HERBERT, TIMOTHY D., Scripps Inst. Oceanography, La Jolla CA; I. PREMOLI SILVA and E. Erba, Univ. Milan, Milan, Italy; S. L. D'HONDT, Univ. Rhode Island, Kingston, RI; A. G. FISCHER, Univ. Southern California, Los Angeles, CA; and J. PARK, Yale Univ., CT

ABSTRACT: Orbital Chronometry of Cretaceous and Early Paleogene Strata

The rarity of biostratigraphically useful, reliable, radiometric dates of Cretaceous age, and the longer duration of magnetic polarity zones relative to the Cenozoic, severely limit the time resolution of Cretaceous strata. Many deep-sea sections of mid-Cretaceous to early Paleogene age may contain an internal clock in the form of rhythmic carbonate or other sedimentary oscillations. The periods of these cycles consistently match predicted periods of the earth's orbital elements to within the error of independent

dating (10-50%, depending on the time interval), and show frequency ratios characteristic of orbital cycles. Because the mean periods of astronomical cycles can be calculated by celestial mechanics to within a few percent uncertainty, they measure out elapsed time in suitable stratigraphic sections to a new precision.

Time-series analyses from three intervals of the Cretaceous will be presented, along with their chronometric implications. Campanian-early Paleocene DSDP/ODP coring locations show cyclic variations in carbonate content whose mean periods can be estimated from magnetic reversal stratigraphy to be very close to that expected of the earth's precessional (21 kyr) cycle. Placed in the paleomagnetic framework, the cycles offer a high resolution chronology of elapsed time across the K/T extinction boundary, and of evolutionary recovery after the event. Periods of Barremian black shale-carbonate cycles can be calibrated by the upper M-series timescale at approximately 20 and 100 kyr repeat times. An accurate precessional and eccentricity "cyclostratigraphic" determination of Tethyan foraminif ral and nannofossil zonal durations within the Long Normal interval appears possible.

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