Relationship between eustacy and stratigraphic sequences of passive margins

Abstract

It is commonly thought that transgressive or regressive events that may have occurred simultaneously on geographically dispersed continental margins have been caused by worldwide sea-level rise or fall, respectively. Instead, it is shown here that these events may be caused by changes in the rates of sea-level rise or fall. The subsidence of an Atlantic-type (passive) margin may be modeled as a bordering platform rotating downward about a landward hinge line. The rate of subsidence is greatest at the seaward side of the platform and decreases landward to zero at the hinge line. With the exception of sea-level changes due to glaciation, dessication, and flooding of small ocean basins and other sudden events, the rate of subsidence at the seaward edge of the platform (shelf edge) is greater than the rate at which sea level may possibly rise or fall. Thus, if sea level is falling, the shoreline will seek that point on the subsiding platform at which the rate of sea-level fall is equal to the rate of subsidence minus the sedimentation rate. If the rate of sea-level fall decreases, the shoreline will move landward; if the rate increases, the shoreline will migrate seaward. If sea level is rising, the shoreline will move to that point where the rate of sea-level rise is equal to the sedimentation rate minus the subsidence rate. Thus, if the rate of sea-level rise decreases, the shoreline will move seaward; if the rate increases, the shoreline will move landward. The position of the shoreline is also a function of the sedimentation rate. These relationships have been quantified so that the position of the shoreline and the thickness of the sediments deposited during discrete time intervals may be computed as a function of the rate of sea-level change and the sedimentation rate. A sea-level curve, based on volume changes of the mid-oceanic ridge system, has been computed. Sea level is seen to fall persistently from Late Cretaceous to middle Miocene time, but transgressions occur in Eocene and early Miocene time because the rate of sea-level fall is slower for these periods. It is concluded also that the presence of the shoreline seaward of the shelf edge of an Atlantic margin should be symptomatic of events that may cause rapid sea-level fall, such as glacial build-up or the sudden flooding of large deep basins.