Parasequences in Third Generation Sequence Stratigraphy

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

Third generation (3G) sequence stratigraphy maintains the main components of earlier sequence methodologies developed by Sloss/Wheeler (1G) and Exxon geoscientists (2G) and avoids the pitfalls that have limited their usefulness. One of the units added to sequence stratigraphy in the second generation phase was the parasequence, defined as “A relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces”. A marine flooding surface was defined as a surface across which there is evidence of an abrupt increase in water depth. For clastic strata this boundary coincides with the contact of sandstone and overlying shale and is thus a lithostratigraphic boundary rather than a sequence stratigraphic one. In 3G sequence stratigraphy maximum regressive surfaces are used as the boundaries of a parasequence. A maximum regressive surface is the stratigraphic horizon that marks the change from shallowing-upward, regressive strata below to deepening-upward, transgressive strata above. This change of boundary definition brings the parasequence into sequence stratigraphy and allows a parasequence to be directly related to a sequence that is in part bounded by maximum regressive surfaces. The critical difference between the two is the occurrence of subaerial unconformities and/or unconformable shoreface ravinements on the boundaries of a sequence and their absence on the boundaries of a parasequence. Furthermore, a maximum regressive surface has a lower diachroniety than a lithologic/facies contact has and thus parasequence boundaries of 3G sequence stratigraphy allow a much better quasi-chronostratigraphic framework to be established than do the parasequence boundaries of the 2G methodology.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005