Geology Reference
In-Depth Information
(a)
(b)
20 cm
10 cm
(d)
(c)
10 cm
10 cm
Fig. 5.
Conophyton
-
Jacutphyton
association of the Atar Formation. (a) High synoptic relief and nearly vertical,
parallel laminae characterize
Conophyton
within the Atar Formation reefs.
Conophyton ressoti
(shown here) bears a single
central axial zone and occurs in R1 and R2 reef intervals;
Conophyton jacqueti
has an expanded axial zone, an elongate
cross-section and occurs primarily in the R3 reef interval. (b) The enigmatic stromatolite
Jacutophyton sahariensis
is
characterized by low synoptic relief branches that surround a conical central core. (c) Detailed cross-sectional view of
Jacutophyton
showing oblique growth of branching elements. (d) Oblique bedding plane view of
Jacutophyton
highlighting
the petaloid nature of branching elements that encompass the central cone.
the continuous walled nature of stromatolite
margins and general absence of erosion resulting
from sediment entrained within this water column
supports the paucity of detrital sediment at the
time of cone growth. Rarely, successive laminae in
the basal 3-5 cm of conical stromatolites record an
abrupt onlapping of fi ne-grained interstromatolitic
sediment, suggesting that at least small amounts
of detrital sediment may have been deposited
during
Conophyton
growth. Combined, these
observations suggest that interstromatolitic debris
represents sediment deposition that occurred pre-
dominantly after stromatolite growth.
Jacutophyton
consists of a central conical region,
indistinguishable from
Conophyton
, that is
surrounded by low synoptic relief, unwalled to
moderately walled, columnar branches (Figs 5b, c
and 7a). Branching elements are moderately to
highly divergent from the central cone, extend up
to 50 cm from the cone margin (Fig. 5b), and are
subcylindrical to petaloid in form, forming ellipti-
cal to elongate cross-sections that encircle the cen-
tral cone (Fig. 5d). With rare exception, branches
initiate along a single laminae of the central cone
(Fig. 7a). In
Jacutophyton
, branching occurs along
the entire height of the central cone, although
in the Atar Formation biostromes, branching
elements variously attributable to
Baicalia
or
Tilemsina
(Fig. 7b) are occasionally concentrated
in the uppermost portion of the central cone or
on pavements of broken or toppled
Conophyton
(Fig
.
7c).
Branching conical stromatolites
Along with
Conophyton,
branching-conical stro-
matolites (
Jacutophyton sahariensis
) form conspicu-
ous elements of the Atar Formation biostromes.
