VARIATION IN DEFORMATION STYLE OF MOLAR-TOOTH STRUCTURE DURING FLUIDIZATION: TAWAZ FORMATION, ATAR GROUP, MAURITANIA

Vermicular structures consist of variably contorted ribbons and sheets of carbonate microspar that occur within an ~4 m-thick interval in the Proterozoic Tawaz Formation, Atar Group, Mauritania (Bertrand-Sarfati et al., 1997). In their undisturbed state, vermicular structures (VS) strongly resemble molar-tooth structure (MT), a widespread Proterozoic carbonate fabric that develops at or near the sediment-water interface from rapid cementation of early diagenetic voids (Furniss et al., 1998). Questions regarding VS include: 1) what accounts for the variation of VS facies within this composite interval, and 2) are vermicular structures a facies variant of the more common molar-tooth structures?

The vermicular interval contains five distinct, interbedded facies: 1) planar to cross-bedded grainstone lacking VS; 2) “undisturbed” VS in a dark grey, fine-grained, indistinctly laminated matrix, 3) sub-spherical clusters of reworked VS floating in a featureless dark grey matrix, 4) clast-supported breccia composed predominantly of VS, and 5) graded, matrix-supported VS breccias. Reworked and brecciated VS suggests that most facies formed from post-depositional deformation processes, and the presence of VS clusters and breccia floating in featureless matrix suggests fluidization as a possible mechanism of deformation. Facies within this unit are cyclically arranged, suggesting storm-induced fluidization episodes rather than a single seismically induced event. Dissimilarities among fluidized facies suggest that the mechanical properties of the substrate at the time of fluidization (e.g., ratio of VS to matrix components, degree of lithification) exert a major control on deformation style.

“Undisturbed” VS are identical to MT fabric in terms of crack morphology and the petrographic and cathodoluminescent character of microsparitic cement fill. Disturbed VS facies, however, typically differ from MT by the presence of a central sparry zone. Restriction of these sparry zones to reworked facies suggests that the central void is a secondary feature, related to deformation processes rather than to VS origin.

References: Bertrand-Sarfati, J., Plaziat, J.C., Moussine-Pouchkine, A. 1997. Facies 36:231-234. Furniss, G. Rittel, J.F., Winston, D. 1998. J. Sed. Res. 68:104-114.