Kaugatuma 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.

Twenhofel (1916) introduced the name Kaugatoma in the sense of the upper subdivision (Zone) of his Oesel Formation (=“Upper Oeselsche Gruppe” by Schmidt 1858, 1881). The present-day limits and stratification of the stage were proposed by Klaamann (1970a) who separated the Kuressaare Stage as an independent unit. The historical stratotype of the stage is the Kaugatuma Cliff supplemented by Kaugatuma-GI drill core in the interval of 0.6 to 37.2 m (Resheniya… 1987, Nestor 1993). The rocks of the Kaugatuma Stage are distributed on the southern peninsulas of Saaremaa Island, and also on Ruhnu and Abruka islands. They crop out in the northern part of the Sõrve Peninsula and on the Roomassaare, Muratsi, Vätta and Leina peninsulas (Fig. 73). The main localities are the cliffs at Kaugatuma and Lõu and the abandoned quarries at Muratsi, Väike-Rootsi and Äigu. The full thickness of the stage varies from 41.6 m in the Ruhnu borehole to 85.7 m in the Sõrve-514 borehole (Table 9).

The Kaugatuma Stage is represented by interbedded marlstones and bioclastic to coquinoid limestones displaying certain cyclicity. In the lower part of the cycle, marlstones are dominating; in the upper part the limestone interlayers become more frequent and the cycle ends with a thick (2-4 m) deposit of crinoidal limestones. In the upper cycles and southwards, the role of limestone layers decreases. The lower boundary of the stage coincides with a notable increase in the clay component. Higher in the sequence, there appear species of ostracodes Amygdalella nasuta, Sleia equestris, Frostiella groenvalliana, Neobeirichia buchiana; chitinozoans Ancyrochitina fragilis; conodonts Ozarkodina remscheidensis eosteinhornensis etc.

The Kaugatuma Stage contains a rich shelly fauna, particularly ostracodes. The guide fossils of the stage are as follows (abbreviations: Ä - Äigu beds, L - Lõo beds): Ancyrochitina fragilis Eisenack (Ä, L), Fungochitina pistilliformis (Eisenack) (L), Densastroma astroites (Rosen) (Ä), Actinostromella vaiverensis Nestor (Ä), Parallelostroma tuberculatum (Yavorsky) (Ä), Favosites pseudoforbesi muratsiensis Sokolov (Ä), Syringopora blanda Klaamann (Ä, L), Cystiphyllum cylindricum Lonsdale (Ä), Atrypoidea prunum (Dalman) (Ä), Stegerchynchus pseudobidentatus (Rybnikova) (Ä, L), Acaste dayiana Richter et Richter (Ä), Pulcherproetus nieszkowskii (Männil) (Ä), Amygdalella nasuta Martinsson (Ä, L), Sleia equestris Martinsson (Ä), Frostiella groenvalliana Martinsson (Ä), Nodibeirichia tuberculata (Klöden) (L), Crotalocrinites rugosus (Miller) (Ä, L), Ozarkodina remscheidensis eosteinhornensis (Walliser) (Ä), O. remscheidensis remscheidensis (Walliser) (L), O. confluens nasutus (Viira) (L), Thelodus admirabilis Märss (Ä), Nostolepis gracilis Gross (Ä, L). The presence of Ozarkodina remscheidensis eosteinhornensis in the lower part of the Kaugatuma Stage shows that its base roughly corresponds to the Ludlow/Pridoli boundary.

In Estonia, the Kaugatuma Stage is represened by the Kaugatuma Formation which is subdivided into the Äigu (below) and Lõo beds (above) (Fig. 73). The Äigu beds consist of two regressive sedimentary cycles, sometimes regarded as the Lower and Upper Äigu beds (Nestor 1995a). In the lower part of these cycles, marlstone layers are prevalent; in their upper part limestones dominate. Among the latter, coarse-grained crinoidal limestones are the most typical rocks, but interlayers of coquinoid or bio-lithoclastic limestones are also quite common, among these bands with Atrypoidea prunum. Both cycles are capped by a thick deposit of crinoidal limestones. The Äigu beds roughly correspond to the ostracode Frostiella groenvalliana Biozone.

The Lõo beds also consist of two sedimentary cycles of the same type but marlstones prevail throughout the whole sequence. Bioclastic to coquinoid limestones occur as thin intercalations. A thicker band of crinoidal limestones occurs at the top of the lower cycle considered sometimes as the Lower Lõo beds. The Upper Lõo beds lack crinoidal limestones at the top. The Lõo beds roughly correspond to the Nodibeirichia tuberculata Biozone.