Fossil contourites: a critical review

Abstract

Despite three decades of study, there is still great controversy over the recognition and interpretation of fossil contourites exposed in ancient series on land. In order to best examine this problem, we briefly review the evidence from modern systems, including the many examples of Cenozoic contourites that have been recovered from DSDP/ODP drilling on major drifts in the present-day oceans. The range of contourite facies described from both deep-water (>2000 m) and mid-water (300–2000 m) drifts are mostly fine-grained, bioturbated and homogeneous, often with a distinct bedding cyclicity, and with some coarser-grained sandy contourites developed under higher-energy bottom currents. There are also a number of current-controlled sediment bodies that have formed in outer shelf/upper slope settings (50–300 m) under the influence of counter currents, underflows and major surface currents. These are not considered contourites sensu stricto, but may be mistaken as such in ancient examples. The most commonly described fossil contourites in the literature have been interpreted by the authors concerned as bottom-current reworked turbidites. However, a critical review suggests that these are the facies most subject to misinterpretation and many of the sediments claimed as fossil contourites are almost certainly fine-grained turbidites, whereas others were more likely formed under outer shelf/upper slope current systems. There remain very few ancient examples that are more closely comparable to modern contourites; these include the Cretaceous Talme Yafe Formation in Israel, the Ordovician Jiuxi Drift in China, and parts of the Paleogene Lefkara Formation, Cyprus and the Neogene Misaki Formation in Japan. We present a set of possible criteria for the recognition of fossil contourites and bottom-current reworked turbidites.