Palaeoenvironmental significance of lacustrine microbialite forms from the Middle Old Red Sandstone of the Orcadian Basin

The form of microbialite accumulations is largely the product of environmental processes and microbial activity. Recent work has largely concentrated on the identification and classification of microbialites with little attention being paid to their environmental significance. This study describes the environmental distribution of the varied microbialite forms recorded from the predominantly lacustrine Middle Old Red Sandstone sequences of the Orcadian Basin. Comparisons are made with Triassic examples from East Greenland and modern microbialite accumulations and a model describing the palaeoenvironmental distribution of the examples studied is presented.

The Middle Old Red Sandstone of northern Scotland was deposited in a predominantly lacustrine setting. Climatically driven lake level fluctuations resulted in the generation of cyclic sedimentary sequences. Microbialites are recorded from both steep, basin margin coincident settings and lower gradient settings where the lake margin is distant from the basin margin. In the latter case microbialite development is largely restricted to transgressive lacustrine sequences during the deposition of which, reduced rates of sedimentation resulted from the migration of sediment input points towards the basin margin. Microbialite sheets, domal mounds, aligned mounds (and associated runnels), sand cored microbialite mounds and reefal microbialite accumulations have been identified representing the transition from more sheltered to more exposed environments. In basin margin coincident settings microbialite accumulation is restricted to areas of low sedimentation where they coat boulders and pebbles.