Time-specific aspects of facies: State of the art, examples, and possible causes

The term time-specific fades (TSF) was introduced to the scientific community by the late Otto H. Walliser to refer to unique facies typical of particular narrow intervals, some of which were related to bioevents. In some senses, however, the concept was recognized much earlier and is even engrained in the very names of some geologic periods. The concept of time-specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time. The recognition of time-specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale. These range from very short-term, but widespread facies that overlap with event deposits, to general fades types that may persist for intervals up to 10s of millions of years in duration. This paper briefly summarizes the history of development of the TSF concept, provides examples to illustrate a few of the key aspects of time-specific facies and offers a few tentative explanations for this phenomenon. Among the factors that control TSFs, abrupt changes in redox conditions and early diagenetic mineralization, sedimentary condensation, often associated with abrupt sea level change, altered climate and paleoceanography, and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of TSFs. Inevitably, however, many TSFs reflect multiple effects and some remain largely unexplained. (C) 2012 Elsevier B.V. All rights reserved.