Cyclostratigraphy of the Middle Jurassic Aalenian Sha-1 member lacustrine record in central Sichuan Basin, southwestern China

Abstract

Cyclostratigraphic studies enable to reconstruct the geological timescales of many marine formations. However, these investigations are still lacking a precise astronomical tuning in lacustrine environments, particularly in the Middle Jurassic, due to the paucity of biological traces and the lack of data. In this study, detailed spectral analyses of natural gamma ray (GR) logging data were conducted on the lacustrine Sha-1 member from two wells in the Sichuan Basin, southwestern China. Core data samples were calibrated to test the lithological changes for proceeding with cyclostratigraphic analysis with more confidence. The spectral analysis and evolutionary spectral analysis of the GR logs reveal that the wavelength ratios of the stratigraphic cycles are ∼20:5:2:1, being consistent with the period ratios of astronomical cycles (long eccentricity, short eccentricity, obliquity, and precession). Correlation coefficient (COCO) analyses were applied to quantitatively measure the fitting of the witnessed sedimentary cycles to astronomical periods and provide a possible sedimentation rate range. The results show that astronomical time scale of the studied interval comprises 58.4–60.6 m cycles, representing the 405 kyr eccentricity cycles, and 14.9–16.6 m cycles, representing the ∼100 kyr eccentricity cycles. This study reveals a ∼2.43 Myr duration for the Sha-1 member. Sedimentary noise modeling reveals that long-term million-year period (1.2 Myr) astronomical forcing may have been a significant driver of lake-level changes in the Sichuan Basin. Moreover, high-frequency (405 kyr scale) lake-level variations associated with the mid-term base-level cycles (4th-order sequences) were linked to climato-eustatic changes. This study suggests comparison between parameters of cyclostratigraphic records and sequence stratigraphic base-level cycles in lacustrine sediments. The stratigraphic surfaces of mid-term base-level cycles correspond to minima ∼405 kyr-long eccentricity cycle curve and high values of DYNOT (dynamic noise after orbital tuning). Integrating cyclostratigraphy with the base-level cycles is, therefore, a vital approach for defining the short-term oscillations of lake-level, and proved to be a useful tool for characterizing thin-bedded lacustrine reservoirs.