Tagasi otsingusse
Bouougri & Porada, 2002

Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)

Bouougri, E., Porada, H.
DOI
DOI10.1016/S0037-0738(02)00103-3
Aasta2002
AjakiriSedimentary Geology
Köide153
Number3-4
Leheküljed85-106
Tüüpartikkel ajakirjas
Keelinglise
Id8386

Abstrakt

Proterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or “syneresis”. Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation between mat exposure and flooding. Respective conditions prevail in the upper intertidal zone. Gas domes and cabbage-head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat. They may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones. Mat-related structures may serve as sensitive facies indicators once their modes of formation are revealed.

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