ORIGIN OF CRETACEOUS LIMESTONE-MARL CYCLES: ORBITAL FORCING OF CENOMANIAN-TURONIAN ORGANIC-RICH SEDIMENTARY ROCKS, EAGLE FORD FORMATION, TX, USA

Hemi-pelagic and pelagic sequences from Phanerozoic epicontinental seas are often characterized by rhythmically inter-bedded limestones and marls, and have been the focus of considerable research and debate over the last century. In many cases, their rhythmic appearance at the 0.5-1 meter parasequence scale has been interpreted to result from orbital forcing, although many alternative mechanisms have also been proposed. Unequivocal proof of a primary depositional origin of the rhythmic intercalation of the two lithologies has been difficult due to potentially significant diagenetic overprints observed on these sedimentary sequences and the lack of suitably precise absolute-age control. Here we present a fully integrated multidisciplinary study of limestone-marl couplets from a continuous and relatively expanded section of the Eagle Ford Formation (Texas, USA) from the Shell Iona-1 research core, which provides a >8 m.yr. distal intrashelf basin record of the earliest Cenomanian to the earliest Coniacian stages. Results show that despite a mild diagenetic overprint, several unambiguous primary environmental signals are preserved and support greater water-mass ventilation, oxygenation, and current activity during the depositional of limestones compared to marls. Furthermore, our astronomical analyses demonstrate that the limestone and marl cycles in the Iona-1 core reflect climatic forcing driven by solar insolation resulting from integrated Milankovitch periodicities. In particular, we propose that obliquity and precession forcing on summer insolation and its impact on seasonality may have been responsible for the observed lithologic and environmental variations through the Cenomanian, Turonian and Coniacian in this mid-latitude epicontinental shelf setting. Our data also suggests that rhythmic lithologic alternations in greenhouse periods, in general, may simply reflect climate-driven cycles related to planetary and Earth-Sun dynamics without the need to invoke significant sea-level variations.