|Mens & Pirrus
|Paala, place in Viljandi town, southern Estonia
Mens, K. & Pirrus, E. (1997). Cambrian. In: Raukas, A., Teedumäe, A. (eds). Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3.
The Paala Formation occurs in central and southeastern Estonia transgressively on the crystalline basement or on the Lower Cambrian rocks the type section is the Viljandi drill core (Fig. 22-203) in the interval of 409.8 to 434 m (Mens et al. 1984). The thickness of the formation varies greatly. Due to the low core yield, the determination of the boundaries is complicated and the thickness of the formation is unclear.
The formation consists of light quartzose non‑glauconitic middle‑ to fine‑grained sandstones with pellets of white kaolinitic clay. The grain-size of the rocks in the lower and upper parts of the formation is coarser than in its middle part where very fine- to fine‑grained varieties dominate. Feldspar and muscovite are not common. The heavy mineral assemblage is mainly composed of allothigenic minerals dominated by ilmenite. In the group of heavy transparent minerals, zircon is always the index mineral.
The character of the basal bed depends on the type of underlying rocks. On the crystalline basement, it consists of conglomeratic sandstone, locally with phosphatized pebbles (Laanemetsa, Fig. 22-269). Resting upon the argillaceous Lower Cambrian rocks, which are often weathered, the basal bed occurs like a variegated kaolinitic clay comprising coarse grains of quartz redeposited from the weathering crust.