Quaternary Lacustrine Complexes of the Northwest East European Platform and Northern Yakutia and Their Deformation Structures

Resumo

The Quaternary lacustrine complexes of two regions considered in this work formed in different paleogeographical conditions. In the Baltic crystalline shield, they accumulated as a result of the admission of thawing glacial waters in depressions, and this was favorable for the formation of a diverse lithological composition and sedimentary structures: from varve silts with dropstones (facies of deepwater glacier dam lakes) to equigranular sands with wavy stratification (shallow lacustrine facies). In northern Yakutia, the lacustrine complexes accumulated in closed thermokarst (alas) depressions at the expense of the redeposition of ice silty sediments of various ages. This is responsible for more a homogeneous composition and minor flowing water structures. The lacustrine complexes of both regions exhibit abundant deformation structures of various types, which are sensitive indicators of sedimentary and diagenetic conditions. Similar structures include the fold deformations of convective and/or cryogenic mixing of poorly consolidated sediments (convolutions) with similar morphology (narrow antiform and wide synform folds). Although these structures are morphologically similar to seismites, they, as a rule, are confined to hiatuses in lacustrine sedimentation, minimizing their possible formation as a result of the seismic dilution of sediments. The presence of ubiquitous pseudomorphoses after wedge polygonal ice is a typical feature of the structure of alas sediments, in contrast to rare pseudomorphoses in lacustrine complexes of the Northwest region. On the other hand, under-moraine lacustrine complexes of the latter exhibit intense glaciodislocations, which are absent in northern Yakutia.