basal surface forced regression

base level

correlative conformity

onlap

regression

regressive surface of erosion

sequence boundary

unconformity

A forced regression is induced by the seaward movement of the shoreline in response to relative sea-level lowering. Catuneanu (2002) defines this type of regression as occurring "during stages of base level fall, when the shoreline is forced to regress by the falling base level irrespective of the sediment supply. This triggers erosional processes in both the nonmarine and shallow marine settings adjacent to the coastline. Fluvial incision is accompanied by the progradation of offlapping shoreface deposits." Forced regressive sediments are those sediments that collect during a forced regression that display diagnostic progradational and downstepping stacking patterns.

Hunt and Tucker (1992) emphasize how the upper surface of downlapping and prograding shoreline of the forced regression is eroded and is expressed as a diachronous subaerial unconformity. Plint and Nummedal, (2000) name the subaerial unconformity that caps the forced regression as the "regressive surface of fluvial erosion". Hunt and Tucker (1992) match this unconformity to a downdip surface they call the marine correlative conformity and relate to the end of base level fall. The Hunt and Tucker (1992) suggest that this "sequence boundary" over the forced regression does not match Mitchum's (1977) original definition of a sequence boundary or its time equivalent marine correlative conformity that was tied to the onset of a sea level fall.

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.

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
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.
Plint, A.G., Nummedal, D., 2000, The falling stage Systems Tract: recognition and importance in Sequence stratigraphic analysis. In: Hunt, D., Gawthorpe, R.L. (Eds.), Sedimentary Response to Forced Regression, vol. 172. Geol. Soc. London Speci. Publ, pp. 1–17.
Posamentier, H.W., Allen, G.P., 1999, Siliciclastic Sequence Stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology no. 7, 210 pp.