Effect and Mechanism of Episodic Sea Level Events: Record Preserved in Late Wisconsinan-Holocene Incised Valley-Fill Sequences

Mark A. Thomas, John B. Anderson

Late Wisconsinan-Holocene sea level curves have been considered as regional phenomena, mainly because geoidal and hydroisostatic effects cause worldwide sea level changes to be nonuniform. In spite of this observation, recent sea level curves suggest a global pattern of episodic sea level events occurring throughout the last post-glacial transgression. Evidence from the northern Gulf of Mexico continental shelf supports an episodic model. These episodic eustatic events must have had a profound impact on the evolution of coasts and estuaries. The consequences of episodic events are manifested in the sedimentary record as rapid translations of the shoreline, displacement of estuaries, generation of shelf sand bodies, and discontinuities in 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 drawdown of marine ice sheets and the associated eustatic rise in sea level during any single event was probably on the order of 5 to 10 m in a few centuries. On a low-gradient shelf such as the North Texas shelf, the shoreline would translate up to 25 km. Several episodic eustatic events are believed to have occurred during the late Wisconsinan-Holocene and were probably instrumental in isolating shelf sand bodies.

The bathymetric 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 transgression is preserved in the incised valley-fill sequences. By applying high-resolution seismic and 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).

Because marine ice sheets have existed in Antarctica since at least early Miocene time, the episodic glacial-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 surely have 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 #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.