Origin of Enigmatic Structures: Field and Geochemical Investigation of Columnar Joints in Sandstones, Island of Bute, Scotland

Columnar joints, resembling those in igneous rocks, are present in Devonian and Early Carboniferous sandstones at two localities on the Island of Bute in southwestern Scotland. New data concerning field relations and geochemical changes constrain processes responsible for generation of columnar fracture patterns in sedimentary rocks. The columns are mostly normal to bedding and occur at several discrete stratigraphic levels. The host rocks are cut by Early Carboniferous volcanic necks or plugs, which acted as heat sources, but development of columnar jointing was strongly controlled by small sills and dikes of a recessive, purple, fine-grained rock. Geochemistry of these rocks points to an igneous origin. Where contacts are exposed, columnar joints appear above subconcordant intrusions and disappear upward. The lowest parts of the columnar-jointed sandstones are Si depleted and enriched in Ca, Mg, Fe, Al, rare earth elements (REEs), and some base metals. REE patterns from columnar-jointed sandstones near contacts with intrusions resemble those of the igneous rocks, probably as a result of hydrothermal activity. Because most of the columns are approximately normal to bedding, intrusion probably took place before folding of the host rocks and possibly before complete consolidation.