Pärnu 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 Pärnu strata were established as an independent stratigraphical unit by Orviku (1930c, 1932). The name “Pärnu” (“Pernu”) was first used by Obruchev (1933). Palaeontologically, it was distinguished as the Schizosteus heterolepis Zone by Gross (1942) and as a stage by Mark-Kurik (Mark 1958). The stratotype is the bank of the Pärnu River near the settlement of Tori. The exposures occur on the banks of the Pärnu and Navesti rivers in central and southwestern Estonia, including Oore dairy — the boundary outcrop with the Narva Stage (Fig. 81).

The Pärnu Stage is spread in southern Estonia. The outcrop forms two narrow wedgeform areas in the northwestern and northeastern parts of the distribution area. The total thickness of the stage ranges commonly from 15 to 47 m (Figs. 75, 76, 81). In the Võrtsjärv Depression, only the topmost part of the section, up to 8 m in thickness, is represented.

The Pärnu Stage is characterized by light-yellow fine-grained cross-bedded sandstone. In most of the distribution area it lies conformably on the Rēzekne Stage. The topmost layer of the Rēzekne Stage in the southeastern part of the area is represented by dolomitic marl which is overlain by sandstone of the Pärnu Stage. In the western part, the boundary between these stages is difficult to establish because of their similar composition. In the northern part of the of distribution area, the Pärnu Stage lies with a stratigraphical disconformity on the Silurian and Ordovician carbonate rocks.

The majority of the fossils of the Pärnu Stage are confined to the Tori Member. The Tamme Member has revealed gyrogonites (?) of charophyte algae and, probably, unidentified lamellibranchs and rare fish remains (Orviku 1930c). In the Tamme Member Valiukevičius (pers. comm.) has identified scales of acanthodians Cheiracanthus gibbosus Valiuk., Rhadinacanthus primaris Valiuk., Cheiracanthus brevicostatus Gross and Acanthodes? sp. D. Fossil fishes occurring in the Tori Member are: Schizosteus heterolepis (Preob.), Psammolepis toriensis (Mark-Kurik), Tartuosteus sp., Actinolepis tuberculata Ag., Homostius sp., Byssacanthus dilatatus (Eichw.), Archaeacanthus quadrisulcatus Kade, Diplacanthus kleesmentae Valiuk., Acanthodes sp. B? Valiuk., Porolepis sp., Glyptolepis sp., Osteolepididae, Dipnoi?. Invertebrates (lingulates) are extremely rare. Common is fossil flora including macroremains of Hostinella sp., and Psilophytites sp., and miospores: Periplecotriletes tortus Egorova, Emphanisporites rotatus McGregor, Retusotriletes raisae Tchib., R. devonicus Naumova, R. concinnus Kedo, R. incomptus Kedo, R. planituberculatus Kedo, Dibolisporites antiquus (Kedo) Arkh., Hymenozonotriletes marginodentatus Kedo, H. altus Kedo, H. ludzus Kedo, H. longus Arkh., Calyptosporites velatus (Eisenack) Richardson, C. tener (Tchib.) Obukh. var. concinnus Tchib., Camarozonotriletes apertus Kedo, Sinuosisporites sinuosus (V. Umnova) Arkh., Punctatisporites tortuosus (Tchib.) Arkh. (data from Sorokin 1981, Valiukevičius et al. 1986, modified according to Abukhovskaya (pers. comm.)).

In Estonia and adjacent areas, the Pärnu Stage is represented by the Pärnu Formation. In Estonia, the formation (Table 10) is divided into the Tori (below) and Tamme (above) members.

The Tori Member is dominantly represented by yellow, light-grey or purplish-grey loose cross-bedded sandstone. Strongly cemented sandstone with dolomitic matrix forms only a basal layer with a thickness of 0.03 to 2 m on the Silurian or Ordovician carbonate rocks. Commonly, it contains pebbles of the underlying sediments. The sandstone is dominantly fine-grained, in the bottommost part medium-grained sandstone is developed. The thickness of the Tori Member varies greatly and irregularly and is highest in the Tsiistre drill core. In some places it is absent (Taagepera drill core, Fig. 77).

The Tamme Member is represented by interbedding loose and dolomitic-cemented greenish-, pinkish- and purplish-grey sandstone containing thin interlayers of siltstone and clay. The complex is horizontally-bedded. Commonly, yellowish-grey sandy dolomite (dolostone) with a thickness of 0.5 to 1 m occurs in the topmost part of the section. Strongly cemented sandstones with dolomitic matrix contain irregular vugs with a diameter of 1 to 15 cm, fulfilled with loose sandstone. The sandstone of the Tamme Member is fine and very-fine grained. The thickness of the member varies irregularly from 2 m (Valga-324 borehole) to 30 m, which is the full thickness of the Pärnu Formation (Taagepera borehole, Fig. 77).

Mineralogically, the sandstone of the Pärnu Formation is quartzose and feldspatic arenite with the quartz content of 75–85%. The heavy fraction is dominated by transparent allothigenic minerals (about 50%), among which garnet with a considerable supplement of zircon is prevailing (Fig. 78). The sandstones of the Tori and Tamme members are quite similar; only in the Tamme Member the content of garnet is higher and that of zircon is lower.