Rēzekne 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 Rēzekne Stage was established by Lyarskaya (1974) with the stratotype in Akniste-5 drill core (interval 361.8 - 487.8 m) in southeastern Latvia. Earlier it was treated as the Kemeri (Liepinš 1952) or Viesite (Liepinš 1964) Formation. After destruction of Akniste-5 drill core, the interval of 427 - 446 m in Ludza-15 drill core in eastern Latvia was selected for the neostratotype of the Rēzekne Stage (Lyarskaya 1978). In Estonia, the corresponding strata were earlier treated as the Pärnu Stage (Mark & Paasikivi 1960) and the Viesite Formation (Kleesment 1966). As the Rēzekne Stage they were first mentioned in 1975 (Kleesment et al. 1975). The age of the stage was palaeontologically determined by Mark-Kurik (1991a).

The Rēzekne Stage is spread in southern Estonia and covered with the rocks of the Pärnu Stage (Figs. 75, 76, 77). The best examined section is the Mehikoorma drill core (interval 220.3 - 246.2 m, Kleesment et al. 1975). The total thickness of the stage varies from 0.7 m in the Asuküla and Kaagvere boreholes up to 51.5 m in the Kavastu borehole. The stage is at its thickest in eastern Estonia (Fig. 80).

The Rēzekne Stage is characterized by greenish-, purplish- and light-grey sandstone. In southeastern Estonia, the upper part of the section is represented by dolomitic marls. The stage lies with a stratigraphical unconformity on the different stages of Ordovician or Silurian age, in a few cases on the rocks of the Tilžė Stage (Värska, Laanemetsa, Väimela), in the Mõniste High it overlies the basement complex. The lower part of the section consists of sandstones with dolomitic matrix, often conglomeratic (Figs. 75, 76, 77).

The fossils known from the Rēzekne Stage are largely confined to the Rēzekne Formation. An equivalent of the latter unit, the Lemsi Formation contains a few unidentified fish remains (Sorokin 1981). Fossil fishes coming from the Rēzekne Formation include Schizosteus sp., Psammosteidae gen. indet., Cephalaspidida gen. indet., Kartalaspis belarussica Mark-Kurik in litt., Antiarchi gen. indet., Laliacanthus singularis Kar.-Tal., Diplacanthus kleesmentae Valiuk., Acanthodes ? sp. B Valiuk., Acanthodes? sp. C Valiuk., Ptychodictyon ancestralis Valiuk., Cheiracanthus gibbosus Valiuk., Markacanthus parallelus Valiuk., Ectopacanthus flabellatus Valiuk., Rhadinacanthus primaris Valiuk., Nostolepis sp., Pruemolepis wellsi Vieth-Schreiner, Crossopterygii gen. indet. The presence of otoliths is noteworthy.

Invertebrates comprise “Lingula” sp., Ostracoda, Glyptasmussia? sp., Gastropoda. Microfossils include a simple conodont and miospores: Retusotriletes simplex Naumova, Leiotriletes microrugosus (lbr.) Naumova, L. simplex Naumova [rz1], Stenozonotriletes conformis Naumova [rz1], Acanthotriletes perpusillus Naumova [rz1], A. parvispinosus Naumova [rz1], Archaeozonotriletes memorabilis V. Umnova [rz1], Emphanisporites rotatus McGregor [rz1], Dibolisporites cf. eifeliensis (Lanninger) McGregor [rz1], Diatomozonotriletes devonicus Naumova [rz1], Retusotriletes cf. priscus V. Umnova [rz2], Leiotriletes cf. insuetus V. Umnova [rz2], Granulatisporites cf. rudigranulatus Staplin [rz2], cf. Calamospora pannucea Richardson [rz2]. The above list shows that the miospore content differs in the lower [rz1] and upper [rz2] parts of the Rēzekne Formation. The lists of fossils are by Kleesment et al. 1975 and Valiukevičius 1994 (miospores identified on generic level are not indicated).

In Estonia, the Rēzekne Stage consists of two formations, laterally replacing each other. In eastern Estonia, the Rēzekne Formation expands as far as Lake Võrtsjärv, west of it the Lemsi Formation occurs (Sorokin 1981, Figs. 77, 80).

The interval of 220.3–246.2 m of the Mehikoorma drill core has been selected as the parastratotype of the Rēzekne Formation. The thickness of the formation varies commonly from 10 to 30 m (Fig. 80). The lower part of the section is represented by grey, purplish- and pinkish-grey loose sandstone, with interlayers of brownish-black silty platy clay in its basal part. On contact with the underlying Ordovician or Silurian carbonate rocks (3–5 m) the sandstones are strongly cemented with dolomitic matrix. In southeastern Estonia, the upper part of the Rēzekne Formation is represented by grey silty dolomitic marl up to 10 m in thickness, in other regions by an up-to-1–2-m-thick layer of grey siltstone or silty sandstone (Figs. 75, 76, 77). The sandstone of the Rēzekne Formation is fine- and very fine-grained.

Mineralogically, the sandstone of the Rēzekne Formation is predominantly feldspatic arenite with the quartz content of 70–85%. The sand component of the dolomitic marl contains 60–75% of quartz. The heavy fraction is dominated by allothigenic transparent minerals. In sandstone its share is commonly 45–60% and in dolomitic marl it forms 30–50% of the fraction. This group is dominated by garnet (50–70%), accompanied by zircon (15–25%, Fig. 78). The content of garnet is relatively high in dolomitic marls.

The stratotype of the Lemsi Formation is the interval of 69.8–85.8 m of the Kihnu drill core (Sorokin 1981). The thickness of the formation is commonly 11–20 m (Fig. 77). It mostly consists of light grey, yellowish and brownish, most rarely of purplish loose sandstone, which in the basal contact part with Silurian carbonate rocks is strongly cemented with dolomitic matrix. The upper 0.6–2.9 m of the section consist of greenish and purplish-grey siltstones or very fine-grained sandstone, often strongly cemented with dolomitic matrix. The sandstone of the Lemsi Formation is predominantly fine- and medimum grained.

Mineralogically, the sandstones of the Lemsi and Rēzekne formations are similar. Only in the sandstone of the Lemsi Formation the content of zircon is higher, particularly in the Kanaküla – Tõlla – Ipiku area where it dominates over the garnet.