Milankovitch climatic signals in Lower Triassic (Olenekian) peritidal carbonate successions, Nanpanjiang Basin, South China

Meter-scale peritidal carbonate strata from an isolated platform in the Nanpanjiang Basin of South China were tested for the presence of periodic or quasi-periodic climatic signals. These signals provide information on factors controlling cyclic sedimentation in the equatorial eastern Tethys during the Early Triassic greenhouse period. Parasequences are composed of shallowing-upward successions of subtidal facies, including skeletal packstone and grainstone, calcimicrobial boundstone, and oolitic packstone and grainstone, intertidal laminated lutite and ribbon rock, and rare supratidal microbial laminite. Parasequence stacking patterns display three orders of cyclicity, suggesting hierarchical stratigraphic relationships. Gamma analysis of Kominz and Bond [Earth Planet. Sci. Lett. 48 (1990) 233^244] was used to estimate facies-dependent thickness^time conversion factors (i.e. Qs), and to construct Q-tuned and Q-untuned time series for two stratigraphic sections. Spectra of these time series indicate the presence of quasi-periodic signals, with prominent short-eccentricity (94.9^131.2 kyr), short-obliquity (35.8 kyr), and longprecessional index (21.2 kyr) peaks, and minor long-eccentricity (412.9 kyr), long-obliquity (45.3 kyr), shortprecessional index (17.7 kyr), and constructional-tone (9.7 kyr) peaks when calibrated on the Q-tuned spectra. Thus, Milankovitch climatic forcing probably greatly influenced sedimentation. The calibration indicates that the subtidal facies has a sedimentation rate of 24.6^30.7 cm/kyr and the intertidal^supratidal facies has a rate of 2.7^6.0 cm/kyr. The estimated duration of deposition of the two sections is 1139^1423 kyr, and corresponds to a stratigraphic completeness of 32^92%. The completeness is much greater than that of icehouse stratigraphic records. We speculate that variations in carbonate productivity and environmental conditions driven by Milankovitch climatic forcing, combined with low-amplitude sea-level fluctuations, were likely major controls on cyclic sedimentation. Furthermore, evolutional spectra of the two sections show that dominant Milankovitch climatic forcings varied from shorteccentricity, obliquity, to long-precessional index during the course of sedimentation. Some spectral differences between the two sections suggest variations in depositional/recording mechanisms of Milankovitch climatic signals between platform interior and windward platform margin. C 2003 Elsevier B.V. All rights reserved.