Origin and palaeogeographical distribution of the Viruan iron and phosphate ooids in Estonia: evidence from mineralogical and chemical compositions

The occurrence of iron ooid beds in the Ordovician sequence of Baltoscandia has been known for a long time. In Estonia, the best developed ooid beds are known to occur at two levels in the Lower (Oelandian) to Middle Ordovician (Viruan) boundary interval, being restricted to the Kunda and Aseri-Lasnamägi Stages, respectively. According to outcrop and subsurface data, these ooid beds can be traced from Hiiumaa island, Estonia to the St. Petersburg area, Russia, in the east. The geographic and stratigraphic distribution of the Viruan oolite body indicates that the source material was derived from a land area located northwest to north-northwest of Tallinn. The presence of well developed hardgrounds at the base of Viruan oolite suggests that ooid formation took place during a deepening event. Two mineralogically different ooid groups have been identified in this study—brown ooids, composed of iron oxy-hydroxides and apatite, and light-grey ooids, composed primarily of carbonate-fluorapatite. The latter probably formed through early diagenetic alteration of the brown ooids. Brown ooids contain over 60% of iron oxy-hydroxide and remnants of an unidentified silicate mineral. The concentric lamination is composed of alternating laminae of phosphate (francolite) and iron oxy-hydroxide. The trace element abundances in the ooids are strongly influenced by the phosphate content, as is the observed REE distribution pattern. The chondrite-normalized REE patterns are characterized by enrichment of the lighter REE, and have a typical negative Eu-anomaly of about 0.7. Shale-normalized REE patterns have a “hat shape” similar to REE patterns found in conodonts and other biogenic apatite.

Thin volcanic ash beds have been recorded at this level in Sweden. In Estonia, however, there are no visible indications of volcanic material.