Фораминиферы и биостратиграфия апта северного палеобиогеографического района Западной Сибири

Abstract

Several (7) boreholes have been drilled between the territories of the Pur and Taz rivers which are assigned to the Northern Palaebi-ogeographical district on the base of finding Foraminifera. The marine Aptian deposits have been discovered in this district for the first time. The southern boundary of the Aptian foraminiferal finds in the sections of the wells lies between the Pur and Taz rivers. Based on these finds, one may think that the Aptian boreal transgression has spread in the West Siberian basin from the Arctic one. The Aptian assemblages have also been found in the territory of the Yjamal Peninsula. For the most part, transgression may have occurred in the Middle Aptian age because there exist many Foraminifera Assemblages which conditionally are assigned to this age. The author of the paper has studied the Aptian Foraminifera Assemblages from the section wells of the South-Russkaya area, the line sections of wells from the Taseiskaya area (well P-159), the West-Tambeiskaya area (wells 42, 45, 124), the North-Tambeiskaya area (wells 201, 205). In theese sections agglutinated quartz - siliceous Foraminifera related to the Aptian age were discovered. Foraminiferal Assemblages proved to be of a small systematic diversity and a quantity of specimen species along with their rather poor preservation. This made it difficult to determine comparatively well-preserved specimens. Content rocks are mainly grey aleuro-lites with the interlayers of dark-grey argillites and light-grey millstone grit of the Viculian Horizon. The systematic composition of most assemblages of the identified sections allows us to establish the deposits of the conditionally Middle Aptian age in the middle part of the Viculian Horizon. The deposits concerned are documented in the section of borehole 55 (the South-Russkaya area) in the depth intervals 1841,50-1834,30 m. In the assemblage of this section the species Trochamminoides aff. ivanetzi Podobina, Labrospira aff. rotunda Podobina, Haplophragmoides topagorukensis Tappan, Pseudoverneuilina (?) aptica Podobina sp. n., Siphogaudryina rayi (Tappan) are present. Rounded forms of the Labrospira, Haplophragmoides, Trochammina genera are prevalent in established Aptian assemblages. Almost all the foraminiferal assemblages in this and the other established sections are supposedly of the Middle Aptian age. Based on the Foraminifera finds, the Aptian transgression is supposed to have reached its maximum in the Middle Aptian time. In addition, conditionally early and late Aptian foraminiferal assemblages are established in solitary sections. In the well sections of the Yjamal Peninsula in the Viculian Horizon the Aptian microfauna - agglutinated quartz-siliceous foraminifera and ostracod nucleis are also discovered. The foraminifera in all the sections are not abundant, they are approximately of a similar systematic composition and of rather a poor preservation. The paper concerned presents factual data on the first discovered Aptian foraminiferal assemblages related to the rocks of the Viculian Horizon. All Aptian Assemblages (mainly the Middle Aptian ones) are illustrated in 8 accompanying photos (drawing) which to some extent allow us to form an opinion about their systematic composition and quantative content. In addition an illustration of Pseudoverneuilina (?) aptica Podobina sp. n. species are shown in the fig. 9. Also there are brief description of this species.