Advances in characterizing the cyclostratigraphy of binary chert-mudstone lithologic successions, Permian (Roadian-lower Capitanian), Chaohu, Lower Yangtze, South China

Abstract

Successions of Mesozoic and Paleozoic radiolarian-bedded cherts display a “binary” cyclic hierarchy of cm-scale chert and mudstone couplets. Some of these couplet successions contain an apparent imprint of Milankovitch cycles, in which the couplets may reflect changes in silica deposition related to Milankovitch forcing, for example, high insolation for chert deposition, and low insolation for mudstone. The challenge has been how to represent such binary lithologic series in a format that is suitable for signal processing to search for Milankovitch cycles. Here we present an analysis of chert-mudstone binary series by considering, in lieu of a “boxcar” linear interpolation, a “midpoint-triangle” linear interpolation. The procedure is demonstrated first on synthetic data, then on the binary chert-mudstone stratigraphic series of the Permian (Guadalupian: Roadian-lower Capitanian) Gufeng Formation, Chaohu, Lower Yangtze, South China. We assign −1 to the midpoint of each mudstone bed and 1 to the midpoint of each chert bed, then linearly interpolate from midpoint to midpoint along the succession. Time-frequency analysis of the midpoint-triangle series highlights a previously unnoticed stratigraphic alternation of obliquity and precession band power. Finally, we present a revised astrochronology for the Gufeng succession that is anchored at 270 Ma at the base of the Jinogondolella nankingensis conodont zone (base of Roadian stage) and ending at 264.6 Ma in the undifferentiated Follicucullus scholasticus-Ruzhencivisponus uralicas radiolarian zone (mid-Capitanian Stage), for a duration of 5.4 Myr.