Ichnofabric logs and the geochemistry of sapropels (Miocene, Maldives)

Abstract

An ichnofabric log analysis was carried out on the organic-rich sediments (sapropels) alternating with organic poor oceanic carbonates in Late Oligocene and Early Miocene strata in the Maldives Archipelago with the objective of determining the utility of such logs in tracing paleoenvironmental changes recognized by well-established geochemical proxies. Ichnofabric logs proved to correlate well with natural gamma radiation logs (NGR), which in the study case are indicative of the amount of preserved organic matter in the sediment. However, the relationship between different geochemical proxies for paleo-oxygenation/organic-matter preservation in the sediment and the trace-fossil content was still to be determined. The trace-fossil assemblage of the sapropels consists of Thalassinoides, Planolites, Palaeophycus, Zoophycos, Chondrites, Phycosiphon, and Scolicia. This ichnoassemblage is characteristic of Zoophycos ichnofacies and in the study section was attributed to five ichnofabrics according to the intensity of bioturbation, main ichnotaxa and type of sediment. The ichnofabrics characterized by high bioturbation indices correspond to intervals with white sediment and low content in organic carbon, <0.1%. The ichnofabrics characterized by low bioturbation indices and low diversity of ichnotaxa are coincident with organic-rich intervals containing up to 30% organic carbon and trace element signatures distinctive of anoxic bottom waters (Al, Ba, Fe, K, Mo, S, and U). The presence of discrete burrows within the black beds is the result of high-frequency seafloor ventilation superimposed to the orbital scale controlling the occurrence of sapropelic intervals. The ichnofabric logs proved to be a valuable tool to reconstruct in detail the evolution of paleo-oxygenation of sapropels.