On the mineralogy of the Ülgase beds in Maardu district

The present mineralogical determination is based on the study of the mineral composition in the coarse silt fraction (0.1-0.05 mm) of 21 samples collected from two sections, оnе of them being the stratotype of the Ülgase Formation (Fig. 1).

The results of the study, are given in Table 2, and their total average for sand-siltstones (I) and claystone (II) separately in Fig. 3. The content of light minerals (А), heavy minerals (B), and the numerical relations in the group of the nonopaque allothigenic heavy minerals (В) and those in the group оf authigenic heavy minerals (B), are shown.

The most typical features of the mineral composition are represented in allothigenic groups as follows: high mineralogical maturity (see the quartz-feldspar ratio in Таblе 2); zircon-tourmaline-rutile assemblage; the continual presence of apatite and unstable minerals (commonly amphiboles); the simultaneous occurrence of ilmenite and leucoxene, with the former prevailing (see the last column in Table 2). Authigenic mineralization is characterized by an extensive replacement of leucoxene by anatase, limpid overgrowths on detrital feldspar grains and phosphatic pellicles on some detrital minerals (quartz, zircon, ilmenite).

The Ülgase Beds of the studied area differ clearly by their mineral composition from the underlying (the Tiskre Formation, Lower Cambrian) as well as from the overlying (the Maardu Member of the Kallavere Formation, Ordovician) deposits. The difference lies in the content of some minerals, particularly in mineral associations, mostly the heavy fraction.

Thus, the described unit can be recognized on the basis of the peculiarities of its mineral composition, especially in the sequences, where faunal criteria are lacking.