Geology Reference
In-Depth Information
Reinterpreting a Proterozoic enigma:
Conophyton-Jacutophyton
stromatolites of the Mesoproterozoic Atar Group, Mauritania
LINDA C. KAH
*
,
JULIE K. BARTLEY
†
and ALICE F. STAGNER
‡
*
Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA
(E-mail: lckah@utk.edu)
†
Department of Geosciences, University of West Georgia, Carrollton, GA 30118, USA
‡
School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, USA
ABSTRACT
The Mesoproterozoic Atar Group, Taoudeni Basin, Mauritania, preserves a spectacular
diversity of stromatolite morphologies, including stromatolitic biostromes comprised
of the conical form
Conophyton
, the enigmatic branching conical form
Jacutophyton
,
and a variety of irregularly branching forms, including
Tilemsina
and
Baicalia.
Until
now, the peculiar juxtaposition of high- and low-relief stromatolite morphologies has
posed a conundrum for environmental interpretation of stromatolite forms, and has
led to interpretations of strict biological control over stromatolite morphology. Careful
assessment of superpositional relationships among stromatolite elements, however,
suggests that the diversity of stromatolite morphologies in the Atar Group can be
readily explained via parasequence-scale sea-level changes and the incomplete and
laterally discontinuous fi lling of accommodation space. In the Atar Group, biostrome
growth initiates during relative rises in sea level with the widespread, subtidal
nucleation of
Conophyton.
Exposure of
Conophyton
to wave energy during falls in
relative sea level result in disruption of stromatolitic laminae, generation of inter-
stromatolitic debris, and development of both superimposed and laterally adjacent
branching stromatolite forms. In this scenario, the enigmatic stromatolite form
Jacutophyton
represents stromatolite growth through a complete depositional parase-
quence, and the unusual juxtaposition of stromatolite forms refl ects growth of different
forms that is separated by time and depositional environment. With subsequent rises
in sea level, nucleation of new
Conophyton
in subtidal regions, and continued growth
of branching forms and aggradation of the depositional substrate in intertidal regions,
results in continued modifi cation of sea-fl oor topography. In the model presented
here, stratigraphic time is partitioned both vertically and laterally during biostrome
growth, resulting in a complex internal architecture that is not readily discerned
in outcrop. In connecting hydrodynamic variables to stromatolite morphology, this
model provides a more comprehensive understanding of Atar Group stromatolites in
terms of basin geometry, relative sea level and carbonate production.
Keywords
Proterozoic, stromatolites,
Jacutophyton
,
Conophyton
, Mauritania, Atar
Group, biostrome architecture, parasequence development.
INTRODUCTION
and sediment infl ux (Cloud & Semikhatov, 1969;
Semikhatov
et al.
, 1979; Semikhatov & Raaben,
1994, 1996; Andres & Reid, 2006). As a result,
stromatolitic laminae, which record both
microscale and macroscale growth processes, are
arguably the most fundamental aspect of stroma-
tolite morphology (Walter, 1992). Each lamina
represents the active growth surface of the mat at
the time of deposition and therefore records the
topography of the depositional surface (i.e. syn-
optic relief, Hofmann, 1969; Donaldson, 1976),
the relative rates of stromatolite growth and
Whereas stromatolite microfabrics are generally
believed to refl ect a combination of microbial
community growth, decomposition and lithifi ca-
tion (Golubic, 1976; Bertrand-Sarfati, 1983; Turner
et al.
, 1993; Knoll & Sergeev, 1995; Kah & Knoll,
1996; Knoll & Semikhatov, 1998; Grotzinger &
Knoll, 1999; Lee & Golubic, 2000; Reid
et al.
,
2000), stromatolite morphology appears to be
affected more strongly by physical depositional
factors, particularly water depth, wave energy
