The valuation of unconformities

Eliot Blackwelder published a paper with this title in 1909. He was the first to demonstrate that the Phanerozoic record of North America contains continent-wide regional unconformities that divide the stratigraphy into what we would now call sequences. Subsequent studies by Grabau, Barrell, Wheeler, Ager, Dott, Sadler, and others have helped to clarify the issue of missing time in the rock record, and unconformities now play a key role in the definition and mapping of sequences. Calculated sedimentation rates for the ancient record indicate that as little as 10% of elapsed time is recorded by rock at measured time scales in the 105-year range, the remainder being missing at sedimentary breaks. Recent refinements in chronostratigraphic methods and the availability of a reliable Global Time Scale permit a more detailed evaluation of the nature of unconformities and other sedimentary breaks. They may be grouped into four broad classes. 1) Major breaks spanning 10(6)-10(7) years. These are generated by five distinct processes: a) orogenic tectonism. Hutton's classic unconformity at Siccar Point in northern England is of this type. b) Dynamic topography. This is the term for the slow elevation and subsidence of the craton in response to changing thermal properties of the underlying mantle. Example: the great basal Phanerozoic unconformity overlying the Canadian Shield. c) Dynamic unconformities associated with basin formation, including breakup unconformities and flexural onlap in extensional basins, and the onlap/offlap stratigraphy of foreland-basin forebulges. d) Global eustasy, caused by changing rates of seafloor spreading and its effect on the total global volume of the ocean basins. The resulting breaks are the basis for the definition of what have come to be called Sloss sequences. e) Long-term environmental change, including eolian supersurfaces, and drowning unconformities in carbonates. 2) Breaks of two distinct types that span 10(4)-10(5) years: a) Unconformities generated by high-frequency tectonism, including the regional propagation of such breaks by intraplate stresses. Sequences of regional extent may be bounded by breaks of this type. b) Glacioeustatic sea-level changes generated by orbital forcing of global climate change. Example: Cyclothem boundaries in the late Paleozoic record of the US Midcontinent. 3) Hiatuses of 1(0)-10(3)-year duration, the product of autogenic, seasonal to long-term geomorphic processes, which drive the migration and switching of depositional systems, including shelf clinoforms and deltas. 4) Minor breaks of 10(-6)-10(-1)-year duration (minutes to months), the breaks generated by bedform and bar migration. The product of diurnal, monthly (lunar) and normal meteorological changes in runoff; tidal cycles. Identification and classification of unconformities and other sedimentary breaks is an essential component of high-resolution stratigraphic mapping. (C) 2016 Elsevier B.V. All rights reserved.