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Book Chapter

Transgressive-Regressive (T-R) Sequence Stratigraphy

By
Ashton F. Embry
Ashton F. Embry
Geological Survey of Canada Calgary, Alberta, Canada, T2L 2A7
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Published:
December 01, 2002

Abstract

A sequence, as originally defined by Sloss and colleagues, was a stratigraphic unit bounded by subaerial unconformities. Such a stratigraphic unit proved to be of limited value because, in most instances, sequences could be recognized only on the margins of a basin where subaerial 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 has led to the formulation of four different types of sequences, each having a different set of bounding surfaces. Vail and colleagues have defined two types: a type 1 depositional sequence and a type 2 depositional sequence. A type 1 depositional sequence utilizes a subaerial unconformity as the unconformable portion of the boundary and a time line equivalent 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 impossible to objectively recognize a time line that corresponds to the start of base level fall. For these reasons a type 1 depositional sequence has little practical value.

A type 2 depositional sequence also uses the subaerial unconformity as the unconformable portion of the boundary but uses a time line equivalent to 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 time line that corresponds with the end of base level fall (start of base level rise) 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 genetic coherency on the basin margins.

To overcome these major deficiencies in sequence definition, Embry and Johannessen have defined a fourth type of sequence that they term 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.

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Contents

GCSSEPM

Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories

John M. Armentrout
John M. Armentrout
Houston, Texas
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Norman C. Rosen
Norman C. Rosen
Houston, Texas
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SEPM Society for Sedimentary Geology
Volume
22
ISBN electronic:
978-0-9836096-8-1
Publication date:
December 01, 2002

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