Juuru 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 Juuru strata were established by Schmidt (1858) as the Bed (“Jördensche Schicht”), later transferred to the rank of Stage (“Stufe”) (Schmidt 1892). Nestor and Kala (1968) determined the present stratigraphical extent of the stage and worked out its classification. With the Juuru Stage they united the Tamsalu Formation, earlier treated as an independent stage, and the lowermost beds of the Raikküla Stage (now the Karinu Member). The former Hilliste Member of the Juuru Stage was recently expanded and raised into formation rank partly corresponding to the Raikküla Stage (Männik 1992b, Nestor 1995a).

The Juuru drill core in the interval of 0.4-16.2 m has been selected as a neostratotype for the Juuru Stage (Nestor 1993). The Juuru Stage spreads on the islands of Hiiumaa and Saaremaa and in the western, central and southern parts of mainland Estonia. The outcrop extends as a west-eastwards widening belt (4 to 25 km) from midsouthern Hiiumaa as far as the eastern slope of the Pandivere Upland. The main localities are ancient coastal cliffs at Kallasto and Pullapää, quarries at Hilleste, Kirimäe, Karinu, Tamsalu and Rakke (Kamariku) and a well in the ancient Varbola stronghold (Fig. 63). The full thickness of the stage varies from 20.1 m in the Asuküla borehole to 63.7 m in the Viljandi borehole (Fig. 63).

The stage is dominated by biomicritic limestones (packstones, wackestones) rhythmically intercalating with thin layers of marl- and mudstones (argillites, clays) and containing interlayers of sparitic limestones (grain- or rudstones). The proportion of marlstones increases southwards and the number of sparitic interlayers towards the north-west and upwards in the sequence.

The lower boundary of the stage coincides with the base of a thin band of micro- to cryptocrystalline limestone of the Koigi Member or, if the latter is absent, with the base of the marl- or mudstones of the Varbola and Õhne formations overlying various sparitic limestones of the Porkuni Stage, including bioclastic and oolitic grainstones, lithoclastic rudstones of shallow-water origin. Above the boundary, the brachiopod Stricklandia lens, the chitinozoans Ancyrochitina laevaensis and Spinachitina fragilis or the conodont Ozarkodina ex gr. oldhamensis appear.

The Juuru Stage contains a rather rich benthic shelly fauna, whereas planktonic fossils are rare. The most characteristic species are ( abbreviations in brackets: vr - Varbola Formation, tm - Tamsalu Formation, õh - Õhne Formation, pt. - part) Clathrodictyon boreale Riabinin (vr, tm), Paleofavosites paulus Sokolov (vr, tm, õh), Stricklandia lens prima Williams (vr, lower pt.), S. lens lens Williams (vr, upper pt.), Zygospiraella duboisi (Verneuil) (vr), Borealis borealis (Eichwald) (tm), Acernaspis estonica Männil (õh), Calymene ansensis Männil (vr, tm), Aitilia senecta Sarv (vr), Steusloffia eris Neckaja (vr, tm, õh), Ozarcodina ex gr. oldhamensis (Rexroad) (vr, tm, õh), Distomodus kentuckyensis (Branson et Mehl) (vr, tm, õh), Ancyrochitina laevaensis Nestor (õh, basal pt.), Spinachitina fragilis Nestor õh, basal pt.), Conochitina postrobusta Nestor (õh), Dimorphograptus confertus Nicholson) (õh, top), Pribylograptus incommodus (Toernquist) (õh, top). Records of S. lens prima, A. laevaensis and S. fragilis from the basal part of the stage suggest that the base of the Juuru Stage lies on the level of the Parakidograptus acuminatus Zone (Cocks 1971, Nestor V. 1994). Graptolites D. confertus and P. incommodus from the top of the stage confirm that the upper boundary of the stage roughly coincides with the boundary between the Orthograptus vesiculosus and Coronograptus cyphus zones (Kaljo & Vingisaar 1969).

In the Mid-Estonian Confacies Belt, the Juuru Stage is divided into the Varbola (below) and Tamsalu (above) formations. In the South-Estonian Confacies Belt, the Õhne Formation corresponds to both of them (Figs. 64, 65).

The Varbola Formation is represented by nodular biomicritic limestones (skeletal to coquinoid pack- and wackestones) with thin intercalations of marlstone. The formation contains tempestitic interlayers of skeletal grainstones, often with intraclasts, the number of which increases upwards in the sequence and northwestwards in the space. Brachiopods of the Stricklandia Community are characteristic to the formation. The thickness of the formation varies from 8.8 m in the Pusku borehole to 24.6 m in the Käru borehole. The 0.1—3.5-m-thick Koigi Member of micritic (aphanitic) limestones is developed at the base of the Varbola Formation.

The Tamsalu Formation consists of various, prevailingly sparitic limestones (skeletal and pelletal grainstones, coquinoid or lithoclastic rud- and floatstones). The thickness of the formation varies from 8.8 m in the Pusku 2 borehole to 18.5 m in the Rumba borehole. The formation is subdivided into the Tammiku (below) and Karinu (above) members.

The Tammiku Member is typically represented by a bank of coquinoid limestone consisting of shells and debris of the brachiopod Borealis borealis. The thickness of the bank reaches 13.5 m on the Pandivere Upland. In the same area, the Karinu Member consists of skeletal and pelletal grainstones and bio- or lithoclastic rudstones. South- and westwards the latter are replaced by fine-grained grain- and packstones with numerous hardgrounds.

The Hilliste Formation consists of a highly variable assemblage of rock types in which the most characteristic are crinoidal limestones (grainstones) with coral-stromatoporoid bioherms. The formation also contains fine-grained pelletal and skeletal grain- and packstones and micritic limestones. The formation corresponds to the upper part of the Tamsalu Formation (Karinu Member) and to the lower part of the Raikküla Stage (Nestor 1995a). It occurs on Hiiumaa Island and in the vicinity of Haapsalu - Rohuküla and Rapla - Käru in mainland Estonia.

The Õhne Formation is represented by marlstones, mudstones and micritic limestones. It corresponds to the whole stratigraphical extent of the Juuru Stage in southern Estonia. The rather poor fauna corresponds to the brachiopod Clorinda Community. The maximum thickness (63.7 m) has been fixed in Viljandi 91 borehole. The thin, up-to-2.7-m-thick Puikule Member of marlstones and the overlying, up-to-8-m-thick Ruja Member of micritic limestones occur in the basal part of the Õhne Formation along the southern and eastern margins of the area of distribution of the formation.