Origin and diagenesis of the organic matter in Miocene freshwater lacustrine phosphates (Cerdanya Basin, Eastern Pyrenees)

Abstract

A suite of samples from the lacustrine facies of the Cerdanya Basin (Miocene) containing early diagenetic phosphatized sediments in several horizons has been examined for phosphate mineralogy and organic content. The phosphates consisted of anapaite and farifeldite occurring as veins, spaherulite beds and septarian-like nodules. The hydrocarbon and fatty acid distributions of the samples studied show that terrestrial inputs were generally predominant throughout the sedimentary record. Furthermore, the good correlation of higher molecular weight n-alkanes and n-alkanoic acids with total organic carbon (TOC) suggests that the productivity of the lake was determined to a large extent by the amounts of detrital organic matter entering into the system. Thus, the phosphate accumulations studied here are different from those of marine origin usually described in the literature. However, several common characteristics are found when comparing these terrestrial phosphate deposits with those of marine origin, namely an intermediate TOC content (0.5–1.3%) and important microbial inputs, suggesting that phosphatogenesis occurred at rather mild oxidizing/reducing conditions and was mediated by intense microbial degradation. Furthermore, the results presented here extend previous observations from marine phosphate basin studies in the sense that phosphate generation seems to be a phenomenon much more dependent on the environmental conditions of deposition and early diagenetic processes rather than on a specific type of sediment or organic matter source.