic strata
contain abundant thin (<1 m thick) interbeds of
fi ne-grained, fl at-laminated microbial carbonate
and fl at-clast breccia, and mark shallow-water
lowstand deposition following regional fl ood-
ing and deposition of shallow-marine carbonate
strata of the Ksar Torchane Formation (unit I-4).
Similarly, stratigraphically higher siliciclastic
intervals represent lowstand deposition follow-
ing craton-wide fl ooding and development of the
Atar Formation
Conophyton-Jacutophyton
bio-
stromes (Bertrand-Sarfati & Moussine-Pouchkine,
