Tillybrachty Sandstone Formation

This lithostratigraphic unit comprises the lower part of the sequence of the basin fill and is laterally variable in character. In much of the basin the unit is dominated by coarse-grained clastic rocks, primarily locally derived sandstones and conglomerates. In the area of Rhynie itself much of the unit appears to comprise a significant thickness of tuffs with interbedded sandstones (Rice et al. 2002). Some of the tuffs appear to represent direct air-fall deposits, but most appear to have been water-lain and were probably derived from eroding ash cones.

Thin section of altered andesite

Above: Thin section of andesite showing vesicles (v) and plagioclase phenocrysts (p). Most of the plagioclase and mafic minerals have been hydrothermally altered (scale bar = 1mm). Note: The blue colour in the vesicles is dyed epoxy resin to show void space in the thin section.

Concurrent volcanic activity is also recorded in the rocks of the Tillybrachty Sandstone Formation in the form of extrusive andesite lava flows. The andesites show many primary textures that were preserved at the time the lava cooled and solidified such as vesicles and phenocrysts, however, due to later passage of hot fluids through the lavas the original minerals that make up the rock have been highly altered (see inset left).






The sedimentary rocks of the Tillybrachty Sandstone Formation  appear to represent localised deposition in an alluvial environment. The conglomerates and coarse-grained sandstones were probably initially deposited as alluvial fans at the basin margins with sediment being distributed further into the basin by sheet flood events. Localised volcanic activity is also evident by the presence of andesitic lava flows and tuffs.