Stratigraphy and its Evolution

William Smith's 1815 map

William 'Strata' Smith (23 March 1769 – 28 August 1839) was an English geologist and surveyor who generated the first geological map of England. When one thinks of how little had been published on formalized sedimentary stratigraphy and how little was known of the relationship of fossils to depositional setting and time, this was a miraculous creation of intuitive genius. By the standards of his time, William Smith had had a relatively simple education and his family connections in a vertical society prevented him from mixing easily with learned society.

Sequence stratigraphy is a recent methodology for stratigraphic interpretation, pioneered by Peter Vail in the mid 70's (Vail et al 1977), that explains the complex geometries that sediments acquire as they fill accommodation in response to changes in rates of sedimentation, tectonic and eustatic movement. This discipline is built on a solid foundation of five geological paradigms that include:

• The dating of the sedimentary section using bio-stratigraphic (recognized in the late 1800's but better defined and used in the post 1950's) and radioactive minerals (intensively used in the post 1950's).
• Recognition that some global changes in base level are glacial eustatic (identified in the late 1800's but greatly refined in the 1970's onwards).
• Continental drift and plate tectonics cause local and some global changes in base level (identified in the late 1800's but greatly refined in the 1960's onwards).
• Sedimentary depositional systems, and their geometries are the products of well-defined physical processes (recognized in the late 1800's but greatly refined in the post 1950's).
• Unconformities are a response to drops in base level (recognized in the late 1800's but better defined in the late 1970's and 80's when marine and land based seismic, coupled with bio-stratigraphic data collected at wells, became widely used).

The text that follows traces the historical development of each of these five paradigms.