Depositional Environment Of Zhiluo Formation In Fuxian Shaanxi Province

Zhiluo Formation is firstly named by Li Desheng(1951) in zhiluo Town Fuxian, and the holostratotype is Hulu River Strata Section in zhiluo Town Fuxian.Zhiluo Formation is above Yan’an Formation and underlying Anding Formation, has an unconformable contact with the underlying strata and conformable contact with overlying strata. Its lithology is mainly composed of massive feldspathic sandstone and the variegated mudstone, siltstone and so on.There is a lot of research achievements of Zhiluo Formation,but it is all lack of classification. Some basic problems still in doubt, such as where is Qilizhen,the boundary line between Zhiluo Formation and Anding Formation,the lack of facies marker materials and so on. To solve these problems,we do this study and the mainachievement made in the institute:1. according to the research level,collect, classify and inductive a number of research of Zhiluo Formation, some conclusions are drawn: the name, definition, evolution, holostratotype, geological time, geological characteristics, regional changes and other aspects of the material genesis.At last we use Qilizhen sandstone under Zhiluo Formation, Yan’an Formation’s coal streak, Gaoqiao sandstone in the middle of Zhiluo Formation, violet red sandstone under Anding Formation to division the Zhiluo Formation. Combined early research with the actual data,We suggest that Qilizhen is in huangling county and its current name is Diantou Town, located alongside of Jushui River where Jianzhuangchuan flow into.The lithology is yellow-green medium-coarse massive feldspathic sandstone, a stable distribution in this region, the erosion surface is on the bottom with gravel and the trunk fossils.2. The study found that Zhiluo Formation’s lower boundary line with Yan’an Formation is the coal sector, its upper boundary line with Anding Formation is violet red thick layer of feldspar quartz sandstone.Its can be divided into two cycles. The lower cycle is starting from the east of Liujiabian to the west of Dujiabian, Its lithology in the lower part is yellow-green thick layer of medium-coarse sandstone, It’s also called "Qilizhen sandstone ",a stable distribution in this region and gradually being thine form south to north to. There are erosion surface on the bottom with gravel and the trunk fossils; the upper part is yellow-green mudstone, siltstone interbedded with yellow-green middle layered sandstone. The upper cycle is starting from Caozigou to the southt of Jiangjiachuan,Its lithology in the lower part is yellow-green, yellow- gray blocks fine-grained sandstone, thers is the trunk fossils at the bottom; the upper part is yellow-green, blue, purple and other variegated mudstone, siltstone sandstone interbedded with yellow-green middle layered sandstone.3. Sandstone developed very well in the study area,the curve of cumulate to probability match well with channel sand, combine with other evidence,such as sedimentary structure, characteristic of Paleontology, we draw the conclusion that, it is braided river sedimentary facies in Dujiabian, meandering river sedimentary facies in Caozigou and lacustrine facies in Zhiluo Pagoda Mountain, braided river sedimentary facies and meandering river sedimentary facies can be further divided into channel facies and alluvial flat facies, lacustrine facies mainly developes shore-shallow lacustrine.In the early depositing stage of middle jurassic Zhiluo Formation the paleoclimate is semiarid climatic and mainly develops river deposition, In the latter depositing stage mainly develops shore-shallow lacustrine and belong to the variegated mudstone formation.4. The lower part of Zhiluo Formation is yellow-green medium-coarse massive feldspathic sandstone, Dolomite cement is feldspathic which comes from clay mineral‘s recrystallization, The main pore type are intergranular pore and solution typed secondary pores.The sandstone’s average porosity is 56.71% and average permeability is 49.16x10-3μm2,which suggest the reservoir quality of the lower part of Zhiluo Formation is low permeable sandstone reservoir(IIb).