Middle-late Permian Fossil Woods From Northern Tuha Basin:Implications For Palaeoclimate

Tuha basin is one of the three petroliferous basins in Xinjiang. This basin is rich in oil and gas resources and has vast exploration prospect. The hydrocarbon source rocks of Tuha basin are mainly concentrated in the Permian stratum, especially the Middle Permian. As we known, climate has close relation with the formation of oil and gas, so the research on the Permian palaeoclimate of Tuha basin is very important for the forecast of oil-generative stratum.Most of previous studies, however, on the basis of animal fossils, fossil plants, climate sensitive sediment and so on, made a simple summarization in the form of overview. Meanwhile, the study area of other investigators mostly included whole Xinjiang or northern Xinjiang. There are still a lot of problems, such as too wide survey region, low accuracy and large time scale.Gymnosperms fossil woods of seven deposites from Permian strata in Tianshan village, northern Tuha basin are studied. Based on stratigraphic correlation, characteristics of biological assemblage, and combining the isotopic dating data, we have established and improved the chronostratigraphic framework of study area. Finally, the age of fossil woods are inferred to be Middle Guadalupian to Early Lopingian. On this basis, combining the fossil wood rings, leaf physiognomy analysis, sporopollen assemblage and climate sensitive sediments, the Permian palaeoclimate characteristics and evolution in the area are summarized.In this paper, we did large number of internal anatomy for fossil woods that were collected from the Middle Permian Taerlang Formation and the Upper Permian Quanzijie Formation. A lot of thin sections from different sections have been obtained. By observation under a microscope, five genera and seven species are identified. The taxonomic study shows that all fossil woods collected in this investigation have been identified as coniferale. Among fossil woods, a new genus with only secondary xylem, Prototianshanoxylon Wei et Zhang gen. nov., and two new species (P. hamiense gen. et sp. nov.,P. erdaogouense gen. et sp. nov.) are proposed. The main characteristics of the new genus include below several aspects:The tracheid walls pitting of the new genus is a transitional type of araucarioid and protopinoid; The cross-field pitting is of typical window-like type; Tracheid walls has large number of septa instead of helical thickenings; The xylem ray are uniseriate and homogeneous, accidentally biseriate, and the walls of ray cells are smooth. Besides, we have established a new conifers fossil woods with pith, primary xylem and secondary xylem, Xinjiangoxylon hamiense sp. nov. Some hard-walled cells scatter in the pith of new species; Primary xylem is endarch, and its radial walls possess scalariform thickening; Secondary xylem is of Protophyllocladoxylon type, with transitional pitting between araucarioid and protopinoid. The discovery of these conifers fossil woods provides new materials for the classification of fossil woods of Angaran flora.A large number of fossil plants were collected from the Lower Permian Yierxitu Formation. The identified result shows that these fossil plants are typical Early Permian Angara flora, well comparing with other reported floras. Leaf physiognomy analysis indicates that the palaeoclimate in the study area are warm and humid during Early Permian, but intermittent droughts were also occurred.Using statistical analysis of fossil wood rings, for the first time, we have rebuilt the palaeoclimate of our research area during middle-late Middle Permian to early Late Permian. The high average mean sensitivity (0.4) and different width of continuous tree-ring show that the climate is frequently fluctuant, with uneven annual rainfall and cyclical wet and dry change. The occurrence of false rings and growth interruptions in the secondary xylem among the fossil woods samples also reflects the existence of intermittent drought at that time. Low late wood rate suggests the cold and dry period that trees experienced by was not too long. Compared with the typical North temperate zone, study area was location in the warm temperate zone with warmer climate. From Middle Permian to early Late Permian, fossil woods from different stratas, have obvious change in development of rings, experiencing a process from obviousness-slight obviousness-absence. Rings change completely recorded the change process of palaeoclimate, which varies from warm temperate zone with obvious seasonal variation to middle-low latitudes subtropical zone without seasonal variation. The seasonality of climate distinctly weakens.As for the sporopollen assemblage of Late Permian in the study area, conifers and pteridosperms pollens occupy a considerable amount, which reflects the relatively dry climate conditions. The occurrence of large number of "red layer" in the Upper Permian also indicates that the climate has become arid. In addition to "red layer", there are many calcretes or masses suggesting drought climate in the Upper Permian. However, from the point of the whole Junggar region, plant fossils and sporopollen extremely developed at that period, which shows that vegetation is prosperous, unlikely to extreme drought climate. Therefore, we think that the climate of study region may be semi-arid to sub-humid, with periodic dry-wet shift during the Late Permian.Combining the statistics of wood rings, the discussion of flora composition and property, leaf physiognomy analysis of fossil plants, sporopollen composition and climate sensitive sediment, we put forward the Permian palaeoclimate evolution in study area. During the Early Permian to middle Middle Permian, our survey region was located in the northern hemisphere warm temperate zone, with relatively high latitude. The climate had obvious seasonal variation, so trees developed strong rings; Beginning from middle-late Middle Permian, the study area moved to subtropical zone with no obvious seasonal variation. Due to the change of climate, most trees rings were not obvious even absent; In the middle-late Late Permian, affected by the formation of pangea, study area became relatively dry, with semi-arid to sub-humid climate condition and periodic drought-wetness shift. The Siberia plate where study area located gradually moved to the south, and eventually collision with the Tarim-North China plate, just like a "scissors", which should be the main factors that leaded to changes of palaeogeography, palaeoclimate and phytogeography.