Sequence and Cyclostratigraphy of the Basal Cambrian (series 2) Xinji Formation in North China

The basal Cambrian (Series 2) Xinji Formation in North China is a continental-scale stable stratigraphic unit that resides on the Great Unconformity and preserves abundant phosphatized small shelly fossils. High-resolution cyclostratigraphy within the solid sequence stratigraphic framework enable exploration of continuous cyclicity and possible orbital forcing during the peak Cambrian explosion. Here we summarize a field-based study of the regional sequence stratigraphy, and report astronomical signals measured by portable X-Ray Fluorescence Spectroscopy and Magnetic Susceptibility (MS) Meter from a drill core sited near the weathered stratotype section with a resolution of 5 cm. Outcrop and drill core studies show that regional sequence boundaries only developed at the base and top, and can be traced over a distance of 800 km, suggesting relatively stratigraphic continuity of the Xinji Formation. Spectral analysis of the geochemical and geophysical variations recorded by Ti content and MS yield a comparable pattern of orbital cyclicity. The ratios between thickness of observed cycles fit with the known Milankovitch periods, suggesting that the most likely match to the observed stable cycle with a thickness of around 19.31 m correspond to the long eccentricity of 405-kyr. Furthermore, the cyclostratigraphically constrained duration of ~0.938 Myr for a third-order depositional sequence within the Xinji Formation is significantly shorter than the ~5 Myr interval delineated for the rapid, craton-wide Sauk II transgression in North America.