Transgressive-Regressive (T-R) Sequence Stratigraphy

Ashton Embry, Geological Survey of Canada

Abstract

A sequence was originally defined as a stratigraphic unit bounded by unconformities and thus most sequences were of limited value because they were confined to the margins of a basin where unconformities were present. Vail and colleagues greatly expanded the utility of sequences for basin analysis when they redefined the term as a unit bounded by unconformities or correlative conformities. The addition of correlative conformities allowed a sequence to potentially be recognized over an entire basin.

This revised definition led to the formulation of four different types of sequences, each with a different set of bounding surfaces. Vail and colleagues defined two types, depositional sequence type 1 and depositional sequence type 2. Depositional sequence type 1 utilizes a subaerial unconformity as the unconformable portion of the boundary and a stratigraphic surface which closely ties to the start of base level fall for the correlative conformity. Because the subaerial unconformity migrates basinward during base level fall, much of it is therefore included within such a sequence rather than being on the boundary. Also it is commonly very difficult to objectively recognize a correlative surface which corresponds to the start of base level fall. For these reasons a type 1 depositional sequence has little practical value.

A depositional sequence type 2 sequence also uses the subaerial unconformity as the unconformable portion of the boundary but uses a stratigraphic surface which approximates the end, rather than the start, of base level fall for the correlative conformity. This resolves the problem of including a portion of the unconformity inside the sequence. However, it is essentially impossible to objectively recognize a stratigraphic surface that corresponds with the end of base level fall and thus this type of sequence also has no practical value.

Galloway proposed the use of maximum flooding surfaces as sequence boundaries and named such a unit a genetic stratigraphic sequence. This alleviated the problem of major subjectivity in boundary recognition because maximum flooding surfaces can be determined by objective scientific analysis. However this sequence type founders on the problem that the subaerial unconformity occurs within the sequence and thus it lacks any genetic coherency.

To overcome these major deficiencies in sequence definition, Embry and Johannessen defined a fourth type of sequence that they termed a T-R sequence. This sequence uses the subaerial unconformity as the unconformable portion of the boundary and the maximum regressive surface as the correlative conformity. This methodology keeps the subaerial unconformity on the boundary and also provides for a correlative conformity that can be objectively determined. It thus avoids the fatal flaws of previously defined types. A T-R sequence can be divided into a transgressive systems tract below and a regressive systems tract above by using the maximum flooding surface as a mutual boundary. T-R sequence stratigraphy, unlike the other proposed methodologies, has maximum practical utility with a minimum of stultifying jargon.