Episodic event deposits versus stratigraphic sequences—shall the twain never meet?

Abstract

In the early half of the 20th century, questions were raised about different rates of deposition reflected in the stratigraphic record and of small gaps or diastems. Even more discussion centered upon rhythms and cycles of deposition—at one extreme were glacial varves and at the other were Carboniferous cyclothems. After World War II, interest in stratigraphic cycles declined. Then the turbidity current revolution stimulated interest in event deposits, which interest has surged again recently with a focus upon storm deposits. Meanwhile, the recent dramatic growth of sequence stratigraphy has rekindled interest in both cyclicity and eustasy. The two themes—events and cycles—should be better integrated, for there is considerable confusion about the interpretation of high-frequency sequences. There is also a need to reconcile the current fad for Milankovitch-related sedimentary cycles versus more or less random event deposits. The most familiar event deposits are turbidites in deep-water and tempestites in shallow-water environments. More subtle are the diastems, which include non-depositional surfaces as well as scoured surfaces. Other processes that can produce event deposits include avalanches and tsunamis. Potentially, any type of event deposit could be mistaken for a sequence boundary. For example, submarine megabreccias could be formed either by a seismic event unrelated to any sea level change, or by slope failure resulting from a eustatic fall associated with a sequence boundary. To surmount the intellectual barrier to alternate interpretations requires careful attention to processes, time resolution, and objective tests for periodicity.