ABSTRACT

Late Wisconsinan-Holocene sea level curves have been considered as regional phenomena, mainly because geoidal and hydro-isostatic effects cause worldwide sea level changes to be non-uniform. In spite of this observation, recent sea level curves suggest a global pattern of episodic sea level rises occuring throughout the last post-glacial transgression. Evidence from the northern Gulf of Mexico continental shelf supports an episodic model. These episodic eustatic rises must have had a profound impact on the evolution of coasts and estuaries. Episodic sea level rises are manifested in the sedimentary record as rapid translations of the shoreline, displacement of estuaries, generation of shelf sand bodies, and discontinuities within incised valley-fill sequences.

Recently gathered evidence from the Antarctic continental shelf indicates that late Wisconsinan ice sheets were formerly grounded at the shelf edge and that retreat of these ice sheets from the shelf was rapid and sporadic. The subsequent "draw down" of marine ice sheets and the associated eustatic rise in sea level during any single event was probably on the order of 16 to 33 feet (5 to 10 meters) in a few centuries. On a low gradient shelf, such as the north Texas shelf, the shoreline would translate up to 15 miles (25 kilometers) landward during a rise of this magnitude. Several episodic eustatic events are believed to have occurred during the late Wisconsinan-Holocene deglaciation, and were probably instrumental in isolating shelf sandbodies.

The shoal banks on the north Texas shelf, which have been interpreted as former shorelines, are likely the result of rapid "overstepping" events. Sea level reconstructions based on the position of and dates from these banks may be inaccurate due to later reworking of the banks. A more complete and continuous sedimentary record of the transgession is preserved within the incised valley-fill sequences. By applying high resolution seismic stratigraphic techniques to incised valley-fill sequences, episodic sea level events (manifested as discontinuities in estuarine evolution) can be accurately dated and related to shelf sand deposits (banks).

The results of drilling on the Antarctic continental shelf indicate that marine ice sheets have existed in Antarctica since at least early Oligocene time. So, the episodic glacial-eustatic mechanism has been operative for at least that long. While the processes responsible for delivering sands to the offshore environment may not have changed over geologic time; the frequency and rate of eustatic variation has changed and surely has 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 versus non-glacial periods of geologic time.