Identification of Milankovitch Signals in Middle Triassic Platform Carbonate Cycles Using a Super-Resolution Spectral Technique

L. Hinnov, R. K. Goldhammer

The Middle Triassic Latemar carbonate buildup of the Dolomites (northern Italy) contains 500 meter-scale cycles (average 0.65 m/cycle) each composed of subtidal deposits overlain by a thin vadose diagenetic cap. The cycles record depositional and early diagenetic responses to glacio-eustatic sea level oscillations with superimposed ~20,000 year and ~100,000 year periodicities. Previously, documentation of these Milankovitch periodicities was limited to autocorrelation analyses of truncated sequences (i.e., < 45 consecutive cycles) of cycle thicknesses and modeling sedimentation dynamics.

To improve the resolution and statistical measure of nonrandom variability in cycle thicknesses, we approach Latemar cyclicity using a spectral technique that produces super-resolution spectra of discrete time series. The technique differs from other spectrum estimation procedures in that the time series is "multitapered" as a consequence of expanding its Fourier transform by an ordered set of spheroidal functions. The resulting high-resolution spectrum estimates have a greatly enhanced statistical stability that permits the detection of extremely narrow-band signals in noisy data sets that are likely to be missed by traditional spectral analysis.

The application of this multitapering algorithm on 140 consecutive Latemar cycles reveals nine statistically significant harmonies controlling cycle development, all with periodicities corresponding to those of Berger's modeled series for the Earth's orbital eccentricity and axial tilt. We believe this to be the first report of evidence for multiple, superimposed high-resolution Milankovitch spectra in the pre-Pleistocene shallow marine, platform carbonate record.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.