Cyclostratigraphy of the Kimmeridge Clay Formation (Late Jurassic) in Southern England
The Late Jurassic Kimmeridge Clay Formation (KCF) is an important regional organic-rich mudstone of Late Jurassic (Late Kimmeridgian-Early Tithonian) age which has long been recognized as the major petroleum source rock in the North Sea. The main rock types of the KCF in Dorset, UK, are medium to dark grey marl, dark grey shale, dark grey to black laminated shale, pale grey coccolithic limestone, and grey to pale yellow limestone and dolostone; this lithologic variation occurs with an obvious sedimentary cyclicity. Untuned total organic carbon (TOC) and formation microscanner (FMS) data both show very strong ~8 m and ~32 m bundled cycles in the Lower Kimmeridgian which respectively correspond to ~100 kyr and ~405 kyr eccentricity when calibrating the ~32 m cycles to the 405 kyr long-eccentricity cycle. Power spectra of the untuned data further define a hierarchy of cycles throughout the KCF with ~36 m, ~10 m, ~3.7 m and ~1.6 m wavelengths. Assigning the 32 m cycles to the 405 kyr long-eccentricity in both the TOC and FMS data clearly reveals the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. There is a very strong 405 kyr long-eccentricity peak (the result of the tuning), ~100 kyr short-eccentricity, ~40 kyr obliquity, and ~22 kyr and 20 kyr precession cycles. The power in the precession band is especially strong. The tuned duration of the Upper KCF (lower part of the Tithonian Stage, Fittoni to Elegans ammonite zones) is 3.25 Myr and the Lower KCF (Kimmeridgian Stage) is 3.63 Myr. The results demonstrate that the short ~100 kyr and long 405 kyr eccentricity cycles in the TOC and FMS are strongly expressed in the KCF, particularly in its lower part. Small-scale cycles interpreted as precession cycles are also well expressed in the lower part of the formation, and are clearly evident in filtered datasets where they bundle into the eccentricity cycles. Our results differ significantly with those of Weedon et al. (2004), who believed that there was no evidence for 405 kyr long eccentricity cycles in these same KCF data, and instead interpreted obliquity-dominated sea-level and climate change.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009