The Effect of Sealevel Change on the Shelfedge and Slope of Passive Margins1
Walter C. Pitman, III, Xenia Golovchenko, 1983. "The Effect of Sealevel Change on the Shelfedge and Slope of Passive Margins", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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Subsidence of passive margins appears to be of thermal origin, and increases from near zero at a landward hinge zone to a maximum value at the ocean-continent boundary. At all points on the attenuated margin the driving subsidence is at a maximum immediately subsequent to rifting, and decreases thereafter as a function of time. The present driving subsidence at the ocean-continent boundary of the U.S. East Coast may be greater than 1 cm/1000 years. Rifting at this margin ceased at least 160 m.a. It is concluded that glacial fluctuation is the only known mechanism that can cause world-wide sealevel to change at rates in excess of 1 cm/1000 yrs and with a magnitude greater than 100 m. We do not preclude the possibility that mechanisms as yet unknown may be sufficient to change sealevel by rates and with magnitudes in excess of the above. Considered here are geological periods during which glacial fluctuation was minimal to non-existent. We have assumed that at most passive margins, the shelfedge lies at (or even seaward of) the ocean-continent boundary. Under these conditions it is not likely that the shoreline can move out over the shelfedge because the driving subsidence at the shelfedge is greater than the rate at which sealevel may fall. We have also shown that if by chance the rate of sealevel fall is several times greater than the rate of subsidence at the shelfedge it may take several million years to displace the shoreline seaward to the shelfedge.
Alternatively, at a starved margin, the shelfedge may lie well landward of the ocean-continent boundary (well up toward the hinge zone). In this case, rate of subsidence may be much less than 1 cm/1000 yrs and rate of sealevel fall may be much greater than rate of subsidence at the shelfedge. In this case the coastline may move seaward over the shelfedge, but this will still require a time interval of a million years or greater. If sealevel drops sufficiently to allow the coastline to move seaward over the shelfedge and if at the same time sealevel ceases falling, then within a time interval of less than two million years the combined effects of both erosion and subsidence will generally cause the coastline to retreat onto the shelf and transgress toward the hinge zone.
Figures & Tables
The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.