OBRADOVICH, JOHN D., U. S. Geological Survey, Denver, CO

ABSTRACT: Advances and Problems in Time Scale Calibration

Recently, tremendous advances in mass spectrometry now allow us to measure precisely isotopic abundances and ratios on increasingly smaller quantities. In addition to the improvements in age measurements, an integrated approach to time-scale studies has evolved. Besides isotopic and biostratigraphic data, use is being made of magnetostratigraphy on sedimentary and igneous rocks and on sea-floor spreading, magnetic lineation patterns.

An example of this approach is the recent time scale of Cande and Kent (CK92), who used a cubic spline function to fit nine calibration points (0-83 Ma) to a composite sea-floor spreading, magnetic lineation profile. However, the small number of calibration points may mask the true rates of sea-floor spreading used to calibrate the paleomagnetic time scale. New isotopic data will be presented that will require significant revisions to this just published time scale for the interval chron 33N to chron 24R.

The astronomical calibration of the late Pliocene-Pleistocene interval based on Milankovitch cyclicities brought into question the radiometric calibration of the Geomagnetic Polarity Time Scale (GPTS). As a result, the GPTS was believed to be too young over the entire interval of 0-2.6 Ma by approximately 5-9%. It was discovered that when Ar{40}-Ar{39} ages on sanidine were compared to conventional K-Ar ages, the Ar{40}-Ar{39} results

were nearly always older. For whatever reason, conventional RF induction heating could not effectively de-gas sanidine. Recent Ar{40}-Ar{39} studies now indicate that a revised GPTS is in near but not perfect accord with the astronomically calibrated time scale.

These and other examples will be discussed in detail.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.