ABSTRACT

Volcanic glass occurring below the base of the Pleistocene in the Gulf of Mexico has been dated at 3.10 ± 0.43 m.y. by the fission-track method.

The Gulf of Mexico ash is from an Exxon Company, U.S.A. offshore Louisiana well at a depth of about 5,000 feet. Stratigraphically, the ash occurs about 1,000 feet below the base of a prominent marine shale section (Upper Marine Shale). Authors, citing different paleontologic criteria, consider that the Pliocene-Pleistocene boundary occurs either above the base, near the base, or below the base of this important, regionally transgressive, shale section.

Biostratigraphically, the ash occurs about 260 feet above the extinction level of Globigerina nepenthes (3.7 m.y.), about 220 feet below the projected extinction level of Globoquadrina altispira (2.8 m.y.) and about 1,000 feet below the extinction level of Globorotalia miocenica (2.2 m.y.) near the base of the "Upper Marine Shale" section. The ash dated at 3.1 m.y., therefore, documents for the first time the numerical age of the very late Pliocene in the Gulf of Mexico and validates the paleontological correlation with deep-sea paleomagnetic stratigraphy.