Reinterpreting a Proterozoic Enigma:Conophyton‐JacutophytonStromatolites of the Mesoproterozoic Atar Group, Mauritania

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 filling 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 interstromatolitic 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 parasequence, and the unusual juxtaposition of stromatolite forms reflects 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 modification of sea floor 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 3 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.

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India

“…While the influence of slight metamorphism affected the overall directionality of many columnar stromatolites of the Jhamarkotra area, resulting in tilting of the columns in the direction of shear (Chauhan, ), the type of microfabrics found in smaller columns appears in many cases to have retained primary textural features (e.g., Figure a,b). The internal lamination and structure of Jhamarkotra stromatolite columns are testimonies of conical growth: Firstly, the Aravalli laminae are steeply convex, although slightly less so than some ancient Conophyton stromatolites (e.g., Kah, Bartley, & Stagner, ). The filamentous mat fabrics have orientations parallel to lamination, a feature that is also described from modern conical microbialites (Walter, ; Walter et al., ).…”

“…Firstly, the Aravalli laminae are steeply convex, although slightly less so than some ancient Conophyton stromatolites (e.g., Kah, Bartley, & Stagner, 2009). The filamentous mat fabrics have orientations parallel to lamination, a feature that is also described from modern conical microbialites (Walter, 1972;Walter et al, 1976).…”

Growth of modern branched columnar stromatolites in Lake Joyce, Antarctica

“…Microbial sediments can also accumulate to form macroscopic deposits like stromatolites, which are laminated structures that protrude away from a limited surface . The emergent morphology of stromatolites is intimately tied to microbial and environmental processes (Kah et al, 2009;Gomez et al, 2014), and these relationships have varied through time (Grotzinger & Knoll, 1999;Bosak et al, 2013). One temporal trend in stromatolite morphology is stromatolite branching patterns through the Proterozoic, which was used in stromatolite classifications from the 1960's into the 1980's (e.g., Cloud & Semikhatov, 1969;Hofmann, 1969;Raaben, 1969;Horodyski, 1977;Bertrand-Sarfati & Moussine-Pouchkine, 1985).…”

“…In some cases, sedi-mentary relationships indicate that branching correlated to a change in depositional conditions. For example, branched columnar stromatolites of the Mesoproterozoic Atar Formation show local erosional disruption of growth surfaces consistent with changing energy levels in the depositional environment; this erosion produced nucleation points for branches (Kah et al, 2009). Similarly, other microbialites contain sediment that buried part of the growth surface and either locally reduced growth or produced local relief that became a high-growth zone (Horodyski, 1977;Dupraz et al, 2006;Planavsky & Grey, 2008).…”

Thrombolite fabrics and origins: Influences of diverse microbial and metazoan processes on Cambrian thrombolite variability in the Great Basin, California and Nevada

“…It is widely accepted that while external microbialite morphology is shaped primarily by depositional conditions (e.g. Semikhatov et al, 1979;Andres & Reid, 2006;Planavsky & Grey, 2008;Kah et al, 2009), internal microbialite textures reflect interactions between depositional conditions and microbial communities (e.g. Kennard, 1994;Grotzinger & Knoll, 1999).…”