This is a very nice cut of sweet water stromatolite of the species Conophyton amantourartensis from the Ediacarian of Morocco.
The stromatolite-bearing carbonate layers of the Ouarzazate Supergroup in the Anti-Atlas (Morocco) represent an exceptional example of continental stromatolite formation during the late Ediacaran. The basins in which they developed were tectonically controlled depressions, fed mainly by groundwater. In addition, there were episodic runoff events, triggered by rainfall or by the erosion of fresh volcanic ash layers, which transported fine sediments into the basins without forming large, permanent river systems. The influx of volcanic ash made the lakes alkaline and rich in silica, conditions that strongly favored microbial carbonate precipitation (Thomas et al. 2002).
The stromatolites themselves display a remarkable morphological diversity: from domal to columnar structures, and even oncoids. These forms indicate shallow, calm, but episodically agitated waters. Reworked stromatolite fragments are preserved in so-called dolorudites. These are coarse-grained dolomite breccias, which formed when waves or storm events destroyed microbial mats and redeposited the material (Thomas et al. 2002).
Another characteristic feature is the alternation of growth and desiccation phases. Stromatolite layers are interbedded with dolostones showing desiccation cracks, evidence of repeated drying of the lakes. This reflects a semi-arid climate with episodic wet phases (Ennih & Liégeois 2008).
A particular strength of this site is that these stromatolites are among the best-dated continental stromatolites of the Ediacaran. Their age is constrained in two ways:
First by chemostratigraphic markers – characteristic d¹³C anomalies that serve as global correlation tools and mark the Neoproterozoic–Cambrian boundary in the Adoudou Formation (Maloof et al. 2010),
Secondly by radiometric U–Pb dating of zircons from volcanic tuffs and bentonites, which yield ages of ca. 580–560 million years (Thomas et al. 2002).
Palaeogeographically, the Ouarzazate Supergroup was deposited at the northern margin of the West African Craton, situated in the tropical to subtropical belt of West Gondwana. The combination of semi-arid climate, volcanically induced alkalinity, and ephemeral inland lakes makes these stromatolites a unique record of environmental conditions immediately before the onset of the Cambrian.
Álvaro, J.J., et al. (2010): The Ediacaran–Cambrian transition in Morocco. Precambrian Research 179, 22–36.
Ennih, N. & Liégeois, J.P. (2008): The Anti Atlas of Morocco: its Proterozoic and Pan African evolution. Geological Society, London, Special Publications 297, 1–17.
Maloof, A.C., et al. (2010): The earliest Cambrian record of animals and ocean geochemical change. Geological Society of America Bulletin 122, 1731–1774.
Raaben, M. E. (1980). Some stromatolites of the Precambrian of Morocco. Earth-Science Reviews, 16(1): 221–234, 5 figures, 3 plates.
Thomas, R.J., et al. (2002): Precambrian evolution of the Anti Atlas belt, Morocco. Precambrian Research 118, 1–57.
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