base level

forced regression

unconformity

The mutual and undisturbed relationship between adjacent sedimentary strata that have been deposited in orderly sequence with little or no evidence of time lapses. True stratigraphic continuity in the sequence of beds without evidence that the lower beds were folded, titled, or eroded before the higher beds were deposited (AGI Glossary of Geology). Mitchum (1977) in his original work describing the identification of seismic stratigraphic surfaces provided the original definition of a marine correlative conformity surface. He traced this surface from the seismically defined unconformity that was identified as sequence boundary. He suggested that this correlative conformity surface is time equivalent to a sequence boundary and tied this to the onset of a sea level fall.

Catuneanu (2002) and Hunt and Tucker (1992) take a different position of and explains that the "correlative conformity forms within the marine environment at the end of Base level fall at the shoreline. This is the paleo-sea floor at the end of forced regression, and correlates with the seaward termination of the subaerial unconformity associated with offlap of the youngest clinoform (paleoseafloor) formed at the end of the base-level fall at the shoreline (end of forced regression).

As indicated above the correlative conformity was originally defined as the paleo-sea floor at the onset of a forced regression (Posamentier et al., 1988), but this choice has been criticized. This occurs when the sequence boundary is defined as the upper surface of a forced regression (Hunt and Tucker, 1992). If this is done then the correlative conformity will be intercepted twice in the same vertical section within the area of forced regression . In this case, the correlative conformity (sensu Posamentier et al., 1988) does not correlate with the seaward termination of the subaerial unconformity. From this perspective then, when dealing with forced regressions the correlative conformity has now been turned into a problem surface in sequence stratigraphy, surrounded by controversies regarding its timing and physical attributes. Undoubtedly it is not a problem at the larger scale of seismic data where the correlative conformity can be traced at the top of the clinoform that correlates with the basinward termination of the subaerial unconformity. Instead the main problem relates to the difficulty of recognizing the correlative conformity in most outcrop sections, cores, or wireline logs. In this case, the shallow marine portion of the correlative conformity separates rapidly prograding and off- lapping forced regressive strata from the overlying aggradational normal regressive deposits. Associated again with forced regressions in the deep marine setting, the correlative conformity has been traced at the top of the prograding basin-floor fan or submarine fan complex (the ‘‘basin floor component'' of Hunt and Tucker (1992)."

Note the correlative conformity on the top of the basin-floor fan as suggested by Vail, 1987, versus the Hunt and Tucker, 1992 & 1995, models.

In summary surface that underlies the forced regressive deposits (correlative conformity sensu Posamentier and Allen, 1999) is, in fact, a downlap surface (internal downlap) that merges with the main downlap surface (Downlap zone or the basal surface of the forced regression sensu Hunt and Tucker, 1992) and is associated with the offlap of the oldest clinoform (paleo-seafloor) at the onset of base-level fall at the shoreline (i.e., onset of forced regression). The surface on top of the forced regressive deposits (correlative conformity sensu Hunt and Tucker, 1992) may be the product of wave reworking due to base level lowering (regressive surface of marine erosion) prior to be subaerially exposed (sequence boundary). Both surfaces bounding, or sandwiching, the forced regressive wedge merge basinward with the correlative conformity and landward with the subaerially exposure surface (sequence boundary) (Fig. Carbonates). One can contend that both models are right but only partially so. Both surfaces, bounding the offlapping wedge are coeval, being formed during the lowering of sea level.

References
Catuneanu,O., 2002, Sequence Stratigraphy of clastic systems: concepts, merits, and pitfalls, Journal of African Earth Sciences, Volume 35, Issue 1, Pages 1-43
Catuneanu, Octavian, William E. Galloway, Christopher G. St. C. Kendall, Andrew D. Miall, Henry W. Posamentier, André Strasser, and Maurice E. Tucker, 2011, "sequence stratigraphy: Methodology and Nomenclature", Newsletters on stratigraphy, Stuttgart, Vol. 44/3, 173–245
Hunt, D., Tucker, M.E., 1992, Stranded Parasequences and the forced regressive wedge Systems Tract: deposition during base-level fall. Sedimentary Geology 81, 1–9.
Mitchum Jr., R.M., 1977, Seismic Stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic Stratigraphy, In: Payton, C.E. (Ed.), Seismic Stratigraphy––Applications to Hydrocarbon Exploration, vol. 26. A.A.P.G. Memoir, pp. 205–212.
Posamentier, H.W., Allen, G.P., 1999, Siliciclastic Sequence Stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology no. 7, 210 pp.