ABSTRACT: Magnetostratigraphic Dating of Pliocene-Pleistocene Seismic Sequence Boundaries, Gulf of Mexico

Dietmar Schumacher

System tracts and their sequence boundaries are dated paleontologically using a series of foraminiferal and nannoplankton species. Although this practice generally provides satisfactory results, it does lead to occasional ambiguity due to the absence of diagnostic species or conflicting age determinations. In the Gulf of Mexico, such uncertainties can result in significant miscorrelation of sand-prone intervals and can seriously impact both the risk and cost of deep-water exploration and development.

The frequency of magnetic reversals during the Pliocene-Pleistocene has potential to form an ideal time scale for correlation and absolute age dating of seismic sequences. More than 20 major and minor magnetic reversals have occurred since the beginning of the Pliocene, 5.4 Ma. More significantly, each of seven major sequence boundaries appears to coincide with a paleomagnetic reversal. For example, sequence boundaries at 0.8, 1.6, and 2.4 Ma are associated with magnetic reversals at 0.73, 1.61, and 2.45 Ma. The 3.0, 3.8, 4.2, and 5.4-Ma sequence boundaries are equally close to magnetic reversals.

The near coincidence of sequence boundaries and magnetic reversal is remarkable but may not be entirely fortuitous. Recent research has suggested that sudden changes in global climates and sea level might trigger reversals of the earth's magnetic field. The required sea level changes are of the same magnitude believed responsible for the formation of sequence boundaries. Preliminary magnetostratigraphic results are encouraging and suggest that direct correlation of seismic sequences with the magnetic polarity time scale is an attainable goal.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990