The stratotype of the Orasoja Member (Tremadocian, Northeast Estonia): lithology, mineralogy, and biostratigraphy

The Orasoja Member constitutes in Northeast Estonia the upper part of the Kallavere Formation and forms some kind of transition into the Türisalu Formation. The member belongs to the Lower Ordovician part (Karepa Substage) of the Pakerort Stage. The Orasoja Member is composed of alternating beds of light quartzose siltstone and black shale. The share of shale is cyclically variable, but mainly siltstone is prevailing. Mineralogically quartz is dominant, K-feldspar (up to 15%) forming a minor component of the rock. Mineral and grain size composition allows us to distinguish two fairly distinct cycles in the Orasoja Member, which are in correlation with changes in conodont associations. Three conodont biozones are distinguished in the section: Cordylodus proavus, C. lindstromi, and C. angulatus. The first appearance of Iapetognathus sp. in the Rannu Member is interpreted here as indicating the beginning of the Ordovician System. Graptolite occurrences give evidence of the presence of the Rhabdinopora flabelliformis socialis and R. f. anglica subzones. Earlier dendroids are missing, but this is mainly attributed to a taphonomical bias.