Manganese nodules in the Miocene Bahía Inglesa Formation, north-central Chile: Petrography, geochemistry, genesis and palaeoceanographic significance

Abstract

Manganese nodules recovered from two stratigraphic horizons of Tortonian–Messinian (late Miocene) age in the Bahía Inglesa Formation of north-central Chile were studied using XRD, SEM and geochemical analysis. The dominant mineral in the nodules is todorokite, which suggests a diagenetic, marine environment. This is supported by field observations of nodules replacing Ophiomorpha burrows. Preliminary, traditional statistical analysis of the nodule geochemistry, including single element, binary and ternary ratios, suggests that the nodules are of the supergene, deep marine type, as also indicated by the presence of foraminifers typical of the upper continental slope, as well as debris flow and turbidity current deposits in an associated submarine palaeocanyon. However, abnormally low Cu concentrations seem to contradict this interpretation, so that additional analyses were carried out. This included multiple discriminant analysis (MDA), as well as a new technique applied for the first time to manganese nodules, namely artificial neural network analysis (ANN). In both methods central log-ratio (CLR) normalization was applied to the raw data. The results, in particular those of the ANN analysis, suggest that the Bahia Inglesa nodules present a chemical signature distinct from that of nodules described to date. A new class is therefore proposed, namely supergene intermediate marine (partially restricted basin).