Deep-marine environments of the Middle Eocene Upper Hecho Group, Spanish Pyrenees: Introduction

The Middle Eocene sediments of the Ainsa Basin, Spanish Pyrenees, provide world class outcrops and a natural laboratory in which to study ancient deep-marine sedimentary environments and the controls on deposition. Over the past 50 years, the Ainsa outcrops have been extensively studied and visited by countless students, academics and hydrocarbon industry personnel, all wishing to understand various aspects of deep-water sedimentology and stratigraphy, as well as those studying the orogenic processes that created the Pyrenean mountain belt. Throughout the Pyrenees and into the Basque Basin, subaerial, shallow- and deep-marine sedimentary rocks of the same age as those in the Ainsa Basin are well preserved and exposed along a transect that includes an eroding mountain belt, coastal plain, shelf and deep-marine environments connected farther west with the open Atlantic Ocean basin. This, therefore, provides one of the best natural laboratories globally for investigating source-to-sink processes and controls on deposition. The Middle Eocene is also a particularly dynamic period in Earth history as it represents the time when global climate showed a significant long-term shift from greenhouse to icehouse conditions. Considerable controversy exists as to the presence of long-lived versus more transient continental ice sheets and therefore, whether glacio-eustasy could have been a possible control on stratigraphic architecture (e.g., Pekar et al., 2005; Miller et al., 2011). There is also little agreement about the relative importance of tectonics, global climate change (e.g., Milankovitch cyclicity) and/or autocyclic processes in driving deposition in what was clearly a tectonically very active convergent orogenic system.