Microbial crusts as indicators of stratigraphic diastems in the Cambrian Brèche à Micmacca, Atlas Mountains of Morocco
Résumé
The presence of microbial crusts interrupting high-energy clastic sediments in carbonate and mixed (carbonate-siliciclastic) platforms can be considered as distinct indicators of stratigraphic diastems, characterized by either very low rates of deposition or non-deposition. This analysis is based on a microfacies study of the Cambrian Brèche à Micmacca (Atlas Mountains, Morocco), which contains some stratigraphically condensed limestones characterized by: (i) disarticulation and fragmentation of skeletons; (ii) abundance of volcanigenic debris; (iii) rarity of muddy fraction; (iv) dominance of a sessile, epibenthic shelly fauna dominated by echinoderms, chancelloriids and sponge spicules; and (v) a complex diagenetic history due to erosion and reworking of lithified substrates. The Brèche à Micmacca limestones are composite event-concentration, low-relief shoal complexes composed of parautochthonous and allochthonous bioclastic assemblages. These units can be desribed as "hiatal accumulations" because of the subdivision of the coquinas by minor discontinuities or hiatal surfaces, and the composite, multiple-event nature of the grainstones/packstones. The growth of microbial mats can be used as a record of time interrupting background-sedimentation patterns characterized by the amalgamation of high-energy events. The Brèche à Micmacca was deposited in an environment in which some intervals of non-deposition (or under extremely low-sedimentation rates) alternated with the succession of sedimentary events. The biological response of microbial communities to coat stratigraphic discontinuities can be considered as an integral part of event stratigraphy. They enhance the preservation and identification potential of interruption in the background sedimentation on substrates devoid of burrowing activity. A distinct event of non-deposition is also recognized, based on the presence of microbial fabrics, at the top of the Brèche à Micmacca. Its microkarst structures reflect a major fall in the relative sea level of a reduced region of the platform resulting in dissolution and colonization of coelobiontic cavities by microbial mats and branching filaments.