Fiona Gallacher

Over the past decade, microbial mats have been identified as major players in sedimentology. The mats are thin crusts, largely of bacterial origin and form rubbery mats on sediment surfaces, which they protect from erosion. Domal stromatolites are also microbial in origin. Microbial mats and stromatolites have been recognized as far back as 3.5 billion years but until recently, mats on flat-lying surfaces have been difficult to identify and have been overlooked.

Stromatolites and remnants of microbial mats have been discovered at Blue Beach in the Mississippian Horton Bluff Formation of Nova Scotia. The domes range in height from 13 to 30 cm and have horizontal bases ranging in width from 15 to 60 cm. The domes are spaced individually and in clusters along a gently dipping bedding surface on the tidal platform. The stromatolite bed can be correlated with strata in the adjacent cliff approximately 47 m distant, and the spacing of each group on the bedding surface ranged from 3.4 m to 10.6 m. The domes have oversteepened laminae at the margins of approximately 35o, steeper than the angle of repose for any wet sediment, suggesting sediment fixation with possible microbial origin.

The remnants of microbial mats have been found on an equivalent bedding surface of very fine-grained sandstone in the cliff face. The mats, which are also termed wrinkle structures, have no distinct orientation and do not cover the entire surface, forming patches with an average diameter of 8-10 cm. The largest wrinkles are up to 4 mm in relief. Domal stromatolites are also seen in the cliff face, and in one instance a dome surrounds remnants of a tree stump with wrinkle structures on the bedding surface around the feature.

Petrological analysis identified four micofacies (A to D) with a similar upward succession within several stromatolite domes. The domes grew from a greenish layer with discontinuous dolosiltite laminae (A), overlain in turn by finely laminated quartz- and dolomite-rich silt (B), massive dololutite (C) and banded dolosiltite and dololutite with quartz grains (D). The domes are penetrated by roots. XRD results show that dolomite is the most abundant mineral, along with quartz and muscovite in the finely laminated layers. Based on a sedimentological analysis of the associated strata, the microbial structures grew in shallow water and on periodically exposed flats, probably in bays that experienced fluctuating salinities and probably hypersaline conditions.

Keywords: Stromatolites, blue beach, microbial mats,calcite, dolomite, xrd work, avon, Mississippian, fossils, hyper saline basin
Pages: 158
Supervisor: Martin Gibling