Adavere Stage

by H. Nestor

Original text from: Raukas, A., Teedumäe, A. (eds). 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3. Available online at: sarv.gi.ee/geology.

The Adavere Stage as a stratigraphical unit was established by Schmidt (1858) as the uppermost unit (zone 6) of the group of smooth pentamerids (“Gruppe der glatten Pentameren”). Afterwards it was termed the Esthonus-Schicht (Schmidt 1881), Addifer Formation (Twenhofel 1916), Adavere Stage (Bekker 1922). Kaljo (1962) fitted the upper boundary of the stage with the Llandovery and Wenlock boundary and included in it the marlstones of the present Velise Formation. Recently, Perens (1992) and Nestor (1995a) excluded the Mõhküla beds and replaced them into the Raikküla Stage.

The Päri quarry in western Estonia has been selected as the neostratotype of the stage (Nestor 1993, 1995a) and the Kirikuküla core at the depth of 50.3 m may be treated as the boundary stratotype of the stage. The Adavere Stage is distributed in the southernmost part of Hiiumaa Island, on Saaremaa and Muhu islands and in the southwestern part of mainland Estonia as far as the Viljandi fault. The outcrop extends as a 10—15-km-wide belt from the southernmost Hiiumaa Island and the Soela Strait over Matsalu Bay up to the vicinity of Türi - Vändra being denudated eastwards the Paide-Pärnu belt of disturbances. The main localities are the Saastna coast, Päri quarry, river banks at Päärdu, Jädivere, Velise, Valgu, Vändra and ditches at Lätiküla and Valgu (Fig. 67). The thickness of the stage increases westwards — from 10.7 m in the Ristiküla borehole to 56.3 m at Nässumaa.

The Adavere Stage is represented by thin-bedded to nodular biomicritic limestones (wackestones to packstones) with Pentamerus oblongus (below) and marl- to mudstones (above). The former unit is treated as the Rumba Formation and the latter as the Velise Formation. The clay content increases westwards. The lower boundary of the stage coincides with the strongly pyritized erosion surface at the base of the nodular biomicritic limestones of the Rumba Formation, transgressively overlying different strata of the Raikküla Stage. The Adavere Stage contains rather rich shelly fauna of Pentamerus (below) and Clorinda (above) communities. Microfossils (chitinozoans, ostracodes, conodonts) are more frequent in the mud- and marlstones of the Velise Formation, almost devoid of corals and stromatoporoids. The most characteristic species are as follows (abbreviations in brackets: rm - Rumba Formation, vl - Velise Formation): Clathrodictyon variolare (Rosen) (rm), Mesofavosites obliquus Sokolov (rm), Angopora hisingeri (Jones) (vl), Palaeocyclus porpita (Linnaeus) (vl), Prodarwinia speciosa (Dybowski) (rm), Pentamerus oblongus (Sowerby) (rm), Stricklandia laevis (Sowerby) (rm), Dicoelosia baltica Musteikis et Puura (vl), Encrinurus (Nucleurus) rumbaensis Rosenstein (rm), Calymene frontosa Lindström (vl), Beirichia valguensis Sarv (rm), Longiscella caudalis (Jones) (vl), Conochitina emmastensis Nestor (rm), Eisenackitina dolioliformis Umnova (rm, vl), Angochitina longicollis Eisenack (vl), Pterospathodus celloni Walliser (vl), P. amorphognathoides Walliser (vl), Spirograptus turriculatus (Barrande) (vl), Monograptus discus Törnquist (vl), Monoclimacis griestoniensis (Nicol) (vl).

The presence of the index species of graptolites (S. turriculatus, M. griestoniensis) and conodonts (P. celloni, P. amorphognathoides) in the upper half of the Adavere Stage demonstrates that most probably the stage corresponds to the Monograptus sedgwickii to Monoclimacis crenulata biozones.

In Estonia, the Adavere Stage consists of the Rumba (below) and Velise (above) formations. The Rumba Formation spreads on the islands of the West-Estonian Archipelago and in the southwestern part of mainland Estonia. It is represented by horizontally-bedded to nodular biomicritic limestones (wackestones, packstones) with clayey partings and scattered shells or tempestitic accumulations of the brachiopod Pentamerus oblongus. The formation consists of twelve low-grade sedimentary cycles beginning with argillaceous rocks (marlstones, argillaceous limestones) and ending with a layer of pure, hard limestone (Einasto et al. 1972). Westwards the clay content of the rocks increases and on Saaremaa Island marlstones are prevailing in the sequence of the Rumba Formation. A characteristic yellowish-green tuffaceous (metabentonite) interlayer (8 to 18 cm) occurs at the level of the base of the upper third of the sequence.

The thickness of the Rumba Formation is mostly 15 to 19 m and it decreases at the western and eastern margins of the distribution area. Local hiatuses occur in the Ohesaare and Are sections.

The Velise Formation overlies the Rumba Formation and consists of different marlstones and mudstones up to plastic clays. The mostly greenish- to bluish-grey rocks are south- and eastwards replaced by red-coloured (purple) varieties. In the southwesternmost sections (Ohesaare, Ruhnu) graptolites are present in the dark-grey interlayers of argillite. Thin (0.5 to 5.0 cm) metabentonite interlayers are characteristic to the formation. The thickness of the formation is greatest in northwestern Saaremaa, reaching 37 - 38 m in the Viki and Eikla sections. In the southeast direction, it decreases until thinning out in the Ristiküla section, eastern Pärnumaa.

Number	M-6674
Locality	Võhma H-40 borehole, Viljandi County
Country	Estonia
Coordinates	null, null
Depth / thickness (m)	143.2
Date collected	77
Weight (g)	0
Institution	Department of Geology, TalTech