Aruküla Stage

by A. Kleesment & E. Mark-Kurik

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 Aruküla strata were distinguished as an independent stratigraphical unit by Gross (1940b, 1942) and transfered to the rank of regional stage by Mark-Kurik (Mark 1958). The stratotype is the river bank near the Tartu Jaani Cemetery (Photo 24); not far from it are the Aruküla caves (Photo 13). The other important localities are Kallaste, Viljandi, Tamme and Õisu (Fig. 84). The rich material collected there during more than a hundred years contains the majority of the fauna characteristic of this unit.

The Aruküla Stage is spread in southern and southeastern Estonia. The outcrop area forms a 17–55-km-wide belt extending from Ruhnu Island and Ikla Settlement in the west to Pala and Mehikoorma settlements in the east (Fig. 84). The total thickness of the stage in Estonia ranges from 66.0 to 97.2 m.

The Aruküla Stage consists of reddish-brown cross-bedded sandstone, interbedded with siltstone, clay, and dolomitic marlstone. It lies everywhere above the Narva Stage. The lower boundary of the Aruküla Stage coincides, in general, with the lower surface of the first significant uncemented reddish-brown sandstone layer above the dolomitic siltstone or marl of the Narva Stage (Figs. 75, 76). The topmost part of the Narva Stage often shows a greenish-grey siltstone layer; the overlying Aruküla sandstone is mostly inequigranular. In the Võrtsjärv Depression, this boundary is often difficult to establish (Mark & Tamme 1964).

The Aruküla Stage is rich in fossil fishes known from its all three subdivisions (Table 10). Characteristic are psammosteid heterostracans and several placoderms, both arthrodires and antiarchs. Fishes from the Viljandi [vl] and Kureküla [kr] beds are better known than those from the Tarvastu beds [tr]. The Aruküla fish fauna includes: Tartuosteus giganteus Gross, Pycnosteus palaeformis Preobr. [vl], Ganosteus artus Mark-Kurik, Psammolepis proia Mark-Kurik [vl, kr], cephalaspidids [vl], Actinolepis tuberculata Ag. [vl,kr], Holonema obrutshevi Mark [vl], Homostius latus Asm., Heterostius ingens Asm., Coccosteus grossi O.Obr. [vl,kr], Millerosteus? sp [tr], Byssacanthus dilatatus (Eichw.) [vl,kr], Asterolepis estonica Gross [vl,kr], Archaeacanthus quadrisulcatus Kade, Haplacanthus marginalis Ag., Homacanthus gracilis (Eichw.), Rhadinacanthus balticus Gross [vl], R. multisulcatus Valiuk., Diplacanthus sp. [vl], D. carinatus Gross, D. gravis Valiuk., Markacanthus costulatus Valiuk. [vl], Minioracanthus laevis Valiuk. [vl,kr], M. alius Valiuk., Acanthodes? sp. A, Acanthodes? sp. B, Acanthodes? sp. D, Cheiracanthus brevicostatus Gross, C. longicostatus Gross, Ptychdictyon rimosum Gross, P. sulcatum Gross, Gyroptychius pauli Vorob., Glyptolepis sp. sp., Dipterus sp. sp., Conchodus sp. [kr], Orvikuina sp.n., Cheirolepis sp., Tartuosteus? luhai Mark-Kurik [kr, tr], Pycnosteus pauli Mark [kr,tr], Nodocosta pauli Gross [kr], Thursius estonicus Vorob. [kr], Hybosteus sp. [kr], Tartuosteus maximus? Mark-Kurik [tr], Nostolepis sp.n. Valiuk. [tr], Ptychodictyon distinctum Valiuk. [tr], Porolepis? sp. [tr].

Invertebrates of Aruküla Age are mostly known from the Viljandi beds. These are: ostracods Leperditia tartuensis Öpik, Aparchitellina taehtverensis (Öpik), Drepanellina orvikui (Öpik), Pontocypris rulescens (Öpik), lingulates Bicarinatina bicarinata (Kut.), B. ugalana Rõõmusoks et Gravitis and unidentified lamellibranchs. Trace fossils are fairly numerous in the Kureküla beds. Lingulate brachiopods occur also in the Tarvastu beds.

Plant remains consist of gyrogonites? of charophyte algae (Viljandi beds) and some poorly preserved fragmental branches. The list of fossils is from Sorokin (1981) and Valiukevičius (1994, modified).

In Estonia, the Aruküla Stage is represented by the Aruküla Formation. On the basis of lithological and mineralogical evidence, three cyclic units are distinguished in the Aruküla Formation. These cycles occur in all sections of Estonia and adjacent areas and are defined as the Viljandi (lower), Kureküla (middle) and Tarvastu (upper) beds of the Aruküla Formation (Table 10). Each unit begins with relatively coarse and poorly sorted sandstones of a mature mineral composition, but ends with a clayey-silty complex (Figs. 75, 76, 85, Kleesment, 1994).

The lower, Viljandi beds are dominated by very fine sandstones, often platy or slaty-bedded. The Kureküla beds are characterized by irregularly cemented interbeds of variegated siltstones, pockets of white sandstone, lenses of conglomeratic sandstone and interlayers with large clay pebbles. The section of the Tarvastu beds contains typically conglomeratic interbeds and surfaces and crusts of Fe hydroxide.

Mineralogically, the rocks of the Aruküla Formation are predominantly quartzose and feldspatic arenite with the quartz content of 60–90%. Micaceous arenites (content of micas 20–50%) occur as thin interbeds. The heavy fraction is dominated by ilmenite (30–60%) and transparent allothigenic minerals (15–40%). Among the latter, garnet and zircon are most significant. Tourmaline and rutile are also important. On the lower boundary of the formation the content of zircon and apatite increases significantly, and staurolite appears (Fig. 78).