Deeply incised and backfilled paleo-canyons in lower Paleogene shelf strata along the western and northern Gulf of Mexico margin attest to large relative sea-level fluctuations but pre-date the accepted age for the onset of Cenozoic continental glaciation. Using Pleistocene canyons as a crude yardstick, the scale of these paleo-canyons suggests relative sea-level changes at least as large as the Pleistocene fluctuations. Therefore, we speculate that the water level in the Gulf of Mexico was drawn down while the Gulf was isolated from the world ocean during the Late Paleocene/Early Eocene interval. We suggest that the cause for isolation was the progressive collision of the Cuban arc with the Yucatan and Bahamas carbonate platforms, thereby temporarily closing off the southeastern Gulf of Mexico. In the Miocene Mediterranean and the Holocene Black Sea examples of marine basin isolation, evaporation greatly exceeded rainfall and runoff, and our examination of the Gulf of Mexico case suggests that the water level may have dropped at least once by at least several hundred meters, and possibly much more, below the level of the world ocean.

Implications for geology and hydrocarbon exploration in the Gulf may include

triggering and/or acceleration of salt evacuation (basinward “squeegee” effect of slumping sediments);

release of gas hydrates from sediments under shallower and warmer water, thereby contributing to the ~100,000-year-long worldwide Paleocene/Eocene boundary heating event;

The recognition that the early Paleogene relative sea-level changes seen in the Gulf may pertain to basin isolation is grounds for treating “eustatic” curves derived for or from the Gulf with suspicion. In addition, catastrophic basinward transfer and collapse of mass near the shelf edges would have caused isostatic unloading (rebound) of shelf margins that was proportional to the mass transfer. In the case of a discreet slumping event, such as the Lavaca “Megaslump” event of south Texas, this effect may have caused uplift of several to a few tens of meters of footwall areas within about 100 km from the slump. Larger downslope movements such as that related to the collective Wilcox fault province would have caused far larger isostatic rebounds on the shelf, perhaps in excess of 100 m if sedimentation did not keep pace with faulting.

A body of circumstantial evidence continues to grow in support of this hypothesis, whose potential implications, both academic and commercial, merit further

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investigation. Integration of information from Cuba, Mexico, the United States, and the Bahamas will be required to fully test the hypothesis.

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