A MODEL FOR VERTICALLY ORIENTED SURFACES IN MICROBIAL MATS

Fenestrate microbialites from the Neoarchean Carawine Formation, Western Australia, and Reivilo Formation, South Africa, preserve evidence of microbial responses to their depositional environment in their morphology. These fenestrate microbialites contain centimeter to decimeter tall, vertically oriented, corrugated growth surfaces composed of carbonate and organic inclusions. No modern analogs for this type of microbial growth have been reported. However, cyanobacterial biofilms grown in our lab form similar vertically oriented surfaces when microbial mats are physically disturbed. Disrupted laminae provide highs that become a locus for continued microbial growth, producing a vertically oriented surface. We propose that similar physical disruption of growing mats may have resulted in some vertically oriented surfaces in ancient fenestrate microbialites.

Serial sectioning and 3-D reconstruction of ancient fenestrate microbialites demonstrate that the vertically oriented surfaces are often associated with underlying cement filled voids. The cement filled void may represent a bubble produced in the laminated mat. The rising bubble may have disrupted part of the laminated mat, creating a high. As in the modern analog, this high would serve as the locus for continued microbial growth, creating a vertically oriented surface. In ancient facies containing fenestrate microbialites, rolled up laminae, contorted laminae, and irregular bases of vertically oriented surfaces attest to physical disturbance. However, vertically oriented surfaces with no supporting evidence for disruption may require a different model for formation.