EPISODIC EVENT DEPOSITS VERSUS STRATIGRAPHIC SEQUENCES - SHALL THE TWAIN NEVER MEET

In the early half of the 20th century, questions were raised about dif ferent rates of deposition reflected in the stratigraphic record and o f small gaps or diastems. Even more discussion centered upon rhythms a nd cycles of deposition-at one extreme were glacial varves and at the other were Carboniferous cyclothems. After World War II, interest in s tratigraphic cycles declined. Then the turbidity current revolution st imulated interest in event deposits, which interest has surged again r ecently with a focus upon storm deposits. Meanwhile, the recent dramat ic growth of sequence stratigraphy has rekindled interest in both cycl icity and eustasy. The two themes-events and cycles-should be better i ntegrated, for there is considerable confusion about the interpretatio n of high-frequency sequences. There is also a need to reconcile the c urrent fad for Milankovitch-related sedimentary cycles versus more or less random event deposits. The most familiar event deposits are turbi dites in deep-water and tempestites in shallow-water environments. Mor e subtle are the diastems, which include non-depositional surfaces as well as scoured surfaces. Other processes that can produce event depos its include avalanches and tsunamis. Potentially, any type of event de posit could be mistaken for a sequence boundary. For example, submarin e megabreccias could be formed either by a seismic event unrelated to any sea level change, or by slope failure resulting from a eustatic fa ll associated with a sequence boundary. To surmount the intellectual b arrier to alternate interpretations requires careful attention to proc esses, time resolution, and objective tests for periodicity.