Origin of limestone-marlstone cycles: Astronomic forcing of organic-rich sedimentary rocks from the Cenomanian to early Coniacian of the Cretaceous Western Interior Seaway, USA

We present an integrated multidisciplinary study of limestone-marlstone couplets from a continuously cored section including parts of the upper Buda Limestone, the entire Eagle Ford Group (Boquillas Formation) and lower Austin Chalk from the Shell Iona-1 research borehole (Texas, USA), which provides a >8 million year (myr) distal, clastic sediment-starved, intrashelf basin record of the early Cenomanian to the earliest Coniacian Stages. Results show that despite variable yet minimal diagenetic overprints, several unambiguous primary environmental signals are preserved and support greater water-mass ventilation and current activity promoting increased silica/carbonate productivity during the deposition of limestone beds compared to deposition of marlstone beds which reflect greater organic matter productivity and preservation. Furthermore, our astronomical analyses demonstrate that the limestone-marlstone couplets 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 the latitudinal distribution of solar insolation may have been responsible for the observed lithological and environmental variations through the Cenomanian, Turonian and Coniacian in this mid-latitude epicontinental sea setting. Our data also suggests that rhythmic lithological alternations deposited in Greenhouse periods, in general, may simply reflect climate-driven cycles related to Earth-Sun dynamics without the need to invoke significant sea-level variations. (C) 2015 Elsevier B.V. All rights reserved.