Event stratigraphy

Event stratigraphy comprises the study of stratigraphical traces of relatively short-lived events (instant to thousands of years) compared to those normally observed on a geological time-scale. Events may be represented by depositional, erosional or geochemical features.  They may be of local significance (e.g. a debris flow), or more extensive (e.g. a volcanic ash deposit), or even global (eustatic flooding surface). They may be random (discyclic) or regular (cyclic).

Virtually instantaneous events (e.g. rainprints, footprints) are mostly of local significance, but a bolide impact may produce tektite bands or isotopic excursions in sediments covering wide areas.  Other short-term events may record periods lasting from minutes to a few days, such as tempestites, some turbidites (gravitational flow deposits), tsunamiites (tidal wave deposits), lava flows and flood deposits.

Medium-scale events (days to several years) include glacial varves and some flood deposits. Palaeobiological events include dinoflagellate or nannofossil blooms, which provide regional markers and may cause anoxic events. Mass mortality may be represented by unusually fossiliferous horizons (e.g. some lagerstätten).

Long-term events (tens to several thousand years or more) are represented by palaeosols, transgressive flooding surfaces, hard grounds, omission surfaces, palaeomagnetic polarity reversals (about 3-4 kyr duration), most evaporite deposits and diamictons. Evolutionary or migratory appearances of organisms form the basis for biostratigraphical event correlation (see biostratigraphy).

In the Quaternary setting the growth of stratigraphy recognised from short-duration, often highly characteristic events has led to attempts to use these features as a basis for correlation.  This event stratigraphy (e.g Lowe et al., 1999), typically includes changes of sea level, climatic oscillations or rhythms and the like.  These occurrences, often termed ‘sub-Milankovitch events’, may be preserved in a variety of environmental settings and thus offer important potential tools for high- to very high-resolution cross-correlation.  Of particular importance are the so-called ‘Heinrich Layers’ which represent major iceberg-rafting events in the North Atlantic Ocean.  These detritus bands can potentially provide important lithostratigraphical markers for intercore correlation in ocean sediments and the impact of their accompanying sudden coolings (‘Heinrich Events’) may be recognisable in certain sensitive terrestrial sequences (summary in Lowe & Walker 1997).  The term stadial has been adopted for these short-lived cold phases, whilst the intervening warmer phases are referred to as interstadials in the ice-core sequences (cf. climatostratigraphy).

A further application of event stratigraphy is in Illinois, in the USA, where the Illinois Geological Survey (a pioneer in the application of formal stratigraphy to the Quaternary) has adopted this approach to the subdivision of its glacial-interglacial sequences.