The Valuation of Unconformities

Abstract

Eliot Blackwelder published a paper with this title in 1909. He was the first to demonstrate that the Phanerozoic record of North America contains regional unconformities that divide the stratigraphy into sequences. Subsequent studies by Barrell, Grabau, 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 siliciclastic record indicate that as little as 10% of elapsed time is recorded by rock, 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 classes. 1) Major breaks spanning 106-107 years. Generated by three distinct processes: a) orogenic tectonism. Hutton's classic unconformity at Siccar Point (UK) is of this type. b) Dynamic topography. 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) 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. 2) Breaks of two distinct classes that span 104-105 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 100-103-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.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016