Iron ooids in the lower Ordovician Huk formation, Mj⊘sa district, Norway

Abstract

Abstract Most ooids in an iron oolitic limestone of Llanvirnian age from the Mj⊘sa district, Norway, have been diagenetically altered, and the original iron silicate (berthierine) has been replaced by carbonate minerals such as calcite, ferroan calcite, and ankerite. Remnants of the original ooid lamination and nuclei show similarities with chamositic iron ooids in Sweden. REE pattern of the ooids shows a pronounced negative Eu anomaly and an enrichment of LREE, similar to most chamosite ooids in Baltoscandia. Low oxygen isotope values of the carbonates indicate low‐temperature interaction with unstable volcanic ash. The oolite was deposited in a period of iron oolite formation of short duration, which created coeval oolites not only in Baltoscandia but also in many places in western and central Europe. It is suggested that the Mj⊘sa oolite has a volcanic origin. Sturesson, U., 1994: Iron ooids in the Lower Ordovician Huk Formation, Mj⊘sa district, Norway. GFF, Vol. 116 (Pt. 4, December), pp. 249–253. Stockholm. ISSN 1103–5897.