AV¾ãÀÖ²¿

The Hurd Creek Member of the Horton Bluff Formation comprises cyclic tidal deposits of early Mississippian age.  Near Hantsport, microbially induced sedimentary structures, formed by the growth of cyanobacterial mats, are unusually well preserved on the bedding surfaces of quartz-rich siltstones.  The structures match those documented from modern sediments at Mellum Island, North Sea and Archean strata at Barberton, South Africa.  The sediments display wrinkle structures and leveled depositional surfaces, in association with wave ripples, planed-off ripples, and desiccation cracks that imply shallow-water and periodically exposed conditions.  A single, well exposed surface was mapped in one-metre segments with estimates of the areal percentage of microbial features, descriptions of the main types present, and photographs.  The outcrop is ~70 m long 1 m wide, selected for its high quality and quantity of microbial features.  Microbial structures cover 12.6% of the exposed surface. Wrinkles are sub-parallel or lack preferred orientations, and have 1 mm height and 1-2 mm spacing – much smaller than associated ripple marks.  The wrinkled patches often occur on ripple crests, especially on flattened, planed-off areas, and are commonly associated with coarse, probably windblown, sand.  Some wrinkled patches cover sediment that fills ripple troughs, where mat growth has contributed to levelling of the sediment surface.  Desiccation cracks are present, implying that these quartz-rich strata were unusually cohesive, perhaps due to microbial binding.  Similar microbial features are present on many other bedding surfaces, and microbial effects were pervasive during deposition.

            Microbial effects indicated by wrinkle structures, leveled depositional surfaces and desiccation cracks are complemented by observations from hand specimens and thin sections that suggest an unusual degree of cohesion shortly after deposition.  Dark, 1 mm thick wavy-crinkly laminae drape over mounds of sand in ways that are not characteristic of physical deposition.  Large, 0.5 mm quartz grains are embedded in these dark laminae and may reflect the microbial process of sediment-grain separation.  Individual quartz grains rest in unusual positions, probably bound by dark laminae.  Broken fragments of dark laminae are folded and display frayed edges.

Keywords: Horton Bluff, Carboniferous, Mississippian, microbial mat, Kinneyia, Nova Scotia, Microbial mats, Wrinkles,Tidal Flats
Pages: 79
Supervisor: Martin Gibling