Observations from submersible dives and rock sampling from the lower continental rise, in incised canyons and oin steep escarpments and deep sea drilling, provide evidences of depositional and erosional processes that do not match those of the existing models of sequence stratigraphy.
Three case studies are presented:
Site 397/397A is on the lower continental rise, in a water depth of 2900 m. It lies on a transect SE-NW from the Tarfaya Basin of Morocco through Site 367 to the Canary Islands off Cape Bojador. At 1300 meters subbottom the well encountered a major unconformity with a hiatus of almost 100 Ma that separated Lower Cretaceous from Lower Miocene sediments. Interpretation of the seismic reflection profiles for the offshore and on the upper continental rise calibrated with wells, show no evidence of an event of this magnitude, though slope instability in the Oligocene is documented by allochthonous, highly deformed sediments of Oligocene age.
Heezen Canyon, off Georges Bank in the NW Atlantic, is deeply incised from about -1700 to -1100 m and cuts into a carbonate platform of Early Cretaceous age. The shallow water rocks are well bedded, undeformed and horizontal. They are followed both upstream and downstream by two discrete Middle Eocene formations. The downstream strata dip seaward, are terrigenous and document a progradation on an eroded substrate. Horizontal Eocene pelagic facies (chalk) overlie Mesozoic strata. Their age is just one biozone younger than the Eocene strata cropping out where the Heezen Canyon debouches on the continental rise. Two episodes of submarine erosion followed by deep-sea transgression are hypothesized, probably controlled by global paleoceanographic phenomena.
The Bahamas Escarpment reaches the highest gradient off Cat Island (20° to 40°). In 1978 three dives were made from circa -4000 to -2000 m and sampled (51 sampling stations). The exposed limestones of Early to mid Cretaceous age are cyclically bedded and mostly deposited in a back reef setting. The subsidence rate of the Bahamas decreases as a function of time. The most important erosional process seems to be related to spallation, as documented by joints, open fissures, isolated pinnacles: the carbonate margin collapses in response to the difference in lithostatic versus hydrostatic pressure. Neptunian dykes and borings filled by pelagic oozes are recorded in the upper part of the less steep escarpment and document possible worldwide paleoceanographic events.
These examples indicate that important erosional phenomena, not considered in sequence stratigraphic models, are present well beyond the shelf break. The possible origin of these events with global sea-level changes or other controlling factors cannot be deciphered without an accurate data set, including lithofacies and biofacies interpretation.
Felix qui potuit rerum cognoscere causas (Virgilio, Georgicae).
