Development of joint-controlled rock basins in Bohus granite, SW Sweden

The granite area of Bohuslنn along the Swedish west-coast is characterised by an abundance of rock basins of different sizes. Within 163 km2 large part of the area over 400 basins occur, ranging in size from less than 0.005 to almost 2 km2. These basins were measured, described and classified from air photos according to their length, width, area, altitude, shape, boundaries, openness and relation to joint configuration. The depth of basins was collected from digital elevation data. Their distributions and shapes are clearly associated with the visible jointing patterns, as shown by their location along joints or joint intersections, by joint-guided enlargement and similarity between directions of long axis of basins and principal tectonic lineaments in the Bohus granite. The patterns and characteristics of the basins suggest that deep selective weathering had been the primary process involved in the origin and evolution of the basins. Later, they have been reshaped and possibly stripped of saprolites by glacial processes in the Pleistocene. Dating of deep weathering responsible for basin initiation and origin is difficult, although the complex story of Mesozoic weathering, Late Cretaceous burial and Late Tertiary exhumation is not without support.