Microbial Mounds from the Middle Jurassic of the High Atlas (Morocco)

Tomás, Sara ¹; Homann, Martin ¹; Mutti, Maria ¹; Amour, Frédéric ¹; Christ, Nicolas ²; Immenhauser, Adrian ²; Frijia, Gianluca ¹; Agar, Susan ³
(1) University of Potsdam, Potsdam, Germany. (2) Ruhr-University Bochum, Bochum, Germany. (3) Exxon Mobil Upstream Research Company, Houston, TX.

Microbial mounds are being studied in the last years as possible exploration targets due to their first and secondary porosity. Therefore, it is important to characterize their lateral extent, geometry and internal structure. In the southern margin of the High Atlas, Bajocian Assoul carbonate ramp, thrombolites, a specific type of microbialites, are well-exposed along the Amellago canyon, 70 km W of Errachidia. The mounds are embedded in a 4 m-thick level of ooidal grainstones and represent the base of the 100 m-thick shallowing-upward succession of the Assoul Formation. The mounds have dome morphologies attaining thicknesses of ~2 m and extending laterally along 80 m. The thrombolites show both, upward and sideward accretion likely indicating their growth under limited accommodation. The occurrence of isolated thrombolites (muddy lithologies) surrounded by grainy lithologies, wich should have different petrophysical characteristics, reology and diagenetic behaviour, makes crucial to study the stratigraphic and lateral relationships between both deposits.

The thrombolite fabric grades from homogeneous micritic to peloidal clotted with scarce incorporated allochthonous grains and large growth cavities. Calcite pseudomorphs after gypsum occur inside these cavities, suggesting periods of evaporation. The thrombolites grew upon planar cross-bedded ooidal grainstones, which are capped by a condensed surface. Above this surface a thin layer of echinoid floatstones occur that likely acted as hard substrate for the mounds. The mounds are surrounded and overlied by an upper shoal with through-cross bedding. The thrombolites and the shoals show complex geometrical relationships, making difficult to evaluate in the field if both deposits were or not synchronous. In principle, thrombolites could hardly grow in such high-energy settings. Furthermore, the absence of truncated or flat-topped mounds supports this statement. Petrographic observations document a lack of ooids trapped within the mounds and a sharp contact between both deposits, pointing out they were not coeval, or they developed in different settings of the ramp. The occurrence of these microbial mounds is very local. They are restricted vertically to this 4 m-thick interval and laterally they pass into shoal deposits in other areas. Small oscillations in relative sea level combined with local climate and environmental changes would explain the local occurrence of these thrombolite mounds.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.