544 UNCONFORMITIES Table Names that have been given to different types of unconformity Angular Parallel Non depositional Angular discordance Angular unconformity Clinounconformity Discordance Discordant atmodialeima Discordant discontinuity Discordant hydrodialeima Nonconformity Unconformity Accordance Concordant trachyatmodialeima Concordant trachydiscontinuity Concordant trachyhydrodialeima Disconformity Eroded surface Evident disconformity Parallel unconformity Paraunconformity Concordant leuroatmodialeima Concordant leurodiscontinuity Concordant leurohydrodialeima Diastem Marine unconformity Nonevident disconformity Non sequence Paraunconformity Surface of non deposition unconformity This variation in the nature of the surface reflects local differences in the processes of formation The unconformity complex developed during the transition from the synrift stage (active stretching) to the post-rift stage (thermal subsidence and sediment loading) in the development of the northern North Sea basin A transgression coincided with the transition This combination of differential subsidence, block rotation, changing patterns of sediment input, and sea-level rise caused local differences in patterns of erosion and sedimentation, which are reflected in the spatial variation of the type of unconformity Although varied in detail, there is a general distribution pattern of the different types of unconformity At the rift margins the rising sea covered the previously exposed basement, producing nonconformities On the rift flanks, where faulted blocks subsided and rotated, angular unconformities were normally developed In the centre of the rift, subsidence dominated, generally giving rise to disconformities This example, produced during the development of a passive margin, helps to demonstrate that unconformities can originate in a variety of tectonic and sedimentary settings and are not just products of erosion at the end of a geostrophic cycle Unconformities and the Stratigraphic Record Once the Huttonian theory of geostrophic cycles became commonly known, geologists started to apply the reasoning in their efforts to understand and classify the rock record It became clear that there were major periods of deformation, uplift, and erosion, known as orogenies, which could be recognized over large areas, and the consequent unconformities were used to subdivide the geological column It soon became apparent that much of Britain and Scandinavia had been affected by the Caledonian orogeny, which was originally thought to have Overlying igneous or metamorphic rock Heterolithic unconformity Nonconformity culminated in the Late Silurian The three classic unconformities discovered by Hutton were all produced by deposition after Caledonian deformation South-western Britain, and much of the adjacent continent, had been affected by an orogeny that culminated in the Late Carboniferous, which was variously termed the Armorican, Hercynian, or Variscan (see Europe: Variscan Orogeny) Geologists in North America recognized a similar pattern of orogeny It was also apparent that there is an ongoing Alpine–Himalayan orogeny Although the causes of these orogenic episodes were unclear and were to remain so until the development of plate-tectonic theory in the 1960s, the practical result was the rapid development of the broad outlines of the stratigraphic column In addition to the major unconformities associated with the final phases of uplift and erosion, other unconformities were discovered that helped in the processes of subdivision and classification Stratigraphers and palaeontologists could then look in more detail at the rocks bounded by these unconformities In many cases the first rocks deposited on an unconformity surface are conglomerates, often containing pebbles eroded from locally weathered rocks These pebbles can give us information about the rocks that were exposed at that time For example, the basal Carboniferous conglomerates that lie unconformably on Silurian shales in the east of the English Lake District contain distinctive fragments of the Shap granite We know that the granite is intrusive into the Silurian (up to and including the Upper Ludlow), so we have some constraints on the timing of cooling, crystallization, uplift, and erosion of the granite At a higher stratigraphic level, although in the same area of England, we find the Lower Brockram of Permian age unconformably overlying the Carboniferous Limestone The Lower Brockram is formed of pebbles that are mostly Carboniferous Limestone The Upper Brockram, found slightly higher in the