Geophysical logs as proxies for cyclostratigraphy: Sensitivity evaluation, proxy selection, and paleoclimatic interpretation

Abstract

Correct selection and interpretation of proxies are key to cyclostratigraphic research; however, the uncritical use of geophysical logs as proxies for cyclostratigraphy could be widespread. In most studies, paleoclimate changes are directly interpreted based on the spectra of single geophysical log curves, with little attention being paid to the sensitivity of geophysical logs to the paleoclimate. This practice could lead to systematic bias and misleading conclusions in our understanding of the system of ancient Earth. Accordingly, we first summarized the analysis methods of log cyclostratigraphy and the factors influencing geophysical logs and, subsequently, reviewed the response mechanism and sensitivity of geophysical logs to astronomically forced climate. Further, we conducted systematic evaluations of geophysical logs from the SK-2 borehole in the Songliao Basin (Northeast China) and those of the Newark Basin Coring Project (NBCP) in the Newark Basin (USA) as proxies for cyclostratigraphy by combining spectrum classification and power ratio accumulation analysis. Based on a comprehensive analysis of previous research and two case studies, we discuss the premises and pitfalls of using natural gamma-ray, spectral gamma-ray, porosity, resistivity, and caliper logs as proxies for cyclostratigraphy. The sensitivity of geophysical logs to astronomically forced climate could be affected by events unrelated to such astronomically forced climate, the instability of phase relationships caused by multiple astronomically forced geological processes or diagenetic–epigenetic features, and the wellbore environment. Sensitivity evaluation of multiple geophysical logs and proxy optimization help ensure the accuracy of paleoclimatic interpretation in log cyclostratigraphy research. This study provides valuable insights into log cyclostratigraphic practices and promotes the development of log cyclostratigraphy in an appropriate direction.