UNCONFORMITIES 545 succession, contains a large proportion of fragments of Ordovician and Silurian sedimentary and volcanic rocks, demonstrating that by the time the Upper Brockram was deposited an area of these older rocks was exposed and being eroded With the development of stratigraphic thinking and knowledge it became clear, however, that unconformities were a poor choice for defining widely correlatable boundaries An unconformity necessarily implies that there is a gap in the record This gap may represent different durations of time in different areas For good correlation, detailed information about fossil occurrences and other temporal markers, such as ash bands, is needed A gap cannot provide this kind of detail So a lot of effort was put into searching for areas where a continuous sedimentary record, deposited during the time-span represented by the gap, could be demonstrated In many cases this involved looking outside the European area, where most of the stratigraphic units had first been defined Unconformities and Sequences For many practical problems, the refinements of stratigraphy are less important than the local rock distribution, so not all geologists abandoned the use of unconformities From the 1930s onwards Arville Irving Levorsen (1894–1965) interpreted the geology of the mid-continent region of North America in terms of large-scale unconformity-bounded tectonostratigraphic units Levorsen did not propose names for these units, referring to them as layers of geology, but he demonstrated their importance in petroleum exploration It was in the late 1940s that Laurence Louis Sloss (1913–96) began to use the term sequence for such major unconformity-bounded units, eventually proposing a formal name for each sequence In order to distinguish these sequences from standard chronostratigraphic units they were given the names of Indian tribes (Table 2) Sloss, and many other North American stratigraphers, had encountered problems when attempting to apply the mostly European-based stratigraphic divisions to North American rocks However, a small number of major craton-wide unconformities could be recognized, based on an integrated study of outcrop and subsurface data Sloss stressed that at the scale of an individual exposure there was no obvious characteristic by which these inter-regional unconformities, which were used to separate sequences, could be distinguished from the many local unconformities This was graphically illustrated when the unconformity at the base of the Absaroka Sequence was redefined The inter-regional unconformities represented major breaks in the depositional record and were associated with a great degree of overstep and overlap For example, the rocks at the base of the Sauk Sequence range in age from latest Precambrian to Late Cambrian They were deposited on an unconformity that cut across rocks of a great range of Precambrian ages The boundary at the top of a sequence was interpreted as representing a time of major regression of the sea from the continental craton, with associated subaerial erosion The base of the next sequence represented the re-flooding of the craton Local unconformities were thought to be produced by minor fluctuations in the rate of sea-level rise or fall, by local tectonics, or by local changes in sediment supply Sequences, as promoted by Sloss, are major units, with demonstrably diachronous boundaries Although they are useful as major subdivisions of North American strata, they were not intended to replace the standard chronostratigraphic units They were erected because of the perceived differences between the rock record in North America and that in Europe By no means all stratigraphers agreed with this approach, arguing that better-defined chronostratigraphic boundary sections and improved techniques of correlation would eventually help to solve the difficulties Unconformities, Seismic Stratigraphy, and Sequence Stratigraphy The development of high-quality seismic-reflection profiles, primarily as a result of the intensive search for hydrocarbons, and the calibration of these profiles Table North American sequences Sequence name Age of rocks included in the sequence Tejas Zuni Absaroka Kaskaskia Tippecano Sauk Late Paleocene to Holocene mid Jurassic to mid Paleocene latest Mississippian (post Chesterian) to Early Jurassica late Early Devonian to Late Mississippian mid Ordovician to Early Devonian latest Precambrian to Early or possibly early mid Ordovician a The Absaroka was originally defined as from the Chesterian