Lower Ordovician iron ooids and associated oolitic clays in Russia and Estonia: a clue to the origin of iron oolites?

On the Baltic platform a lower Llanvirn (Ordovician) iron oolite can be tra ced for a distance of 1200 km from Norway to the east of Lake Ladoga in Rus sia. This oolite is usually thin (seldom exceeding 0.5 m) and is dominated by goethite (limonite) type ooids. The easternmost part of the oolite, from Tallinn to Ladoga, is examined here. The oolitic limestone is intercalated with oolitic clay beds. The mineralogical, chemical and isotopic compositi on and other indicators point to volcanic ash being the source for the clay . Similarities in REE distribution patterns and immobile element contents b etween ooids and the oolitic clay suggest that the ooids were also formed f rom volcanic ash. (C) 1999 Elsevier Science B.V. All rights reserved.