El Hafid Bouougri

Kinneyia-Type Wrinkle Structures—Critical Review And Model Of Formation

Abstract Kinneyia structures are among the most typical wrinkle structures observed on ancient siliciclastic sediment surfaces since the Archean. Recently, Kinneyia structures have been grouped together with other microbially induced, crinkly decorations on ancient bedding surfaces as wrinkle structures. They are mainly preserved on upper surfaces of ancient siliciclastic-event deposits and are characterized by millimeter-scale, winding, flat-topped crests separated by equally sized round-bottomed troughs and pits. The structure resembles small-scale interference ripples including crest-dominated linear and pit-dominated honeycomb-like patterns. The steep slopes of the crests, however, exclude their formation at the air or water-sediment interface. Thin sections across Kinneyia structures reveal their formation beneath microbial mats. They formed at an early stage and do not arise from loading and other processes related to burial. Based on the close relationship to event deposits, a genetic model considering the specific hydraulic conditions on siliciclastic tidal flats after storms or floods is proposed. Numerical calculations show that, after microbial mats have been reestablished on the new sediment surface and groundwater is still flowing downslope, the top portion of the sediment confined beneath mats may be liquefied, thus allowing grains to move with the groundwater. Oscillations of groundwater flow owing to periodic reversals of flow direction at rising tides, and a tidal signal of oscillating pore pressure may enhance formation of ripplelike structures along the boundary with the overlying mat. The model applies primarily to Kinneyia structures presumed to be formed beneath cohesive microbial mats in peritidal zones.

Bio-sedimentary structures evolving from the interaction of microbial mats, burrowing beetles and the physical environment of Tunisian coastal sabkhas

Microbial mats are widespread biotic systems in Tunisian coastal sabkhas. In these environments, staphilinid beetles are important bioturbators, making vertical burrows and depositing characteristic hillocks of excavation pellets on the mat surface. The intimate interplay between microbial and beetle activities results in a structured sedimentary surface. Small photosynthetic domes or bulges develop which in turn become microbially overgrown and gypsum-encrusted. Other examples are mat expansion domes above vertical burrows, and excavation pellets overgrown by biofilms trapping aeolian sand grains. In many cases the structures are too complex to explain clearly their formation. Since modern microbial mat-related surface structures form the base for interpretation of fossil record, it is essential to decipher what is contributed by the different structuie-forming factors.

Organisation stratigraphique et structure de la marge Anti-Atlasique du craton Ouest-Africain au sud du Siroua (Néoprotérozoïque, Anti-Atlas central, Maroc)

Abstract The Neoproterozoic calcaires et quartzites group at the southern margin of the Proterozoic Siroua massif is subdivided into two sedimentary parts separated by a volcanic episode related to pre-Panafrican extension. The vertical and lateral evolution allows us to recognize two palaeogeographic domains separated by the major panafrican Anti-Atlasic suture. The southern domain records the incipient stage of an extensional margin, that corresponds to the continental and platform systems of a lower sedimentary ensemble. The northern domain along the Anti-Atlasic suture records a complete sequence of a mature extensional continental margin that includes furthermore, the flood tholeiitic basalts event followed by a turbiditic system of the flyschoidal upper sedimentary group. The functioning of the Anti-Atlasic suture would have controlled this palaeogeographic disposition along the northern margin of West African craton.

Sedimentology and Palaeoecology of Ernietta-Bearing Ediacaran Deposits in Southern Namibia: Implications for Infaunal Vendobiont Communities

The Ediacaran was a period of wide soft-bodied biota development in marine siliciclastic environment. The reconstruction of palaeoecology, lifestyle and environment settings of these organisms are among the challenges in characterising the earliest metazoan life on Earth. The biota colonised sea bottoms and were living as benthic communities occurring in a wide spectrum of environmental settings (e.g. Gehling 2000; Waggoner 2003; Narbonne 2005; Grazhdankin 2004; Droser et al. 2006). In the classical localities (e.g. Newfoundland, Namibia, White Sea and Australia), the fossils occur in rock successions with distinct depositional settings ranging from distributary mouth-bar of braid-delta systems to deep-water slope (Gehling 2000; Grazhdankin 2004; Narbonne 2005).

Biolaminated Siliciclastic Deposits

“Biolaminated deposits” (Gerdes and Krumbein 1987) originate from microbial growth and activity interacting with environmental, physical and chemical processes like sedimentary deposition and mineral precipitation. The resulting “primary” sedimentary structure was named “growth bedding” by Pettijohn and Potter (1964), with clear reference to in situ formed limestones and with “stromatolitic bedding” as the most prominent example (see also Pettijohn 1975).