Study On Tertiary Sequence Biostratigraphy In Qigequan-Hongliuquan Region, Western Qaidam Basin

Tertiary strata of Qaidam basin include Lulehe Formation, Lower Ganchaigou Formation, Upper Ganchaigou Formation of Paleogene, and Lower Youshashan Formation, Upper Youshashan Formation, Shizigou Formation of Neogene from bottom to top.The sequence biostratigraphy is a multidiscipline integrated sequence stratigraphy with biostratigraphy. The interface of the base level cycle of high-resolution sequence stratigraphy and the seismic sequence boundary jointly make up of an isochronous stratigraphic frame. After dating a member of stratum, using palaeontological fossils, the stratum borders can be determined according to the base level. The time span of the formations in each stratum is in accord with the age of the second-order sequence without exception, which is defined as a long-term cycle. Therefore, the long-term cycle is benchmarked on an existing lithostratigraphic unit, whose interface is equivalent to T0~T5 seismic markers. The disciplinarian fluctuation of GR curve and the change of superposition mode of short-term base level cycle are an effective basis for identifying the medium-term base level cycles.The number of the medium-term base level cycle identified in high-resolution sequence stratigraphy of Qigequan structure in Qaidam basin includes two cycles in Shizigou Formation (named as S1-2 ), three cycles in Upper Youshashan Formation (named as SY1-3), four cycles in Lower Youshashan Formation (named as XY1-4), three cycles in Upper Ganchaigou Formation (named as SC1-3), five cycles in Lower Ganchaigou Formation (named as XS1-5), and three cycles in lower member of Lower Ganchaigou Formation (named as XX1-3). The cycles in Hongliuquan Structure resemble those of the Qigequan structure.We chose four seismic profiles across wells in Qigequan and Hongliuquan regions, composing a group of closed netlike sections, and dated using fossils, confirmed the boundary by base level cycles. In south and north of well Hongshen 10 there at least exists a Paleocene depositional depression. In the region of well Hongshen 10 and well Qi 24 there likely exists an Paleocene fault zone, whichseparated the two depositional depressions of Qigequan and Hongliuquan. In a sequence biostratigraphic framework, the fossil Mediocypris distributes in the Lower Youshashan Formation and Upper Ganchaigou Formation and the fossil Cyprideis distributes in the Upper and Lower Youshashan Formations.Ecological system tract is a sedimentary body reflecting the environmental attribute of sequence system tracts using a proper palaeontology assemblage during a sequence stratigraphic evolutional process. The four ecological system tracts (biochore) in this paper are divided as follows: No.1 Ilyocypris ecological tract; No. 2 Cyclocypris ecological tract; No. 3 Cyprideis ecological tract; No. 4 Charophyte ecological tract. The Ilyocypris ecological tract mainly corresponds to the TST in the third-order sequence, and the Cyprideis ecological tract to the HST or the lower TST. The Cyclocypris ecological tract is close to the changing point of decline and rise of the base level. The Charophyte ecological tract can be formed in both TST and HST, while its salinity of water body is fresh to somewhat saline.The fossils of ecological tract reflecting an entire study of the relation of the fossils and environments indicate the existence of the tract which can support the confirmation of the tract of sequence stratigraphy and demonstrate its environmental attribute as well. This is the significance of the ecological system tract in the research of the classification of the system tracts of stratigraphic sequence.