Glacial Eustatic Mechanism (Marine Ice Sheet Decoupling) for Isolation of Shelf Sand Bodies

John B. Anderson, Louis R. Bartek, Mark A. Thomas

The formation of shelf sand bodies requires a mechanism for delivering sand to the offshore environment (below wave base or ravinement depth). The distribution of these sand bodies (sheetlike versus patchy distribution) is greatly influenced by the rate of shoreline translation. Holocene sand bodies of the north Texas shelf illustrate these influences well, as does a map depicting the thickness and lateral extent of sand bodies that would be formed were sea level to rise rapidly again.

Preliminary observations imply that wave-dominated deltas have actively (via plume mechanisms) and passively (via transgressions) been key factors in delivering very fine to fine sands to the offshore environment. Other sand bodies may be relict shoreline deposits that, like the modern Galveston Island barrier, prograded into the offshore environment so that their roots were preserved beneath the ravinement surface.

Recently gathered evidence from the Antarctic continental shelf indicates that ice sheets were formerly grounded to the shelf edge and that the retreat of these ice sheets from the shelf was rapid but sporadic. The subsequent "drawdown" of marine ice sheets and the associated eustatic rise in sea level during any single event was probably on the order of 5-10 m in a few centuries. Such a sea level rise would manifest itself in rapid translations of the shoreline of about 10-25 km on the Texas shelf. Several of these eustatic events are believed to have occurred during the Holocene, and were probably instrumental in isolating shelf sand bodies. Sand bodies associated with the most recent rapid rise occur within 30 km of the present shoreline and their origin is being investigated using high-resolution seismic (uniboom) profiling and coring.

Because marine ice sheets have existed in Antarctica since at least the early Miocene, this episodic eustatic mechanism has been operative for that long. Although the processes responsible for delivering sands to the offshore environment may not have changed over geologic time, the frequency and rate of eustatic variation have changed and have surely been influential in controlling sand body geometry and distribution on the shelf. This implies that the size, shape, and orientation of shelf sand bodies may differ for glacial vs. nonglacial periods of geologic time.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.