Amata 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 Amata Stage was formally established by Liepinš (1951), although the corresponding stratigraphical unit as Stage existed in the scheme of Kraus (1934) and had also been distinguished as the “Podsnetogor-Stufe” by Gross (1942). The stratotype of the stage is situated at the lower course of the Amata River in Latvia. In Estonia, the main outcrops are the banks of the Piusa (Loosi) and Peetri rivers (Karisöödi), and in the vicinity of Vastseliina (Fig. 88).

The Amata Stage is spread in a restricted area in southeastern Estonia. The outcrop area forms a 5–10-km-wide belt from Mõniste and Ape in the west to Petseri and Dekshino in the east. The total thickness in boreholes ranges from 12–21 m (Fig. 88), but in the outcrops on the banks of the Piusa River it reaches 30 m.

In Estonia, the Amata Stage is represented by sandy-silty sediments alternating with frequent clay interbeds. The stage lies everywhere on the grey clay of the Gauja Stage and starts with a layer of breccia-like sandstone (Figs. 75, 85). According to Kuršs (1992), in the lower Amata layers the cross-bedded series are inclined to the north which is not typical of this part of the Devonian.

The Amata Stage contains Psammolepis undulata (Ag.), Psammosteus praecursor Obr., P. maeandrinus Ag., Asterolepis radiata Rohon, Bothriolepis prima? Gross, B. cellulosa? Pand., Panderichthys rhombolepis Gross and Laccognathus panderi Gross (Sorokin 1981).

In Estonia and adjacent areas, the Amata Stage is represented by the Amata Formation. According to borehole data, the predominating rock type in the Amata Formation is the greenish- and purplish-grey siltstone which forms on average of 45% of the section. In outcrops, however, the sandstones are prevailing. The sandstones of the Amata Formation are light to yellowish-grey, more rarely reddish-brown, fine-grained, medium- to strongly-cemented, with indistinct cross-bedding, the inclination of which varies in wide limits. The sandstones often contain pebbles and lens-shaped interlayers of clay, more rarely quartz pebbles. Clay interlayers are usually purplish-grey and -brown and form 30% of the section as an average.

Mineralogically, the sandy-silty rocks of the Amata Formation are predominantly quartzose arenites with the quartz content being 80-90%. The heavy mineral suite is dominated by ilmenite, the share of transparent allothigenic minerals is relatively great, varying from 26 to 40%. Among this group zircon is predominating. It is followed by tourmaline, staurolite and rutile in almost equal amounts ( Fig. 78, Kleesment 1995). The assamblage of clay minerals is dominated by hydromicas with the average content of 95%.