--> Abstract: Depositional Sequence Boundaries in Carbonate Strata - Problems and Proposals, by Ashton Embry; #90078 (2008)

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Depositional Sequence Boundaries in Carbonate Strata - Problems and Proposals

Ashton Embry
Geological Survey of Canada, Calgary, AB, Canada

A depositional sequence boundary is defined as a subaerial unconformity (SU) and correlative surfaces. Correlative surfaces are surfaces which join with the end(s) of the SU, and with each other, so as to form a single, through going sequence boundary. A number of proposals have been published regarding what combination of surfaces constitutes a scientifically valid and practical depositional sequence boundary in carbonate strata. One of the first ones was based on a carbonate shelf/slope/basin physiographic setting and joined the SU with the facies boundary below all strata which were deposited after base level had fallen below the shelf edge. As clearly shown by Hunt and Tucker (1992), such a boundary is not scientifically reasonable.

Another popular choice is a combination of the SU and the base of a shallow water carbonate unit as proposed by Burchette and Wright (1993) for a ramp setting. This combination is also not reasonable because of the high diachroneity of base of the carbonate unit and the fact that it forms during base level fall. Another proposal used part of the SU and the time surface at the start of base level fall. This has no practicality because such a time surface has no characteristic physical attributes which allow its recognition. Yet another proposal joins the SU with the time surface at the start of base level rise. Again this sequence boundary is not practical because such a time surface has no characteristic physical attributes which would allow it to be recognized consistently and with reasonable objectivity.

It appears the only practical and scientifically acceptable depositional sequence boundary for carbonate strata is a combination of an SU with an unconformable shoreline ravinement surface and a maximum regressive surface.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas