Genesis and Diagenesis of the Phosphatic Hardground of the Holm Dal Formation (Late Middle Cambrian), North Greenland

Abstract: The phosphatic hardground of the Holm Dal Formation (late Middle Cambrian) of North Greenland is composed of two parts: one basal with abundant allochthonous material and an upper part without an allochthonous component. It was formed by release of phosphate, by bacterial degradation, from organic matter which accumulated in lime mudstones on which the hardground developed.Following its release to pore water, the phosphate precipitated as apatite forming the hardground, in which cyanobacterial assemblages fossilized. Echinoderm holdfasts are a conspicuous feature of the hardground.REE analysis and the well preserved cyanobacterial fossils point towards an early diagenesis. The same REE analysis and the existence of bitumen in the samples, indicate dysaerobic conditions during the formation of the hardground.Isotopes analysis (143Nd and 144Nd) underline the marine biogenic character of the apatite.