Forced Regression on Epeiric Seas: Examples from the Pimenteira Formation (Devonian, Parnaíba Basin, Brazil)

Charles Young¹ and Leonardo Borghi^2
1 Baker Hughes INTEQ, Rio de Janeiro, Brazil
² Federal University of Rio de Janeiro, Rio de Janeiro, Brazil

The concept of a forced regression involves a relative sea-level fall, which results in a basin-ward migration of the coastline with subsequent shoreline progradation, followed by a marine transgression. On epeiric sea settings this process is of special importance, since any small sea level fall could potentially cause the shoreline to migrate kilometers basin-ward due to a low gradient shelf. Such mechanism is of great importance to the oil industry since it places potential reservoir rocks in direct contact with source rocks. The Parnaíba basin is a large (600.000km²) intracratonic basin located in the Northeast of Brazil. Its Devonian record is represented by the Itaim, Pimenteira, Cabeças and Longá Formations. The shaly Pimenteira Formation is considered by the literature to have been deposited in an ancient eperic marine shelf, with eventual sand transport by storm processes. During the study of over 400m of cores from this formation, three 15m thick and laterally continuous (over 100km) sandstone beds were identified at different stratigraphic levels of this formation. These beds show typical characteristics of beds deposited by events of forced regression, a process that has never been considered for this formation. Also, the contact with the overlying sandy Cabeças formation, usually understood as the result of a deltaic progradation, has characteristics of another event of forced regression, with strong incision features. This suggests that the stratigraphic record in this area is punctuated by events of forced regressions, and that this is also the most important process of sand transport and deposition.

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