Ichnological analysis of the Cenomanian–Turonian boundary interval in a collapsing slope setting: A case from the Rio Fardes section, southern Spain

Abstract

The Cenomanian–Turonian Boundary interval (CTB) at the Rio Fardes section (Betic Cordillera, southern Spain) is characterized by pelagic and hemipelagic deposition mixed with mass-flow conglomerates and sandstones. The Rio Fardes section lends itself to high-resolution ichnological analysis of the late Cenomanian Oceanic Anoxic Event (OAE-2) on a collapsing slope affected by gravity-flow sedimentation. The trace fossils include Alcyonidiopsis isp. A., Alcyonidiopsis isp. B., Chondrites intricatus, C. stellaris, C. targionii, Planolites isp., ?Pilichnus isp., Thalassinoides isp., and Trichichnus isp. This ichnoassemblage differs from most CTB sections given the presence of small Alcyonidiopsis isp., Chondrites stellaris, and ?Pilichnus isp., and the absence of Zoophycos. The particular depositional setting, affected by coarse clastic gravity flow sedimentation, may be responsible for these differences by affecting palaeoenvironmental conditions. The lower part of the Rio Fardes section shows a more or less continuous record of diverse and abundant trace fossils, generally produced under aerobic/low dysaerobic conditions. The middle part is characterized by a diminution in trace fossil diversity and abundance, related to moderately/highly? dysaerobic conditions disrupted by short intervals of anoxia. Both the lower and middle parts represent pre-Bonarelli event conditions. The upper part reflects the most significant change in ichnological characteristics. The absence of discernible trace fossils caused by decreasing oxygenation corresponds to the Bonarelli Level. In comparison with previously studied CTB sections, the lower number and thickness of anoxic layers in the Rio Fardes section point to generally better oxygenation on the sea-floor. A variable influence of factors limiting the macrobenthic trace maker community during the OAE-2 can be related to depositional or palaeogeographic contexts. Oxygenated water in the Rio Fardes section was delivered from shallower areas along with gravity-flow deposits. The usually bioturbated background pelagic or hemipelagic deposits above the coarse gravity-flow deposits suggest rapid re-establishment of the macrobenthic trace maker communities following each mass depositional event.