_Toc174709357">Pirgu Stage

L. Hints & T. Meidla

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 Pirgu Stage (Jaanusson 1944b) is a lithologically variable (Table 7) and thick (up to 66 m, Fig. 58) stratigraphical unit (=upper part of the “Lyckholm’sche Schicht”, Schmidt 1858). The notable changes in the thickness of the stage, sometimes within a short distance, are due to various reasons, e.g. the development of mud mounds, denudation during late Pirgu and/or Porkuni time, intensive tectonical movements and changes in sea-level.

In northern Estonia, within the stage two succesive rock units of grey-coloured limestones are distinguished - the lower, Moe and the upper, Adila Formation (Rõõmusoks 1960) which correspond approximately to the former Nyby (Jaanusson 1944b, Männil 1966) and Piirsalu (Jaanusson 1945) substages.

The Moe Formation, up to 40 m in thickness, consists of micritic and bioclastic nodular or bedded limestones with argillaceous intercalations. The lower part of the formation contains abundant calcareous alga Palaeoporella? (=Dasyporella in Männil 1966) and in some places (Hoitberg on Vormsi Island, Võhma core in central Estonia) typical carbonate mounds are developed, quite similar to the Boda mounds in the Siljan district of Sweden.

The Adila Formation comprises predominantly bioclastic limestones with a thickness of 10-15 m. Numerous discontinuity surfaces and cyclically alternating pure and argillaceous limestones are characteristic to the upper part of the formation. In this topmost part of the formation, the pentamerid brachiopod Holorhynchus has been recorded in the Island of Hiiumaa (Hints 1993). The boundary between the Moe and Adila formations coincides with the boundary between the rugata and bergstroemi chitinozoan zones (Table 7).

Within a rather large area in central Estonia, the Pirgu Stage is characterized by the interfingering of different rock units (Oraspõld 1975a, Table 7), whose correlation with the northernmost and southernmost sequences is complicated. In this transitional area, the lowermost part of the stage consists of argillaceous bioclastic limestones with glauconite (Tootsi Member), overlying the upper part of the Vormsi Stage, corresponding to the chitinozoan barbata Zone. The Tootsi Member contains a distinct association of shelly fauna (Fig. 59). This unit is succeeded upwards by the grey-coloured, sometimes red mottled marls and highly argillaceous limestones of the main part of the Halliku Formation whose relationship with the Moe Formation is not yet very clear (Männil & Meidla 1994). The diverse and abundant ostracode fauna of the Halliku Formation comprises taxa (Fig. 60) common with the upper part of the Moe Formation (Meidla 1996). Most uncertain is the age of the Kabala Member on the transition between the Pirgu and Porkuni stages in central and westernmost Estonia. According to the ostracode record, the formation contains two different associations. The older association comprises the Pirgu species Brevibolbina pontificans Schallreuter, Bullaeferum tapaensis (Sarv), whereas the species Apatochilina falacata Sarv and Gryphiswaldensia plicata Schallreuter from the supposedly younger part are common with the fossiliferous part of the Ärina Formation of the Porkuni Stage.

In southern Estonia, in the limits of the central confacies belt, the Pirgu Stage is represented by red-coloured or mottled argillaceous limestones and mudstones of the Jonstorp and Jelgava formations, including the Kuili Member (Table 7).

According to the traditional understanding, the lower boundary of the Pirgu Stage coincides with the lower boundary of the Moe Formation in northern Estonia. This level is underlain by the chitinozoan Acanthochitina barbata Zone. The zone has also been established under the red-coloured limestones of the Jonstorp Formation (Ruhnu core). On this basis, the lower boundary of the latter formation has been equalized with the lower boundary of the Pirgu Stage in southern Estonia. In terms of graptolite zonation, the stage boundary corresponds to the base of the D. complanatus Zone (Männil 1990). The presence of Climacograptus supernus Elles et Wood in the upper part of the Pirgu Stage suggests its correlation with the Dicellograptus anceps Zone (Männil 1976, 1990).

The lower boundary of the Pirgu Stage is poorly reverberated in the distribution of most shelly fossil groups. As a rule, the new faunal elements appear 1 - 2 m above (or even higher) of the distinct lithological changes (Fig. 59). In northern Estonia, the biostratigraphical boundary is best expressed by the appearance of the concentrations of the alga Palaeoporella?. In central and southern Estonia, the faunal renovation is best revealed in the ostracode record (Meidla 1996, see also Fig. 60).

The Pirgu Stage comprises three different assemblages of macrofauna, related to different facies zones of the palaeobasin. In northern Estonia, in the grey-coloured Moe and Adila formations a rich assemblage of large articulated brachiopods Plaesiomys solaris (Buch), Equirostra gigas Schmidt, Triplesia insularis (Eichwald), Luhaia vardi Rõõmusoks, corals Sarcinula, Catenipora, Palaeofavosites, and stromatoporoids, together with different molluscs, is distributed (Fig. 59). In the ostracode composition nonpalaeocopes are dominating: the associations of Steusloffina cuneata-Medianella blidenensis and S. cuneata-Olbianella fabacea (Meidla 1996, Fig. 60) occur. In central Estonia, in the Halliku Formation the most common representatives of the shelly fauna seem to be brachiopods and rugose corals (Fig. 59). In the red-coloured deposits of southern Estonia, only a few macrofossils, mainly brachiopods and trilobites, have been recorded, while trilobite and echinoderm fragments dominate in the skeletal sand (Männil et al. 1968).

Gasconsia gigantea Hints et Rong, 2024