| Popis: |
The Anti-Atlas belt of Morocco preserves exceptional record of an Ediacaran microbial biosphere. The Amane Tazgart Formation of the Ouarzazate Group consist of an Ediacaran volcanic alkaline lake depositional system (ca. 571 Ma) were microbial buildups accreted in an extreme environment. These microbial accumulations are exceptional not only for their wide scope of extreme setting but also for their significance for understanding the early biosphere and earth habitability. A description of these buildups provides insights into their spatio-temporal distribution, in a 11 m-thick section. Specifically, the lower part consists mainly of thrombolitic limestone, usually displaying irregular to patchy mesoclots and occasionally arranged in dendritic pattern. The upper part dominated by clastic stromatolites, exhibit a variety of morphotypes ranging vertically from planar wrinkly laminated to large domes. The transitional morphotypes are made of linked and vertically oriented or inclined columns, grading upward to cone-shaped domes. The change from planar to columnar forms has been considered to indicate a shallowing trend, whereas the transition from columnar to domal morphotypes indicate a deepening trend. Spherulitic carbonate particles usually found within thrombolites, comprise radiating, wedge-shaped crystals. The analyses of spherulites-bearing samples using diluted acetic acids reveal the presence of microbial aggregates. They preserve spherical or globular shape and often irregular morphologies showing alignment along specific direction. Microfabric typical of Extra-polymeric substances (EPS) is preserved within these carbonate aggregates, suggesting their biological origin. The mineralogy of Amane Tazgart microbialites was studied using microscopical observation and XRD analyses. XRD show significant change in fabric composition from carbonate-dominated to clastic- and epicalstic-dominated microbialites, revealing the role of calcium carbonate saturation on microbialites genesis. Several features are preserved in the microbialites fabrics including micro-tufts and gas-bubbles and gas escape structure, forming evidence for mat growth and metabolic processes related to oxygenic photosynthesis and oxygen production. |
