Structural stage

Structural level or structural floor (Russian структурный этаж, Polish piętro strukturalne, translated by Sloss (1963) as structural stage) is a structural geology unit defined as a succession of strata characterized by a tectonic style which is different from other units (overlying or underlying it) and separated from them by angular unconformities. According to the definition by Depciuch and Lis (1971) it is "a set of tectonic structures formed by a single, distinct tectonic phase".

Formations which together form a structural stage are not a result of continuous sedimentation, but were tectonically deformed together. They are also characterised by a similar stage of metamorphosis. Therefore, any structural level is a product of one phase of the tectonic evolution of a continental platform (which relates both to the crystalline basement and the sedimentary cover). This structural stage constitutes a single sedimentary-diastrophic cycle. The term has been introduced in the context of geosynclinar theory, and was related to one cycle of a closing geosyncline. Comparison of the age of sedimentary strata belonging to a structural stage indicates the age of tectonic processes responsible for it.

Structural floor unit is currently used in the geological literature of Russia, Poland and other former Eastern Bloc countries as well as some Asian and Latin American countries. The term was used in 1948 by Schultz in his geological memoir of Tien-Shan. To some extent it corresponds with sequences introduced in North America by Sloss (1949, 1963), also associated with tectonic cycles.

In Poland, the concept of a structural level was applied by Sokołowski and Znosko (1959) in their classification of tectonic units of Poland, carried out as part of the wider program of developing the International Tectonic Map of Europe. In 1962, in the context of the same program, the term was described by Bogdanov. According to his classification, in continental platform areas there are three types of structural units:

  • megacomplexes (unit of the first order), two megacomplexes are the crystalline basement and the sedimentary cover
  • fold complexes (unit of the second order) associated with the main orogenies (such as Caledonian Variscan, Alpine)
  • structural levels (unit of the third order) associated with major orogenic phases, and characterised by similar tectonic deformation, stage of metamorphism, magmatism and separated by angular unconformities.

Sloss (1963) compared his sequences to Bogdanov's fold complexes (or structural complexes). However, in practice, the distinction between structural level (bound by regional-scale unconformities) and fold complex is difficult, and often these two terms are treated as equivalent. Many authors also apply structural stage in relation to what is described by Bogdanov as a megacomplex (two structural levels: the crystalline basement and the sedimentary cover).

A structural level may consist of sublevels (pol. podpiętro strukturalne) which are bound by lower-order unconformities, not angular discordances. The term tectonic level is also used as a subordinate unit to structural level.

Literatura

Богданов А. А., 1962. О термине "структурный этаж" (в связи с составлением международной тектонической карты Европы масштаба 1:2 500 000). Бюллетень Московского общества испытателей природы. Отдел геологический 38, № 1.
Depciuch, T., Lis, J. 1971. Izochronowa kontrola i interpretacja wyników oznaczeń. Kwartalnik geologiczny 15, Wydawnictwa Geologiczne
King, P.B. 1969. The Tectonics of North America - A Discussion to Accompany the Tectonic Map of North America Scale 1:5000000. Geological Survey Professional Paper 628
Książkeiwicz, M., Sokołowski, S., Ruhle, H. 1965. Zarys geologii Polski. Wydawnictwa Geologiczne
Mizerski, W. Sylwestrzak, H. 2002. Słownik geologiczny. Wydawnictwo Naukowe PWN
Sloss, L.L. 1963. Tectonic Cycles of the North American Craton. W Merriam, D.F. (ed.) 1964. Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, 449-459 available on Kansas Geological Survey Website
Sokołowski, S., Znosko, J. 1959. Projekt mapy tektonicznej Polski jako części mapy tektonicznej Europy. Kwartalnik geologiczny 3, Wydawnictwa Geologiczne
Stupnicka, E. 1997. Geologia regionalna Polski. Wydawnictwa Uniwersytetu Warszawskiego
Шульц, С.С., 1948. Анализ новейшей тектоники и рельеф Тâнь-Шанâ. Огиз Государственное издательство географической литературы
Komitet Nauk Geologicznych PAN 2009. Regionalizacja tektoniczna Polski II. Lista terminów tektonicznych dla celów regionalizacji, online