Orbital forcing of Upper Jurassic (Tithonian) shallow-water carbonates, Tethyan Adriatic Platform, Croatia evaluated using synthetic vs. real data sets

Abstract

A continuing debate surrounds the relative dominance of random vs. Milankovitch (astronomical) forcing of the stratigraphic record, particularly in the genesis of shallow-water carbonate platform successions. Using time series analysis, we examined a ∼ 7 myr duration, 700 m thick, internally conformable Upper Jurassic (Tithonian) section from the Adriatic Platform, southern Croatia. The Tithonian succession has a lower interval of dominantly subtidal parasequences and an upper interval of oolitic, laminite-capped peritidal parasequences. We combined this investigation with an analysis of synthetic data sets of water depth derived from numerical models of carbonate sedimentation containing differing amounts of random variance with superimposed Milankovitch forcing. Average Spectral Misfit (ASM) was calculated for each synthetic time series of preserved water depth and our Tithonian data to determine the most likely sedimentation rates and significance levels for rejection of the null hypothesis of no astronomical forcing. The ASM-determined sedimentation rates could then be compared to known long-term rates to again reject or accept the null hypothesis of no astronomical forcing. Using this approach, random synthetic data series showed considerable difference from astronomically-forced synthetic data sets. Multitaper method (MTM) spectral analysis of the Tithonian Dunham rank series tuned to the 100 kyr eccentricity cycle shows variable forcing by short (100 kyr) eccentricity, obliquity, and precession. The Tithonian parasequence thickness series was examined using wavelet analysis for bundling of parasequences and showed that long- (405 kyr), short-eccentricity and obliquity, plus obliquity modulation (∼200 kyr) influenced parasequence bundling. The results indicate that the ASM analysis can differentiate between successions dominated by random processes and those strongly influenced by Milankovitch processes.